I don't know how to do the math but my question is: My car has the stock size tires 215/70R-15, the optional size is 215/65R-15. The later are about 1" smaller in diameter. Will 1" make a big difference in the speedometer being off by much?
A worn 215/70 and a new 215/65 will measure about the same diameter. So it's kind of a wash. The factory allows that other size because it is not much of a difference. But your speedo will indicate a little higher mph with the 65s than with the 70s.
Wag3 iz right Tbird. That one size will be a wash. If you're interested in makin' yer head hurt, Go on: Tirerack.com and dig into the site. Size variations, Support weight per tire compared by brand and size, diameter differences per size and Mfg. It'z all there and more. Yah been warned:banghead3:
215-70x15 circumf= 26.85 rev per mile= 774.6 speedo reads 65 mph 215-65x15 circumf= 26. rev per mile= 800 speed actual now is 62.95
Ditto, why I never corrected mine [60 is really 55], gives me a safety net as I easily forget, so if im 5-10 over on the needle, im safe ticket wise
Cadipacer, since you do obviously have a great math ability, I have a problem also with my speedo due to tire size. I just bought my 87 Crown Vic wagon with LT265/75R15 tires on it, which obviously throws the speedo way off. Being on disability, and the tires having at least 30,000 miles left in them, at what following indicated speeds are my actually doing, if you don't mind helping me out? 35/45/55/67/75 what I have been doing by guess is driving about 15 MPH under what I want to go, IE: I wish to drive 65MPH (the 2 lane limit in Montana is 70, freeway 75) I set my cruise at 50 MPH. BUT, I do not like to hold up traffic, so knowing what it will show at 65-70 actual is needed. Thank You. Glenn MG Merideth
Glenn: No I not a math guru. Use this formula for each mph you question; Example: Example; 65 mph X 4.10 gr ratioX 336 divided by 27 tire OD = 3316 rpm This assumes you have tachometer. With the Crown Vic I'll guess 2:87 gear ratio 50mph X 2:87 X 336 divide X 26 tire od= 1854 rpm If you knew what size OEM tire came on the Vic it would help to figure.
