[Leef]

My '05 GT speedo reads ~5 mph high at 70 mph (via GPS). It's hard to tell for sure, but this may be a combination of an absolute error (e.g. 3 mph high at any speed) plus a proportional error (e.g 2% of the speed).

Has anyone had any experience calibrating these speedos?

 

[shed17]

Manufacturers must produce speedos that over read, it's the law, otherwise everybody would use that as an excuse when done for speeding. I believe that it can be as high as 10%.

Personally use my GPS unit to give me my true speed

 

[Leef]

Hmmm...looks like AMG and Volvo haven't heard about this. My (ex) SLK55 was exactly on and our Volvo XC wagon reads low.

Anyway, I'm investigating using the "Yello Box" speedo calibrator that I've used on a few cases with motorcycles. I need to determine if the speed sensor is a two wire or a three wire senson. Anyone know?

 

[The Head]

Leef if you can figure the gear ratios to outside rolling diameter you should be able to compensate by using a tire with a slightly larger or smaller outside diameter

Try this link it allows you to compare tire sizes and will give you the change in MPH between the two sizes
http://pw1.netcom.com/~sgalaba/tiresize.htm

 

[pganmol]

I put 295/35/18 on my car. The error reduce from around 5% to 2%.

 

[Fastfreddy]

I put 295/35/18 on my car. The error reduce from around 5% to 2%.

Are those tires on the stock rims? How's the fit? Do you feel like you're really pushing the limits of proper fitment or does it seem fine?