Diablosport Forums

nope, you're good to go.. 1-3mph off is acceptable...

hell, my wife's Equinox speedo, totally factory stock, was 3-4mph off compared to my phone gps.... heh...

Ok thanks!

Tires tend to be a little smaller than what they say they are. And they vary from brand to brand. Same size stamped on the sidewall from two different brands will be two different sizes. Like shoes, you may wear one size in one brand of shoes and a slightly different size in another brand.

And in my experience, factory speedometers tend to read a little faster than you're really going anyway.
(I'm pretty anal about this. In the days before GPS, I used a watch, highway mile markers, and a calculator.)

I replaced my factory 245/75R16 with 295/75R16. So my speedo went from reading a little high to reading way low. I'd set the cruise about 70+ for an actual speed of 75.
(I came up with 15/16ths, or 0.9375. Divide what the speedo says by .9375 to find out how fast I'm really going, or multiply how fast I want to go by .9375 to see where I should have the needle on the speedometer. Figured this out by driving past those things that have a radar and display your speed as a warning. Later verified this with GPS, when I joined the 21st century and got a smart phone. GPS confirmed that my original figures were pretty damn close.)

295/75R16, doing the math: (295/25.4)*.75*2+16=33.42"
I had to change the tire size in the tune all the way down to 32" to get the speedo to match the GPS.
Now when I set the cruise on 75, the GPS shows 74-75.

At slower speeds, say 35, the GPS shows 1-2mph slower than what the speedo reads. Tires grow with speed due to centrifical force. (They also grow as they heat up.)

From all the info posted above I think I am satisfied with my marginal 1 MPH difference