Help with inaccurate speedo

**Jglisson** · 10-28-2007, 10:41 AM

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?

**2001NBMZ28** · 10-28-2007, 11:05 AM

I'd go to a RPM/MPH calculator that lets you change tire diameter, gears, etc and calculate. www.f-body.org/gears/ is a good one. Something else to consider is the gears/rear could've been changed - my door code says 2.73 but have 3.42's in the car. Play with that calculator and see if the 7-9 MPH discrepancy could be the difference between 3.23 and 3.42 for example. Just an idea....

**Jglisson** · 10-28-2007, 07:23 PM

I jacked up the rear of the car and had right at 3.25 turns on the driveshaft to get one full rotation on the wheels. That would lead me to believe that I do have the factory 3.24 gearing.

**mrr23** · 10-29-2007, 07:13 PM

well, i'm at a loss if the speedo sensor failed to fix it.

**Jglisson** · 10-30-2007, 05:01 PM

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.

**tigersport14** · 10-31-2007, 08:20 AM

could you just take it to the dealership?

**eautocad** · 10-31-2007, 10:35 AM

since it reads 7-9mph lower, you need to tune your tire size down. Keep tuning with trial and error runs until you get a match.

My max energy said I could change the tires by .25", so I kept tuning by .25"+/- until I got a match.

Hope that helps!

**180ls1** · 11-01-2007, 05:46 PM

maybe you can use this as an excuse to get a programmer or a dyno tune to set it straight.

**jag42420** · 11-02-2007, 05:24 AM

Originally Posted by Jglisson

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. Any ideas?

What were your tire sizes, both before and after? Was tire rollout programmed correctly (physically measured on the ground to include 'tire squish'), or was the total area of the tire just guesstimated? Sounds like either the tire size is off or the gearing is different in the PCM.

**mrr23** · 11-02-2007, 01:20 PM

Originally Posted by jag42420

What were your tire sizes, both before and after? Was tire rollout programmed correctly (physically measured on the ground to include 'tire squish'), or was the total area of the tire just guesstimated? Sounds like either the tire size is off or the gearing is different in the PCM.

i did his tuning for him. the speedo error was there with the stock tires with stock PCM tune.

**jag42420** · 11-02-2007, 01:37 PM

If the gears and tire sizes are accurate, the only thing I can think of would be logging RPMs per gear with the actual speed on GPS to calculate what approximate value you need to plug in for the VSS revs.

Are you sure the GPS device works correctly? I'd drive behind someone at set speeds and verify before worrying about a fix.

**mrr23** · 11-02-2007, 06:20 PM

GPS is correct with his motorcycle and his brother's camaro.

**jag42420** · 11-02-2007, 07:09 PM

That's screwy. It has to affect PCM calculations, hope you have a M6...

Anyway, in your case, I'd use one of the calculators out there to insert my known rpms in 3rd and fourth gears with the miles per hour from the GPS to figure what tire size I needed to input to make it come out right and get it changed. You have to work with what you have...

**Frankthetank** · 11-02-2007, 08:00 PM

lol i have allot of experience using garmin gps's.....including combat experience.....
so trust me dont rely on it too much to be accurate lol

**Jglisson** · 11-04-2007, 04:47 PM

I would be inclined to agree...but less than two months ago my little GARMIN unit proved itself on a 9,000 mile motorcycle trip out West. It only lost its bearings on one occasion. I showed accurate speed when we would meet the occasional radar unit in the middle of nowhere in Montana. It saved me from getting in jail when it showed us that we had just crossed into the Canadian border by mistake (long gravel road up through the mountains). It was right on with hotels, mileage traveled, etc. etc.

Regardless...it is off enough to make it annoying to drive. I guess I will buy a predator or hypertech and play with it from there. Thanks for all your help!