I have a 99 Hatchback with a B18c5 motor/trans swap and ITR guages installed. Problem I have is that at speeds over 25, by spedometer is off by 5 MPH too high. If the needle is at 70, I'm going 65 in reality. I found this out after using my Garmin GPS on a trip. In my 08 Fit, the speed matches exactly so it's not a question of the GPS' accuracy. Tires are 205-45-16 which I believe is right at stock diameter.

Any idea what could be causing it to show a faster speed?

 

Vehicle speed sensor?

 

chances are its whats said above

another problem could be tires/alignment but it would need to be drastic for an 8% difference

 

Well it's already off by .5mph but that's not a big difference for tires. For the other 4.5mph. Bump for someone else with more excperience.

 

I thought that the VSS is more of a mechanical item.... or am I thinking of an older type?

 

On my B18C, the VSS is a "permanent magnetic variable reluctance sensor that provides a pulsing voltage whenever vehicle speed is over 3mph. These pulses are used by the PCM to control fuel injector duration and transaxle shift control"

Black Wire: chassis ground
Black/Yellow: Ignition Power
Blue/White: VSS Out

 

Both speedometers on my 97 and 00 completely stock Civics read 3 mph faster than true speed. My understanding is that this a well known problem of 6th gen Civics. This now leaves an unexplained 1.5 mph.

 

So the speed being off might have an impact on my injectors? Hmm.

 

How?

 

?

 

Manufacturers have been known to make cars read high from the factory.This helps keep them out of trouble when speeding tickets and things of that sort come up. If you have a factory service manual or something similar,sometimes they have a table that shows what is acceptable and what isnt in terms of actual speed

 

Oh, sorry. Now I understand what you are saying. Thats not at all what I meant (that was literally copied and pasted from my Integra manual). What I meant to be saying basically was, It's not simply a mechanical device. It is closer to what the pickup of a guitar is.

A device that measures the fluctuation of the magnetic field induced...

In turn, a change in resistance can effect the output accordingly...

 

You don't think it was just a Honda goof up?

 

He would have to find the table i was talking about and see whats acceptable. I has seen this on other manufacturers vehicles also.

 

wheel and tire combo??

 

I remember seeing something on TV years ago about this. They said that car manufacturers can have up to a 3mph difference from one another, when it comes to speedometers. I want to say it was on Mythbusters, when they did the tests to try and block the speed radar gun; but I watch entirely too much TV so it could have been another program.