Speedometer off....tires?
 

I compared my speedometer to my friends Jetta, and mine seems to read about 4 mph slower. I do have BFG KDW 285/40/ZR17's, would they make my speedometer off? I thought these tires are the same dimensions around as the factory 245's.

How do you correct the speedometer on a 2002, is it a gear you replace like on older ones, or do you do it in the computer?

Thanks,
Chris

I don't know if your package is the same size as the original, but yeah bigger wheels will throw the speedo off. To get it back on, you need to have the computer re-programmed for the bigger wheels. Hypertech or PCM4Less should have you covered. I'm also thinking that ls1speed or thunderracing sells a little module that alters the speedo for aftermarket gears and wheel heights (doesn't cost as much either).

I had the car dyno-tuned in Feb for the heads/cam, so maybe I can take it back and have it fixed for free. I just wanted to make sure it was controlled by the computer and not a mechanical gear.
Thanks!

The outside diameter on the 285/40-17 is 25.98". That compares to a 245/50-16 at 25.65". The "errror" in the speedo will be 1.3%. That means your speedo will read 59.2 MPH at a true 60MPH.... nowhere near the error you seem to be seeing.

Take it out on the interstate, and compare your odometer reading with the mile markers, over a fairly long distance. If you drive 25 miles according to the mile markers, and your odometer only adds 24.7 miles, it would confirm the above numbers.

Or, while using the markers, drive as a constant 60MPH on your speedo, and see how many seconds it takes you to cover one mile. In order to get rid of minor variations in marker locations and your ability to hold the speedo constant, do it for 5 to 10 miles and check the seconds per mile.

Yeah, its reading about 4 mph lower. Must not be the tires then. GRRRRRR

What makes you think your friends is perfect:confused:

Good point. I'm gonna compare it to my Beretta later and see what THAT says.

Thanks

Try the gear/RPM calculator at www.f-body.org/gears/

A magazine did a test of speedos a while back-

GM cars got the best rating, almost right on- imports got the worst rating, they tended to exaggerate significantly, especially bmws.

Sports and luxury cars tended to exaggerate more than other types.

Most speedos are a little off, only a ragar gun would tell for sure. Generally manufacturers try to keep the shown speed higher than the actual speed to avoid legal problems.