inaccurate speedometer

Originally Posted by texomawaves

Results from my lunchtime test drive to get a Sonic burger:

7.1% error against the speedometer (75-70)/70 * 100 = 7.1%
2.9% error against the HVAC diagnostic (106-103)/103 * 100 = 2.9%

I am not sure what to conclude from this other than the HVAC diagnostic is closer to the GPS reading than the speedo, duhh. So perhaps my Pirelli Pzero's are too low of a profile. I guess I might consider bigger tires. The ones I currently have are 205/50 ZR-17 on the front, and 255/40 ZR-17 on the back. Perhaps a "taller" tire will shave off a few percentage points on the error.

So guys... what size tires are you running? ...and what error are you getting? Let's compare.

Chris

I'm running the same tires as you but on 16" wheels and my speedo is exactly 5 mph faster than my GPS or the radar guns they use to warn you of your approaching speed. All this time when I was doing 75 and thinking my spoiler was up it wasn't. Dang!

My bad, 17" wheels with PZero's.

[jmatta]

The speedo may be designed around 18" wheels.

Not; I have 18s and as I post earlier, my speedo is off at least 4 to 5 mph.

Seems like all cars run 3-5% optimistic on their speed readout compared to actual.

[lagrasta]

Here's the thing that really threw me off. I have a ScanGaugeII OBDII scanner that has a gauge mode that can, among other things, display the speed according to the car's computer.

I compared that, the speedo, and a gps unit and got this:
Speedo = 75 mph
Gps = 68 mph
OBDII = 69 mph

That confirms for me that the on board computer knows the correct speed and is intentionally under-reporting it on the speedo. At least, I know that the computer is reading the real road speed correctly.

I know manufacturers calibrate speedos conservatively, but I thought it was interesting to see the difference between the speedo and the OBDII readings.

[RandallNeighbour]

I doubt the wheels are the issue. I measured the circumference and height of my 17's and my 18's and they're very, very close. Not enough to make 5 mph difference.

BTW, on my 97 with 18's on it, my GPS, digital gauge, and needles are all the same. Go figure.

Palo Alto Speedo in central California will calibrate speedos for a couple of hundred bucks including shipping if anyone is genuinely annoyed with this problem. I actually wish mine were calibrated too high because it helps reduce the likelihood of receiving a hand-written invitation to visit with a judge in person, and with checkbook in hand.