I'm aware that all factory speedos under-read for "safety reasons". I've been having a weird problem with my speedo where it reads much higher than the actual speed, and then doesnt settle back to 0 at a stop. After not being able to find any chukis in the yards for the last few weeks, I just bit the bullet and pulled a speedo from an SOHC cluster. Here's what I found:

60MPH=1024rpm on both gauges, so they should work! From eyeballing it though, it looked like the last tick on each gauge lined up, which is odd because one is 110 and the other obviously 115. So I set out marking them with an index card behind the gauge face...

(sorry its kinda washed out) Marking the SOHC gauge

90=95? wtf?

Curious, I tried the lower end of the gauge where the needle is more likely to live. This stuff was off...

It gets a bit closer here.

Ultimately, this is what we get across the whole gauge. DOHC in lead, SOHC in pen.

There's a number of possibilities here:1) the SOHC gauge lies2) the DOHC gauge lies3) they both lie4) they are both CORRECT and somehow are scaled internally to read at the correct speed. i.e. if you put the SOHC face on DOHC electronics it would no longer be accurate.

http://www.courtesyparts.com/b....html There's only 1 part # for the VSS so that is ruled out. Can anyone offer some input on this? I put the gauge in and it seems to me that its spot on (from experience with sped in gear), so i'm leaning towards #4. Thanks for reading.

Umm...what is the big deal?

When an individual makes changes to a vehicle the speedometer does tend to read wrong.

On top of that older electronics some times need to be recalibrated to be more accurate.

When was the last time you had the vehicle speed sensor replaced?

Umm... did you read anything?

The deal was my existing speedo went bad and I couldnt find a replacement, so I popped in one from an SOHC. I noticed a discrepancy between the gauge faces and asked if anyone knew anything more about it. One or both of the speedos may be bogus.

Goldeneagle wrote:Umm...what is the big deal? When an individual makes changes to a vehicle the speedometer does tend to read wrong.

Car is stock. One day the speedo worked, the next day it didnt. Also, is there any change that I could make that would cause the speedometer to register 70mph while at a stop light?

Goldeneagle wrote:On top of that older electronics some times need to be recalibrated to be more accurate.

This is a foolish statement. When was the last time you calibrated your TV? Rebuilt your microwave? Yes, electronics fail (as mine did) but they cannot be recalibrated for age related wear.

Goldeneagle wrote:When was the last time you had the vehicle speed sensor replaced?

Again, if you read what I posted above, you'd see that when I put the SOHC speedo in it read perfectly. In my original thread, I mentioned that the odometer is not affected, so I ruled out the vss.

see if you can figure out what kind of signal it works off from. If its a variable dc voltage and the gauge is functioning like a voltmeter then you can add a potentiometer /variable resistor on the wire between the gauge and sending unit. Then tune it until you get the right voltage offset to set it back to an accurate speed. If the signal is a variable resistance same thing. Its just the gauge is just setup to work as a ohm meter. If however its a square wave or ac sine wave signal of varying frequency then I can't tell you an easy way to shift the frequency of the signal to calibrate your speedo. But I'm sure there's an easy way. Just first use an digital volt ohm meter and see what kind of signal is coming on the connector to your speedo. Then if its volts or ohms you know what to do. If its a varying voltage, then call an electronics shop and ask them for a simple inline device/circuit that can change the frequency with a small adjusting screw on it.

best of luck thats my idea. You seem like you would try something like that. I would probably keep hunting for the right part myself.

avrfan wrote:see if you can figure out what kind of signal it works off from.

http://www.zeroyon.com/index/content/view/20/47 Thats how it works. However, thats not my problem.

avrfan wrote:If its a variable dc voltage and the gauge is functioning like a voltmeter then you can add a potentiometer /variable resistor on the wire between the gauge and sending unit.

Both gauges already come with a pot from the factory.

