Jglisson

iTrader: (1)

I have a 1999 T/A with an inaccurate speedometer and I can not seem to figure out why. The speedometer is around 7-9 mph lower than actual speed. Actual speed is being measured by GARMIN GPS.

I do have GU5 on the door and verified by measuring the rear wheel rotation to the driveshaft rotation that I do have a 3.24 gear ratio. I have a basic HP tune (thanks to Mmjr) and he did verify that the speedometer is showing the right speed as determined by the PCM. In other words, the HP tuning computer is showing the same reported speed that the speedometer is showing. This speedometer error existed on both the stock wheels and the new C6 wheels that I installed. At the same time the C6 wheels went on the PCM was also tuned to reflect the larger wheel diameter. I have replaced the only seemingly culprit - vehicle speed sensor and that has not solved the problem.

Any ideas?

ringram

You can mess with H0105 (in efilive) to adjust the speedo to dial it in exactly. Most are slightly out anyway.

Jglisson

iTrader: (1)

Could you expound on that a little more please?

mrr23

iTrader: (3)

basically we can go in and mess around with gear ratio and tire height until we get the speedo to match.

Eurospec2

iTrader: (2)

with efi live you dont need to mess with tire size/gear ratio to trick it into beeing accurate (this would also screw up shifting in an A4)

EFI Live has a speedo tuning tool, specifically for calibrating in the dash when the PCM and dash are showing different speeds.

Frost

iTrader: (45)

he said that they are the same...

What I usually do for this is, set the proper gear ratio, and play with tire height until the speedo matches the GPS. HPT will automatically rescale all of the auto tranny parameters if you want it to.

supra8

If you have an auto trans leave the tire size and gear at what they really are and change the PPM output from the PCM to a number other than 4000PPM which is stock. I am using a cable-x and it was not driving the speedo at the right speed even though it was set to 4000PPM input. Too hard to get to it to play with the dip switches so I just hooked up EFI live and compared pcm readout to speedo read out and made corrections to the PPM output to compensate. I think I ended up using a number around 3600 PPM for my speedo. Way too easy!

Keith

Frost

iTrader: (45)

AGAIN... he said the PCM's reading was the same as the physical guage's reading...

supra8

Right, if he thinks the Garmin reading is more accurate to true speed than what the PCM is giving him then change the PCM output to give the speed reading he wants!

"A man with a watch always knows what time it is. A man with two watches is never quite sure."

Keith

Jglisson

iTrader: (1)

Frost - that is correct. Speedometer reading and PCM reading is the same - per HP Tuner. GPS is showing much higher than indicated. Almost 10mph at highway speed. I have replaced the VSS and verified that I do have the stock gearing - GU5 (3.23).

My shifting is a bit firm...I would suspect a shift kit. I originally suspected that the PCM had a tune in it, which would have explained the hard shifts and inaccurate speedo. I assumed someone programmed the PCM for lower gears without actually making the gear change. That actually wouldn't have suprised me.

Mrr23 came to my neck of the woods and turned off the EGR/AIR. He didn't see an aftermarket tune onboard...so that dashed my original theory.

mrr23

iTrader: (3)

corect. tune was original untouched. PCM and dash read the same. so, that leaves a signal input to the PCM as the cause. this is why he replaced the speedo snesor.

seems that did not solve it.

ShortChevy

iTrader: (21)

I would make sure the speedo is actually wrong, forget the gps set the criuse on 60 mph on the highway and use a stopwatch on the mile markers.

mrr23

iTrader: (3)

he has tested his GPS on his motorcycle as well as two other vehicles. all matched, except his vehicle. but, the stopwatch is a good idea.

Jglisson

iTrader: (1)

I have done that...my cousin has a 99 Z28 and GPS is right on with his car. When we drive side by side he is reading up to 10 higher than me depending on our speeds.

Jglisson

iTrader: (1)

I guess there really leaves only two possibilities. Either I have a fault at the VSS - like the ring gear that the VSS reads off of is damaged or missing some teeth. Or I am retarded and didn't check my gear ratio right. I guess there is the slim possibility that I have a set of gears that is less than 3.23. I will life the rear end and check the driveshaft / rear wheel rotation tomorrow.