I was cruising the freeway last night/early morning @ 70MPH for 20 minutes. I started counting from 23.7 miles and ended up at 47.4 miles after 20 minutes. So I subtracted the miles to get the total distance in 20 min, Ironically it came out to 23.7 where I started. I was playing with my calculator so many times thinking that it was broken. Then after getting some brain cells back, I realized that it was exactly half.

Anyways, 23.7 miles in 20 minutes equals 71.1 MPH.

So my 03 speedometer seems pretty accurate unless the odometer is wrong. Since I was cruising at 70MPH, it could be close to 70 or 71.

Another thing to note, I have a defi VSD and a link display gauge that displays speed. But for some reason it read about 5% faster than our speedometer. During the cruise it read 73MPH and occasionnaly jumps to 74MPH. The meters are set at a speed pulse of 4. Anyone know what speed pulse value our car has?

Using the VSD tachometer, the average rpm was 3750 +- 30 @6th gear. Using 3750RPM to measure speed, according to calculations, it should be 71.9MPH.

Using the http://geocities.com/kflee wesite, with tire deformation it came out to 71.1MPH.

It looks like that the calculated speeds lie in between the Factory Speedometer and The after market speedometer.

Oh one more note, our tachometer reads 100RPMs or 1bar ahead of its actual tach read from the VSD Tach. Probably the same for external tachs from OBD readers and tuning machines. eg: VSD = 4000 RPM vs. 4100 on the factory tach.

Im not sure how to calculate tire deformation. Let me know what the formula is or how to calculate it.


Summary:

Factory Speedometer: 70MPH
Defi VSD, Link Display: 73MPH
Factory Odometer Calculations: 71.1MPH
Defi Digital VSD Tach Calculation: 71.9MPH
Defi Digital VSD Tach Calc w/ tire deformation: 71.1MPH

 

Dude, don't kill yourself trying to get too many decimals of accuracy here. For one thing, U.S. laws force manufacturers to err on the high side when calibrating their speedos (which may or may not be linked directly to the odos). Then there's tire pressures, tire deformations, tread wear, alignment, etc. etc. etc. Not to mention the fact that the speedo's output is quantized to (presumably) +/- 0.5 mph.

Hence the old saw "YMMV".

 

Something else that might make the ODO higher than in other cars. I was quite surprised to see the speedometer register a speed when driving in reverse. Never saw that before in any other car.

I haven't driven the car backwards for a couple of 10ths of a mile to see if the ODO increases as well. Should be interesting.

 

[QUOTE]Originally posted by rworne
Something else that might make the ODO higher than in other cars.