I don't need to retune it, I now have a working part. The point of this thread is to choice the accuracy of both the stock speedometers. Everything is now in working condition. I am not attempting to fix anything. Just wanna know if anyone has any info on which one is more accurate...

your best bet is to take it to a speedometer calibration shop, they can make sure your speedo is reading correctly , also handy if you get a ticket and the speedo comes out reading low :P

worth calling around about if you "have" to know for sure.

Hype wrote:Umm... did you read anything?

The deal was my existing speedo went bad and I couldnt find a replacement, so I popped in one from an SOHC. I noticed a discrepancy between the gauge faces and asked if anyone knew anything more about it. One or both of the speedos may be bogus.

Car is stock. One day the speedo worked, the next day it didnt. Also, is there any change that I could make that would cause the speedometer to register 70mph while at a stop light?

This is a foolish statement. When was the last time you calibrated your TV? Rebuilt your microwave? Yes, electronics fail (as mine did) but they cannot be recalibrated for age related wear.

Again, if you read what I posted above, you'd see that when I put the SOHC speedo in it read perfectly. In my original thread, I mentioned that the odometer is not affected, so I ruled out the vss.

I guess you seemingly can not comprehend a full sentence.

My first statement was in regards to "When" changes are made. I was not implying you had made changes.

The second statement states the fact that older electronics "Some times" needs to be recalibrated. I was not implying that all older electronics needs recalibration all the time in all situations.

In regards to my third statement, you failed to mention your original thread. So I asked when was the last time you changed your vehicle speed sensor.

Based off of the information you happend to give, I gave the best information I could give. Furthermore, I am glad I posted what I did. The statements i left made you post more, unmentioned, information.

Anyway, if you absolutely have to have perfection, go get your stock speedometer recalibrated.

avrfan wrote:see if you can figure out what kind of signal it works off from. If its a variable dc voltage and the gauge is functioning like a voltmeter then you can add a potentiometer /variable resistor on the wire between the gauge and sending unit. Then tune it until you get the right voltage offset to set it back to an accurate speed. If the signal is a variable resistance same thing. Its just the gauge is just setup to work as a ohm meter. If however its a square wave or ac sine wave signal of varying frequency then I can't tell you an easy way to shift the frequency of the signal to calibrate your speedo. But I'm sure there's an easy way. Just first use an digital volt ohm meter and see what kind of signal is coming on the connector to your speedo. Then if its volts or ohms you know what to do. If its a varying voltage, then call an electronics shop and ask them for a simple inline device/circuit that can change the frequency with a small adjusting screw on it.

best of luck thats my idea. You seem like you would try something like that. I would probably keep hunting for the right part myself.

Closest answer yet. It's not voltage based as it would need too large of a range. It's a frequency based system. It's fine man, the only way your gauge is going to be off is if your speed sensor is screwed, or more likely you have changed the circumference of the wheels where making it smaller will net in an odometer that reads faster than your actual speed, and larger reads slower than your actual speed.

To make it easier for some to under stand, the speedometer is just like an analog clock, and if you look at the back of any wall clock, there is an adjustment to correct a fast or slow clock. Electronic speed sensors are the same way, but the adjustment is with a resistor in the gauge itself.

So there you have it, the gauge face has no bearing on it so long as you keep it DOHC to DOHC and SOHC to SOHC. As far as I know, the speed sensors have the same part number, though you may want to double check.

Pretty intresting I'll be the first to tell you that I am NOT an elctronic man but based on the note card experiment that you did IMO it seems to me that the DOHC is off but then again I suppose you would have to factor in wheel size/weight which both are the same stock to my knowledge or if you a 240 that has rims compared to one with hub caps then obviously the weight of the wheel is different and will cause the speedometer to read wrong(correct me if I'm wrong please) and since you have replaced your speed sensor as stated above the simplest/easiest thing that I would do is if you have a buddy or own a SOHC unmoded have someone drive the SOHC and you drive the DOHC or the other way around and both of you drive until you hit 35 and stay there and see where your at and you might get your answer? Like if the SOHC was a little ways back then it's speedometer is off or the other way around if the DOHC is behind then you have your answer maybe make sure the SOHC has a new speed sensor that way you know it's accurate also this is probably an ignorant question but it shouldn't matter on the hp rating where the speedometer is right?. idk i'm just throwing in my two cents like I said I sit on the sidelines and watch when it comes to electronics but good thread for sure very very intresting
Modified by low91240sx at 8:51 PM 8/3/2008

Goldeneagle wrote:I guess you seemingly can not comprehend a full sentence.

