k...so i upgraded from my stock 17"s, to stock 20"s from an slt. my speedometer isn't the same.

what's the physics on this. it reads slower, correct? how much slower?

 

I can't help w/ math but I would just get the speedo recalibrated.

Speed Safely,
AL

 

Whats the tire size before and after, you said stock wheel size did youesn to say the tire size is stock as well.

 

I wanna say that you have to take it to the dealer and have them set something in the computer.

 

Old and new tire sizes would be helpful. Just changing wheel size wouldn't necessarily change the speedo.

 

20" is the rim size or th inside diameter, if the out side ir the tread part increases your speedmeter will show slower than what you are actually going.

 

You usually don't have to go to the dealership to get the speedo recalibrated. Most shops will be able to do this, or if you have a hand held computer "modifier" (i forget what they are called), most of them have a tire size setting on them. But as stated above, if you get a larger diameter TIRE not wheel, then the speedo will read slower. And the physics on that is actually just math...larger diameter means larger circumference (length around the outside of the tire all the way around).

 

I just did this also. Checkout the pics here

As far as the speedometer goes it really depends on the tire size not the rim size. If you went from stock P265/70R17 to stock P275/60R20 (like I did) the difference is about 4% (i.e. when my speedo says 60mph, I'm actually traveling at 63mph). So the difference is small but like you, I noticed it also.

Check this out: http://www.miata.net/garage/tirecalc.html

I just went to the dealership yesterday to get the speedometer recalibrated. There was no one else in Calgary that did it. They have to hook up to the computer to make the change. They told me it would cost $130 but only charged me $50.

 

where ever u got ur tires installed they should have calibrated the speedo it is part of the service

 

yea guys i'm aware that the rim size has nothing to do with it...but i failed to state or make it clear that the 20's made my truck sit a lot higher, therefore a bigger outer diameter.