Jackies Stang

I understand that if a tire/rim change is made to your car that a tune is needed to correct the speedo! My question is when a car is built from the factory and one care comes with 17" rims and another car comes with 18" rims, does ford set up the stock tune for that particular rim/tire package in every car and if they do, when a customer upgrades the tire package from a lot car, does the dealership change the tune/speedo? Sorry if this is a double post, just looking for the correct answer without a lot of debate. Thanks.

nkx1

Sorry, I do not have a direct answer to your question.

However, your assumption that a tune is needed when you make a rim/tire change is mostly inaccurate. Other than some exceptions, you would generally choose a tire/rim combination that would match your overall factory diameter. That way, your speedometer/odometer is not affected.

For instance, my car came with 235/55/17 tires, but I'll be changing them to 265/35/20 tires when I upgrade wheels. The difference in overall diameter is approximately 0.5%, but certainly not enough to warrant a tune to correct for tire size, especially when you consider stock diameters are always changing slightly anyway due to tire wear.

Jackies Stang

So if i understand you correctly, the stock 17 & 18 's have the same over all tire diameter within a certain percentage?

nkx1

Well, I don't know for sure (that's why I stated I don't really have a direct answer for you). Although it is possible that they would have a different factory tune for a 17" vs 18" wheel, I kind of doubt it. But in all likeliness, the 17" and 18" factory wheels probably have a similar overall diameter.

Here's a tire size calculator that I used to figure out what size 20" tire would match stock best:
http://www.miata.net/garage/tirecalc.html

CliffyDeuce

That's correct. The diameter (and thus the circumference) is the same. So if you want to change wheels and/or tires, just make sure you keep the diameter roughly the same. So the sidewall thickness will be less with an 18" tire (or even less with 19" or 20"), and the percentage (the middle number, i.e. the "55" in 235/55R17) will be lower for a wider tire in order to keep the measurement (in inches) the same.

Warhead711

The generally accepted rule of thumb in the tire business is that a 9% or less change in overall tire diameter has little effect on the speedometer.

Warren

howarmat

9% is a ton......that is almost 6-7 MPH off at highways speeds!!

Warhead711

I know. I was in the tire business for 15 years and am just saying...it's generally accepted as OK there!

Warren

Lees07GT

Sure, because they just want to sell tires. They really don't give a crap if your speedometer is inaccurate and it causes you to get a ticket...

PLstang

If you keep the overall diameter you don't have to mess with anything. You can check online for the tires you have and they should give a diameter (cant remember what the number is supposed t be at the moment for stock). Then just get your new wheels and tires to duplicate the diameter. If you like a tire and wheel combination different then the stock diameter you can adjust this using a hand held tuner. I believe the SCT website has the info for how, not sure about Diablo. I did mine with SCT.