Dude, dont hate. I questioned whether or not you read my post because your comments missed the intent of the OP. While I appreciate your suggestion to look into the vss as a cause for an errant speedo, I had already moved past that at the time I made this thread. I just documented the differences between the gauge faces and asked if anyone knew anything more. Not looking to start any beef with you...

H8tred wrote:Closest answer yet. It's not voltage based as it would need too large of a range. It's a frequency based system. It's fine man, the only way your gauge is going to be off is if your speed sensor is screwed, or more likely you have changed the circumference of the wheels where making it smaller will net in an odometer that reads faster than your actual speed, and larger reads slower than your actual speed.

To make it easier for some to under stand, the speedometer is just like an analog clock, and if you look at the back of any wall clock, there is an adjustment to correct a fast or slow clock. Electronic speed sensors are the same way, but the adjustment is with a resistor in the gauge itself.

I'm familiar with the gauge input. In fact I linked to the article describing it in detail. There was no mention of changing wheel sizes either. This is merely a comparison of the two gauge faces. Since it is known they take the same input, one of them must be off.

H8tred wrote:So there you have it, the gauge face has no bearing on it so long as you keep it DOHC to DOHC and SOHC to SOHC. As far as I know, the speed sensors have the same part number, though you may want to double check.

I put an sohc gauge in a dohc b/c I couldnt find an dohc. The speed sensors are the same. I provided a link to courtesy parts.

low91240sx wrote:Pretty intresting I'll be the first to tell you that I am NOT an elctronic man but based on the note card experiment that you did IMO it seems to me that the DOHC is off but then again I suppose you would have to factor in wheel size/weight which both are the same stock to my knowledge or if you a 240 that has rims compared to one with hub caps then obviously the weight of the wheel is different and will cause the speedometer to read wrong(correct me if I'm wrong please)

Thanks for taking the time to read my post! I know to most it seems like much ado about nothing, but I was just throwing it out there to see if anyone knew anything about it. As for the DOHC being off, who knows. It could be the SOHC thats wrong. The wheel weights however make no difference. While light wheels certainly help performance, they do not affect the gauge reading one bit. The VSS measures rotation in the transmission and rotation @ a given speed is unaffected by this. SIZE however plays a role in this. IIRC stock steelies are 14 inchers, alloys came with 195/60R15s and HICAS models roll on 205/60R15s. To my knowledge nissan made no correction to the gauge display to adjust for this, but if they have I imagine it was via the potentiometer at the back of the gauge....

low91240sx wrote:and since you have replaced your speed sensor as stated above the simplest/easiest thing that I would do is if you have a buddy or own a SOHC unmoded have someone drive the SOHC and you drive the DOHC or the other way around and both of you drive until you hit 35 and stay there and see where your at and you might get your answer? Like if the SOHC was a little ways back then it's speedometer is off or the other way around if the DOHC is behind then you have your answer maybe make sure the SOHC has a new speed sensor that way you know it's accurate also this is probably an ignorant question but it shouldn't matter on the hp rating where the speedometer is right?. idk i'm just throwing in my two cents like I said I sit on the sidelines and watch when it comes to electronics but good thread for sure very very intresting

Nah I haven't replaced the sensor, but i know the sensor was good because the while the broken speedo read funky (sometimes 110+ while actual speed was more like 40 ), the odometer was unaffected. I do have a buddy with an SOHC, i'll have to get him to participate in this, haha.

