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post #1 of (permalink) Old 11-20-2000, 09:31 PM
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How does oversize tires affect mph?

Forgive me for my ignorance, i guess i could spend some time surfing and find out the answer, but i know that there is a tire expert with the answer. Had LT 245/75/16 CHEAP Firestone SteelTex now i got 285/75/16 BFG AT's. How much approx. is my speedo off? I know the side wall height diff is approx 75% of 40mm=30mm or about 1" taller. At least i thinks thats good math.




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post #2 of (permalink) Old 11-20-2000, 11:30 PM
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Re: How does oversize tires affect mph?

well mine are about 1" taller than stock and my speedo is off about 2-3 mph at around 55 and 4-5 around 80mph. i checked it against my GPS thats how i know. so yours should be about the same

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post #3 of (permalink) Old 11-20-2000, 11:56 PM
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Re: How does oversize tires affect mph?

so your saying it reads fast right?

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post #4 of (permalink) Old 11-21-2000, 12:03 AM
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Re: How does oversize tires affect mph?

yea my bad i forgot to say that. it reads a few mph faster than actual

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post #5 of (permalink) Old 11-21-2000, 09:22 AM
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Re: How does oversize tires affect mph?

whoa, i just got all confused. we put 285s on the 2500 in place of the sissy-girl stock 245s. our speedo was way off. the speedometer read 45mph, but the truck was actually going 49-51mph. then on the freeway, the speedo reads 70mph, and the truck's going 75-77mph.

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post #6 of (permalink) Old 11-21-2000, 11:01 AM
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Re: How does oversize tires affect mph?

If you put larger tires on a truck, the speedometer will read SLOWER than what the actual speed the truck is going. The reason for this is that the larger tires travel a farther distance in each revolution than the smaller tires, therefore at the same engine RPM the larger tires cover more distance and result in the truck going faster than what the speedometer reads.

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post #7 of (permalink) Old 11-21-2000, 12:36 PM
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Re: How does oversize tires affect mph?

You will be going 1.08x faster thanwhat the speedo says. If it says 100, you are really going 108.
75=81
50=54
25=27


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post #8 of (permalink) Old 11-21-2000, 12:55 PM
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Re: How does oversize tires affect mph?

well that was my logical thinking too, but mine does read faster, so i dunno [img]/wwwthreads_images/icons/crazy.gif[/img] maybe if was off to begin with??

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post #9 of (permalink) Old 11-21-2000, 01:52 PM
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Re: How does oversize tires affect mph?

<font color=purple>Lets just use your example. Say we started with a truck equipped with 245/75R16 tires & a 4L80E transmission with .75:1 top gear & lets assume 4.10 gears. No other changes being made to the truck, you now switch to 285/75R16's. Now, lets figure RPM at 70 MPH.

245/75's are 30.5" tall & 285/75's are 32.8" tall (advertised height for both).

RPM = FD*MPH*336/TD = (4.1*.75)*(70)*(336)/(30.5)

RPM = 2,371

So, at 2,371 RPM's your truck will indicate 70 MPH regardless of tire size. So, now let's figure the difference in ACTUAL speed:

MPH = (RPM*TD)/(FD*336) = (2,371*32.8)/(4.1*.75*336)

MPH = 75.27, which is roughly 7.5 % off from indicated (as mentioned earlier). The other thing that will be off in addition to MPH is MPG. Since your SPEEDometer is off, your ODometer is also off by the same amount. You're actually traveling 7.5% more miles than indicated & getting 7.5% better fuel mileage than indicated.</font color=purple>

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post #10 of (permalink) Old 11-21-2000, 04:03 PM
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Re: How does oversize tires affect mph?

checkout http://www.4lo.com/ they have tons of calculations that don't require any math.


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