My speedometer in my '93 240sx is reading about 7 mph too fast at highway speeds (I read 77 mph when my friends car reads 70 mph right next to me). Is this a function of the speedometer head unit, or the speed pickup on the transmission? Since I know exactly how far off it is (close enough for me, that is, without taking it in somewhere) is there a way to adjust this?
The wheels and differential are bone stock and the tires are standard size so I dont have any obvious modifications that would cause the problem.
Speedometer overly- optimistic
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guyaverage
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- Car: Nissan 240SX Convertible
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psychic_mechanic
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It's a way for the warranty on your car to run out 10% faster and make you think the car is faster at the same time. 
My buddy had a 96 or so civic that was exactly 10% and honda told him that was within spec.
Chances are there were several different tire/wheel combos offered that model with slightly different sizes and they split the difference between several sizes. Unless you are running the stock brand tire, there can still be a significant size difference in mounted sizes of tires that are labeled the same size.
-Vinny
My buddy had a 96 or so civic that was exactly 10% and honda told him that was within spec.Chances are there were several different tire/wheel combos offered that model with slightly different sizes and they split the difference between several sizes. Unless you are running the stock brand tire, there can still be a significant size difference in mounted sizes of tires that are labeled the same size.
-Vinny
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guyaverage
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- Car: Nissan 240SX Convertible
I was wondering the same thing, if it wasnt within some ultraliberal spec of 'accuracy'. Is there any way to adjust this, other than changing out the little plastic gear in the transmission?
I had read on a different thread somewhere that there was a screw on the head unit somewhere that would adjust the speed reading but this sounded to hokey to be true, and I didnt want to go tearing into the dash just to try to find something that may not even be there to begin with.
I had read on a different thread somewhere that there was a screw on the head unit somewhere that would adjust the speed reading but this sounded to hokey to be true, and I didnt want to go tearing into the dash just to try to find something that may not even be there to begin with.
The way the speedo works is:
Drivetrain spins the speed sensor.
Speed sensor sends on-off pulses to the ECU (2 times per revolution i think but it doesnt matter)
ECU counts the pulses and puts out a certain voltage.
Voltage is read by the speedo and the needle is pushed a certain amount by how much voltage is seen.
If the speed sensor was messed up you would wouldnt get any mph reading at all. If the ECU was messed up most likely your car wouldnt be running very well in the first place.
Usually its just the speedo out of calibration. In fact like stated above i believe all speedos are calibrated alittle higher than their supposed to be, or the mechanism itself just cheapy and isnt very accurate.
My car used to be at 40 and the speedo would read 46.... at 70 it was reading over 80... yikes! That was with 15x6 steelies and 195/60/15 tires, i upgraded to the SE wheels and got 205/60 tires which increase the overall diameter a bit. Speedo is a bit tamer now... reads 43-44 at 40mph and 80 at 74-75mph.
I dont know how much a recalibration will help, or how much it will cost, but you can try... be a guinea pig please?
Drivetrain spins the speed sensor.
Speed sensor sends on-off pulses to the ECU (2 times per revolution i think but it doesnt matter)
ECU counts the pulses and puts out a certain voltage.
Voltage is read by the speedo and the needle is pushed a certain amount by how much voltage is seen.
If the speed sensor was messed up you would wouldnt get any mph reading at all. If the ECU was messed up most likely your car wouldnt be running very well in the first place.
Usually its just the speedo out of calibration. In fact like stated above i believe all speedos are calibrated alittle higher than their supposed to be, or the mechanism itself just cheapy and isnt very accurate.
My car used to be at 40 and the speedo would read 46.... at 70 it was reading over 80... yikes! That was with 15x6 steelies and 195/60/15 tires, i upgraded to the SE wheels and got 205/60 tires which increase the overall diameter a bit. Speedo is a bit tamer now... reads 43-44 at 40mph and 80 at 74-75mph.
I dont know how much a recalibration will help, or how much it will cost, but you can try... be a guinea pig please?
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psychic_mechanic
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- Joined: Wed Jan 07, 2004 6:45 am
- Car: the world's ugliest civic
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guyaverage
- Posts: 224
- Joined: Thu Feb 19, 2004 9:48 am
- Car: Nissan 240SX Convertible
I had a feeling this was the case (there not being an adjustment screw somewhere) but I wanted to make sure before I took it to the speedometer shop down the street from my work. I had a '93 240 about 6 years ago with almost the exact same speedometer inaccuracy and it cost about $60 to have them put it on rollers to check the true speed and to have the VSS gear changed out, but it did solve the problem. Maybe I imagined it but I also noticed afterword that my auto transmission seemed more eager to downshift (an unexpected but welcome change) when I hit the gas, I suppose as a result of the lower vehicle speed signal getting to the ECU...(?) Anyway I dont mind spending the money to get it done again, I just wanted to make sure there wasnt a cheapy way around this first. Thanks for the replies.