115=auto 110=5speed..and if your going to put a sohc gauge for a dohc motor, its not going to get the same reading...

i240sx wrote:115=auto 110=5speed..

nope, my friend has an SOHC 5speed and its 115.

i240sx wrote:and if your going to put a sohc gauge for a dohc motor, its not going to get the same reading...

Why? The TACH is incompatible but what drives the speedo? The VSS does, and we've covered that here.

i240sx wrote:115=auto 110=5speed..and if your going to put a sohc gauge for a dohc motor, its not going to get the same reading...

I can tell ya that it doesn't matter if it's auto/manual the gauges are the same to my knowledge and both guages are the same as stated above they are both 60mph=1024rpm so it matches right up just one of them is giving the wrong reading

Exactly. This isnt a matter of life and death, just a curiosity I had. If no one knows, oh well so be it.

FWIW, driving home today I passed one of those radar speed limit signs... I was doing 35 and the sign said 35... right on the money. So maybe SOHC gauge + stock HICAS tire size = perfect reading? Keep in mind speedos are designed to overread by a few mph for safety reasons.

haha well maybe that's all it needed was a SOHC speedo lol

So much misinformation lol. 115=sohc auto or manual, it does not matter. 110=dohc auto or manual it does not matter.

Like I said, unless you're changing just the faces between the two different models I.E; SOHC gauge face onto the DOHC gauge body, it does not matter. The sohc gauge is internally calibrated to read as true as any oem gauge can. It's plug and play. The only reason the SOHC tach wont work on a DOHC engine is because of wiring differences for the ignition.

On to the speed sensor. Wheel size will matter. As far as I know, most base model 240s were offered with 195/60-15 tires, and SEs got 205/60-15 tires. It may have been available as an option as well, I'm not really sure, and it has no bearing on the end result of the point.

Between the two assuming that Nissan did no additional calibrating, there is a difference of 2.0% larger diameter in the 205. Meaning that your speedometer is reading 60mph but your actual rate of travel is 61.2mph.

Now lets say you go from the stock 195/60-15 to 205/35-18. You now have a difference of 2.3% The 195/60-15 has a diameter of 76.1 inches, and the 205/35-18 has a diameter of 74.3 inches. The stock wheel will complete 833 full rotations in one mile, while the aftermarket wheel does 853. Your speedometer says 60mph, but your only actually going 58.6mph. Now add that on top of the accuracy of the original speedometer.

Now in my case, I have Nissan Data scan, the original speedometer, and Iguideance GPS on the laptop. I notice at most a 1.5 mph difference between the three, granted that you can input your tire size in Datascan so the input from the speed sensor is modified. However, the gps isn't. I have had 195/60-15s, 205/60-15s, and I can't remember the tire size on the 16s, and my trans took a dump on my today, so I'm 30 miles away from my car .

Anyway, summery... You're old speedo was kaput, The sohc speedo will read just as accurate as your original granted tire size. If it acts all retarded like the original, it's the speed sensor or a short in the wires.

H8tred wrote:Like I said, unless you're changing just the faces between the two different models I.E; SOHC gauge face onto the DOHC gauge body, it does not matter. The sohc gauge is internally calibrated to read as true as any oem gauge can. It's plug and play. The only reason the SOHC tach wont work on a DOHC engine is because of wiring differences for the ignition.

On to the speed sensor. Wheel size will matter. As far as I know, most base model 240s were offered with 195/60-15 tires, and SEs got 205/60-15 tires. It may have been available as an option as well, I'm not really sure, and it has no bearing on the end result of the point.

Between the two assuming that Nissan did no additional calibrating, there is a difference of 2.0% larger diameter in the 205. Meaning that your speedometer is reading 60mph but your actual rate of travel is 61.2mph.

