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Thanks

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I use a GPS for accurate distance traveled.

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You could use that 10% as suggested but it's dependent on the speed you travel, filling up near and traveling on the interstate would make this easier and more accurate.

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Look at your door sticker and find out what your stock tire size is. It'll probably be in metric so convert it to inches and find the diameter of your tire.

If they're 235 75 R15s, take (235 / 25.4) = 9.25" (tires are 9.25 inches wide)

Then 9.25 * .75 = 6.94" (tires sidewalls are 6.94")

The diameter would be 6.94 * 2 (since there are two sidewalls) + 15 for the rim.

Diameter = 28.88"

Now take a ruler and measure your current tire, don't believe what's written on it. My 30" x 9.5" tires are only 28" in diameter.

Take whatever that is, lets say it's 31.5"

Your speedometer is calibrated for 28.88"

If it says 70mph, you're really going 70 * (31.5 / 28.88) or ~76mph.

If your tripometer says you just traveled 30 miles, you really traveled 30 * (31.5 / 28.88) or ~33 miles.

If you went 400 miles on a tank with 32 gallons and you calculate 12.5 mpg, recalculate it with the above info. That means you really went 436 miles and got 13.6mpg.

Also, if you changed your rear end, factor that into the equation too the same way.

Like if you had a 3.5

400 miles on the tripometer on a tank of gas.

400 * (31.5 / 28.88) * (3.5 / 4.1) = 372.4 actual miles on a tank of gas, which would mean you'd be getting 11.6mpg when it reads 12.5.

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1,271 Posts

Look at your door sticker and find out what your stock tire size is. It'll probably be in metric so convert it to inches and find the diameter of your tire.

If they're 235 75 R15s, take (235 / 25.4) = 9.25" (tires are 9.25 inches wide)

Then 9.25 * .75 = 6.94" (tires sidewalls are 6.94")

The diameter would be 6.94 * 2 (since there are two sidewalls) + 15 for the rim.

Diameter = 28.88"

Now take a ruler and measure your current tire, don't believe what's written on it. My 30" x 9.5" tires are only 28" in diameter.

Take whatever that is, lets say it's 31.5"

Your speedometer is calibrated for 28.88"

If it says 70mph, you're really going 70 * (31.5 / 28.88) or ~76mph.

If your tripometer says you just traveled 30 miles, you really traveled 30 * (31.5 / 28.88) or ~33 miles.

If you went 400 miles on a tank with 32 gallons and you calculate 12.5 mpg, recalculate it with the above info. That means you really went 436 miles and got 13.6mpg.

Also, if you changed your rear end, factor that into the equation too the same way.

Like if you had a 3.5stockand change it to a 4.1, just add that in.

400 miles on the tripometer on a tank of gas.

400 * (31.5 / 28.88) * (3.5 / 4.1) = 372.4 actual miles on a tank of gas, which would mean you'd be getting 11.6mpg when it reads 12.5.

Great info. I just printed this to keep for my records.

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That will probably be just as acurate. however measureing the circumfrence then dividing by pi seems even better.You want your SLR to do your calcs. Measure from the ground to the center of the hub and double that for your dia.

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perhaps, but it is easier to measure from the center of the hub to the ground and multiply by 2 :thumbupThat will probably be just as acurate. however measureing the circumfrence then dividing by pi seems even better.

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perhaps, but it is easier to measure from the center of the hub to the ground and multiply by 2 :thumbup

Yeah totally. Easier is always better.

Sorry but no.That will probably be just as acurate. however measureing the circumfrence then dividing by pi seems even better.

A mounted tire on a vehicle is not round. If you look close it is quite flat on the bottom where it is in contact with the ground. The Static Load Radius of the tire especially with larger low pressure balloon style tires can be quite different than the marked tire diameter.

Check them both and see.

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will do I am going to try some of these and figure what I am actually getting per gallon.

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So, if the speedometer is calculated specifically for 28.88", it's probably basing it on a perfect circle, which would mean that every time the tire makes one revolution, the speedometer figures the vehicle travels 90.73 inches.

What you're saying is valid to a point except for what we're trying to

If, for example, you measure straight across and it measures 31.5", you calculate 98.96" around, but when you measure the circumference, it's only 98.4" or something, since it's not perfectly round. Since you're calculating the difference of distance traveled per tire revolution, you're going to want to go with the 98.4" and not the 98.96" Not

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Wow, i just had a flash back of my high school geometry class...

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Yeah no joke huh?

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And you probably always wondered where in life you'd ever use that stuff, huh?

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yeah I always thought it was a waste of time. who'd a thunk it?

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How about driving down the interstate at 60 mph, pass a mileage marker and time your trip to the next marker. Should take 60 seconds and be 1 mile. lol

clint

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