hector

I install my toyo 285 70 17's this AM . Turn on GPS to see how far my speedometer 's off. The speedometer is dead on with the GPS. I went from 265-70-17's

txarrowhunter

That probly goes more for like 35" and taller.

AggieJustin

What speed did you check them at? At lower speeds, you're difference will be minor. The difference will increase as speeds increase.

Also, you might want to check all the specs on both of those tires. Even though the size on the side is the same, there are other differences (tread depth) that can affect the overall diameter.

hector

I checked at 40 then 55

kry226

Maybe means you were off to begin with. I have experienced the same thing before.

steelblitzkrieg

Huh? You're still traveling the same distance with the same amount of rotations of the tire, why would the amount of time it takes to get the same distance change anything?

AggieJustin

I'm saying that when you compare the two sizes of tire, at low speeds the difference in MPH for a fixed RPM is smaller than the difference at high speed.

For example, based on just the numeric size of the tire you get the following:

265/70/17 is 31.606" tall with a diameter of 8.270'
285/70/17 is 32.709" tall with a diameter of 8.559'

At a tire rotation of 50rpm, the 265 tire will cover 413.50' in a minute, which is a speed of 4.69mph. The 285 tire will cover 427.95' in a minute, which is a speed of 4.86mph. The difference in actual speed is only 0.17mph. The difference is so small, it would be imperceptible on the speedometer.

With a tire rotation of 500rpm, the 265 tire will cover will cover 4135' in a minute, which is a speed of 46.98mph. The 285 tire will cover 4279.5' in a minute, which is a speed of 48.63mph. The difference in actual speed is 1.65mph.

If we jump the tire rpm to 1000, we see a speed difference of 3.28mph (97.26mph vs. 93.97mph).

All the truck has to establish a speedometer reading is a speed of the tire in revs per minute and the overall diameter of the tire. As you make increases or decreases in tire size, that difference will be multiplied by RPM to establish the difference between the speedometer reading and actual speed.

Or in simpler terms, the 285 is about 3.5% larger than the 265. So your readings will be 3.5% off. 3.5% of 10mph is only 0.35mph, but 3.5% of 100mph is 3.5mph.

steelblitzkrieg

Okay, now I follow you. I was thinking the difference is constant in the sense of percentages.

pronstar

Same thing with my '07.

I didn't have the factory tires on for very long, but it must have been off from the factory. With the 285/70's I have now, my portable GPS and those radar speed limit signs (the ones that display your speed) are all spot-on with my speedometer.

AggieJustin

Wouldn't be surprising. My wife's VW Passat is off by about 4% with OEM tires. The service department won't correct it as they say it's within normal parameters. Maybe I'll have to put some bigger tires on there to bring it in spec.

hector

I didn't think speeds would make any difference either. I'm going out of town tomorrow & will be on the freeway. I'll see how it checks out at 70-75 mph

etn550

I have these exact same tires and they are mostly worn. My speed according to gps is dead on at 65 mph. I think the spedo was off with the stock tires. Speedo with stock tires reads low on other cars i measured with gps.

hector

How many miles do you have on them ??

Asher

The factory speedo's read fast on every Ram I have seen. The 285/70's put it dead on... Kind of makes you wonder if it was alittle bit planned... Sure would be a easy way to get better factory MPG calculations and also quicker way to get you out of the warrenty period...

(AggieJustin mentioned 4% on the VW) 4% may not seem like much, but with 100k warrenty it's 4,000 miles sooner out of warrenty then it should be...

2171CTD

Holy cow Aggie, you blew my mind with that explanation. Had to read it twice to comprehend it.