Now lets say you go from the stock 195/60-15 to 205/35-18. You now have a difference of 2.3% The 195/60-15 has a diameter of 76.1 inches, and the 205/35-18 has a diameter of 74.3 inches. The stock wheel will complete 833 full rotations in one mile, while the aftermarket wheel does 853. Your speedometer says 60mph, but your only actually going 58.6mph. Now add that on top of the accuracy of the original speedometer.

Man you could have saved yourself a whole lot of typing. I know about the tach thing; it was brought up as a counter to some misinformation here in this thread. Also, I commented on tire sizes in my reply to low91240sx. I'm aware that tire size changes will affect the speedo.

H8tred wrote:Now in my case, I have Nissan Data scan, the original speedometer, and Iguideance GPS on the laptop. I notice at most a 1.5 mph difference between the three, granted that you can input your tire size in Datascan so the input from the speed sensor is modified. However, the gps isn't. I have had 195/60-15s, 205/60-15s, and I can't remember the tire size on the 16s, and my trans took a dump on my today, so I'm 30 miles away from my car .

Interesting, the data scan is off from the speedo? Was this with stock tires? I dont know anyone that has a standalone gps, otherwise I'd try that... damn. Sucks about your transmission man. My clutch broke (a spring popped out of the hub) on freaking new years when I had some friends fly into town to visit me.

H8tred wrote:The sohc speedo will read just as accurate as your original granted tire size.

The last picture in the OP shows the markings between DOHC and SOHC gauges are off. Its not simply the difference between numbering, the hash marks are just plain off. The input is the same since the speed sensor is the same. The point of this thread was to find out which one is goofed up.

H8tred wrote:If it acts all retarded like the original, it's the speed sensor or a short in the wires.

Everything works fine, except that it looks out of place

Hype wrote:Everything works fine, except that it looks out of place

I know how you feel, I've been driving my Canadian 240 so it's 180 in kilometers. I got into my 92 that I haven't driven in 6 months or more, and kept looking at the smaller numbers wigging out thinking I was going to fast.

I assure you that they are calibrated to read normally in the speedo itself. When I said they were off, I meant between the datascan/gps, and the speedo. Gps and datascan were the same.

As far as my transmission, I'm not really sure what happened, there was no noise, no warning, no holes or leaking fluid. I got off the interstate in 5th still getting power to the wheels, downshifted to 4th, released the clutch, gave it some throttle, and it just revved freely. Same with every other gear. Got to the side of the road, and started thinking about it. Did the slave go? Nope, I have good pedal pressure, and it goes to where each gear should be with or without the clutch. Did the drive shaft snap? Nope, I would have heard it. Did the flywheel shear its bolts ? Maybe, but there was and is no noise.

It's either something in the flywheel, or clutch, or I snapped the input shaft so clean that it isn't making any noise. Won't know for sure until I drop the trans.

I can start the car, put it in gear with our without the clutch, walk around outside, and hear nothing more than the chain rattle, and exhaust. It's really got me baffled.

I've got my money on the clutch bud could be wrong but sounds similar to what happened to me last October. I guess the car didn't want you figuring out the real truth about the speedo jk bud but sounds like the clutch to me

see the KM/H they are calibrated right. look where tle line of 180km is and see the other speedo.. km matches miles I dont see something wrong...

Look at the pictures again, especially the last one...

Heres my experience, I pulled my needle out of my speedo to put in lights behind the needles and forgot where exactly to put the needle. When I was at a junk yard I found an 89 240 and saw where the needle bottomed out completely and then put my needle to the same place. When driving with my GPS on the stock tires on average it showed that I was going five mph over what I was really going. So my speedometer showed 75, I was really going 70. At that same time I was going through a construction area that had a "heres your speed" thing and it agreed with my GPS about exactly. So all I know is inherently speedo is off 5mph, but thats with my needle's bottom where the SOHC was bottomed. There are two dots near the bottom and I think it was set to the bottom one of those. Hope that made sense lol

If you look at the first picture on this thread, the litle white dot under the needle is where it bottoms out. So i was wrong, after seeing the picture it was that white dot.

you spent way too much time on that...