Thanks lobux for the info. I don't think VDIM will care too much about the reduction in tire diameter, since the rear diameter will be the same as the fronts, and I'm pretty sure lots of other people have done that. I will probably wait to see how much of a change the tires make on the speedo before actually trying anything.

FWIW, there are 5 distinct speed signals on the CAN bus.

 

ALL speedometers err on the side of indicating a higher speed than actual:

- to account for tire wear;
- too help people stay under the speed limit!

 

I would agree. We had this whole big discussion over at the Audi forum. My old 2003 A4 was totally off. I think it ended up 10-15km's over, which amounts to about 6-10mph at high speeds.

But when i made a trip, the odometer was correct, it was just the speedometor. It is known that Audi and many other car companies do this for safety purposes.

 

I would agree. We had this whole big discussion over at the Audi forum. My old 2003 A4 was totally off. I think it ended up 10-15km's over, which amounts to about 6-10mph at high speeds.

But when i made a trip, the odometer was correct, it was just the speedometor. It is known that Audi and many other car companies do this for safety purposes.

 

Yes, I would assume the odometer to be quite accurate, since it's a legal representation of the car's mileage at resale.

 

If the speedo is reading high with full tread, then it will get even worse as the tires wear.

 

From hanging out on the VW forums, Audi/VW are about 8% too fast on the readings.

I believe GPS readings would be the most accurate, thought lobuxracer's method is also accurate because he's doing over a long period and taking an average.

I know there's a yellow box like device for the S2000 (Gernby, you probably know what I'm talking about), but I haven't seen something like that for too many cars.

 

The yellow box that I posted about in my 1st post isn't just for S2000s. Some people are even using them on street bikes. There is another yellow box knockoff made by a guy named modifry on the S2000 forum. It is not as full featured as the yellow box, but might be a better choice for price if multiple boxes are needed.

 

yes your right. It was the first time i had experience a "wrong reading" of my speedometer but i had to make sure that it was just reading that way for a purpose. So as i measured the odemeter and it was correct, i came to the conclusion that the speedometer was wrong on purpose.

 

While browsing through the aftermarket wheel setup thread, I saw that TONS of people are going with wheels / tires that will screw up the odometer and speedo in the opposite direction (ie. reading low).

 

I checked my '00 Audi S4 against the measured mile markers on the Garden State Parkway, and it was pretty close to spot on. The speedo was a little high, but nowhere near 8%, nor even the 5% I'm seeing on my IS.

 

US law requires the speedo to be within 5%. Anybody remember the 85 mph speedos from the late 70's and early 80's? Those were mandated to be 1% accurate. The loosened the standard when they allowed them to indicate more than 85 mph.

 

I had my new rear tires installed today. For reference, the OEM rear tire is 26.1" tall, and my new 255/35-18s are 25.1" tall. The OEM fronts, which I'm currently using, are 25.1" tall also. I never liked how much difference there was in the sidewall height between the front and rears, and I like the 3.8% lower gearing (numerically higher gearing). That's about like 10 extra HP to the ground.

So far, I haven't had any issues with VDIM or rubbing. I was more worried about rubbing due to my rear offsets. They also look a LOT better than the OEM "balloon" tires since my fender gaps are now equal.

I'll see if I can borrow another GPS unit to check the effect it had on my speedo.

 

So now you have a smaller ratio tire, which means you will have a little better acceleration, but also put more miles on your odometer. I personal don't think looks are a better option than higher miles on your car.

 

You've also added caster, changed weight distribution and center of gravity, etc., etc., mostly for the worse because it's only one end of the car moving.