Speedometer Problem?

[Loki]

Recently this has come to my attention, especially on highways. First started noticing the ever increasing number of cars passing me that normally do not do so...then I broke out a timer and measured how long it takes me to travel between each 1/10th mile. Did some math and found that, on average, my actual speed was around 5 MPH LESS than the speedometer reading.

Further proof was later on....I was following a friend who was traveling at 75 MPH, speedo read 80 MPH. What could be the problem? Is it cheap to fix?

Thanks for any and all help guys
Modified by Loki at 2:05 PM 4/12/2007

[nissanconvert]

You know, I noticed the same thing when I first started driving my vert after driving my other car. It seemed like I was getting passed far too often for the speed I was going but I just thought I was crazy and that sitting lower made the feel different. I have factory sized tires and stock rims. I am also curious about this. Are the speedos just set to read higher?

[GoinTopless240]

Just an FYI... Most vehicles produced around the time of the Verts had what was called a "fudge factor". This was built in to the speedo to allow the factory to use same speedo gears and allow of optional tire sizes and still remain at or under actual speed. Also to take into account is new tires compared to worn tires (that can account for a 1-2 mph variance... Vehicle of current have tires sizes programmed into the ECU software in some cases.... older vechicles were not sensitive to tire size other than the speedo... newer vehicles use tire circumference to measure tire pressure, anti-lock braking, traction and yaw control... that being said that may account for your variance... Nissanconvert.. the 240 was actually produced with two tire sizes 195/60r15 and 205/60r15.. if you are running 195's you will notice even more "fudge" because of the smaller circumference... hope this helps you guys.

[Loki]

Well, I already know the speed is off. No real problem, I just have to add five miles once it gets past 60-ish.

But would the tacho be off also? Am I REALLY pushing 3k rpm at 80 (which is really 75)?

[guyaverage]

Boys this is an easy one to remedy. My speedo was about 10 mph off (fast) at highway speeds.

...If you open up the gauge cluster you will find the pot they are showing in this picture. I drilled a small hole in the top plastic part of the gauge cluster so that I could get a small screwdriver into it, to adjust while I drove (you are responsible for you own life while doing this, dont blame me if you get dead doing it). Put the steering wheel all the way down, and pull the cluster out and you can drive (uncomfortably) while adjusting the pot. TINY adjustments make BIG changes to the speed shown, so go easy on it. Your obvious best bet is to have someone ride along in the passenger seat to do the adjusting while you drive, and they can talk on the phone to your pace car next to you so they know how fast to adjust it.

My tires are 195's on the stock rim, with an automatic in 4th I think that puts 3k rpm at 72 mph (I'm going on memory, dont quote me).

[wirelessalpha]

cool - ill have to try this. I have the same speedo problem

[slownslurious]

wow you guys go through so much work to get every little thing on your cars working right lol... reminds me of the person wanting to replace all their trouble light bulbs when they burnt out. But yeah even a correctly calibrated 240sx speedo gets inaccurate at higher speeds, I know on my other cars it would register on the speedo the same speed as those automatic speed detector signs they put up at construction zones if I was going 50 or 60 but if I was going 80 or 90 it would be 5-10 mph off and if I had it pegged the speedo would be as much as 15 mph off. The HUD on my sohc seemed to be much more accurate.

but maybe if I had bothered to adjust it like you said I could have corrected that. not sure.I swapped the HUD into my vert and its never worked since so I dont really worry about that anymore!

[AZhitman]

Edit - Bump for Article Status.

[dickie]

yeesh... while calibrating your speedo can be helpful and keep you out of trouble with the law, i can only imagine what the police officer that pulls you over while attempting this would say. i would imagine you would want to do this on a long flat stretch, possibly an industrial driveway or something...

[DeafMonkey]

Ive always had my suspicion that mine was off. I have a few of those radar zones in my area and either im up by a few mph or theyre down by a few. I was also wondering what everyone's RPM to MPH ratio is.

mine is about... 2200 - 65mph (or something like that)3000 - 80mph3500 - 90mph4000ish - 100mph4600ish - 110+ (bottomed out needle)

this is all of course assuming that my speedo is correct and im really going at those speeds

[dickie]

yours has got to be off... for almost everyone i know 3000 rpm is just under 70mph in 5th gear.

[Loki]

For me, 3000RPM is between 70-75 MPH. Real MPH, not the fudged reading my speedo gives. 60-65 is between 2600-2800. At 80, it jumps a bit above 3000, and I haven't driven faster than 80 enough to really notice was a stable RPM is for higher speeds.

[kc240guy]

I recently swapped a j30 diff into mine and it made my speedo more correct. Now I'm only a couple mph off at 70 to 80. lower speeds read correctly.

[DeafMonkey]

The strange part about it all is that if my speedo is off, then everyone else is going really REALLY slow. I live in the LA area and drivers here are crazy, so every one here does drive fast. My 240 is also AT (automatic) so maybe that makes a difference? I dont know though... im really interested to hear what other people's ratio is.

[ishkabibble]

The strange part about it all is that if my speedo is off, then everyone else is going really REALLY slow. I live in the LA area and drivers here are crazy, so every one here does drive fast. My 240 is also AT (automatic) so maybe that makes a difference? I dont know though... im really interested to hear what other people's ratio is.

Yep, it makes a difference.

My speedo is dead on, and my ratios are similar to yours.