Do rims affect the spedometer?

95Mx-3 · Post by **95Mx-3** » April 9th, 2005, 6:06 pm

i have a set of 16's and some ppl are telling me that it will screw up my spedometer, is this true? i dont think it does, but i may be wrong.

neutral · Post by **neutral** » April 9th, 2005, 6:29 pm

Only screws up the speedo if the aftermarket rim/tire combo outer diameter is different than factory stock. That's why larger rim sizes require narrower profile (height-of-sidewall) tires - so ya keep same as or close to the outer dia. meas. of the stock tires.

Robotaz · Post by **Robotaz** » April 12th, 2005, 11:37 pm

if you add an inch to the rim size, you have to decrease the tire height by 25 mm to keep the same speedo reading. If you can't match them, just find out the rim size, plus or minus .3939 inch for every 10 mm in tire height. Find the percentage difference from stock, and that will be your percentage difference in speedo reading. Smaller than stock will read a higher speed than actual while a larger than stock size will register a lower speed than actual.

A tire covering less ground than speedo reads(smaller than stock), will give higher speed than actual since speed is distance divided by time, thus the speedo thinks it covered more distance than it really did. if new tire is 10% shorter around, then total distance is 10% shorter than what's read, meaning speedo will be reading high by 10%.

Circumference, C, equals 2*pi*r where r is the radius that we're measuring(C = 2*pi*r). Since C is proportional to r because they're both in the numerator(and 2 and pi don't change), a percentage change in r will make the same proportional change in C. Bigger radius = bigger circumference = bigger distance covered = bigger speed = lower than actual reading.

So for a wheel and tire combo that's a total of 17.1654 inches(stock 205/55/15), a switch to 245/35/18s, or 19.378 inches(~12.9% gain), will give a reading of 75 mph which would translate to an actual speed of about 84.7 mph(~12.9%).

A switch from stock to 215/45/15 would mean 16.772 inches instead of 17.1654 inches(~2.4% decrease), or at 75 mph on speedo you're moving an actual speed of 73.2 mph(~2.4%).

Put another way, with bigger than stock sizes, if you drive dead on the speed limit you are technically speeding. With smaller than stock, when u drive dead on the speed limit, you'll never get pulled over for speeding.

Too much Time · Post by **Too much Time** » April 14th, 2005, 12:43 am

I LOVE PHYSICS CLASS!!

hgallegos915 · Post by **hgallegos915** » April 14th, 2005, 3:46 pm

hmm how about 15 in rims plus 205/55/r16.. ? so If it says 120mph..im like at 125?

Robotaz · Post by **Robotaz** » April 14th, 2005, 3:47 pm

r16? I don't understand what r16 and 15 inch rims means.

Redlined · Post by **Redlined** » April 14th, 2005, 4:17 pm

r as in on the tire sidewall would indice the tire is of a radial construction

Robotaz · Post by **Robotaz** » April 14th, 2005, 4:36 pm

The question still makes no sense to me, but if I assume that you mean changing from stock(205/55/15) to 205/55/16(larger than stock) here is what you need. I said that every 10 mm means .3939 inches. Stock is 17.1654 inches from center of rim to edge of tire. Your new setup would be 16 inches, plus .3939 inches x 5.5, for a total of 18.1665 inches. This is ~5.8% larger than stock. So at 120 mph on speedo, you would actually be travelling at ~5.8% faster than speedo, or 127 mph. Something tells me that i'm calculating actual hell ride speeds for somebody. LOL!