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Discussion Starter · #1 ·
I'm horrible at math, so helping one of yall smart folks could help me out.

I should be recieving my Procal soon so I can recalibrate my speedo and get accurate milage, but in the mean time I'd like to try and figre out what kinda mpg I'm actually getting.

I thought my Jeep was completely stock, but after using the gps on my phone to check the speedometer I realised either the gears or tires had been changed by the po. At 60 on the speedo I was actually going 65.

I just got the build sheet for my Jeep and found out it originaly had 225/75R16 tires, it now has 255/70/17. That accounts for the milage differance.

I'd like some help with the math to figure out what my actual mpg is.

On this last tank I drove 160.7 miles (according to inacurate odometer) and used 10.7 gallons. That nets just over 15mpg. So with the differance in tire size what would my actual mpg be?

I know it cant be dont 100% with out knowing the actual circumfrence of the tires, but should be able to get close right?

Can anybody help?
 

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A tire size calculator will show that you had 29.3" tall tires and are now running about 31.1" tall tires.

(Odometer miles) x 31.1 / 29.3 = actual miles

That gives you about 170.6 miles.

So new mileage is just a hair under 16.


To be more accurate, start off with a full tank, use the GPS to track your route as you burn off half a tank or so, go back and fill up the tank to see how much you burned off. Get the distance traveled according to the GPS and divide that by the fuel you used. That will eliminate the speedometer accuracy issues completely.
 

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derf said:
A tire size calculator will show that you had 29.3" tall tires and are now running about 31.1" tall tires.

(Odometer miles) x 31.1 / 29.3 = actual miles

That gives you about 170.6 miles.

So new mileage is just a hair under 16.

To be more accurate, start off with a full tank, use the GPS to track your route as you burn off half a tank or so, go back and fill up the tank to see how much you burned off. Get the distance traveled according to the GPS and divide that by the fuel you used. That will eliminate the speedometer accuracy issues completely.
If you above equation is right(which I won't doubt it is) then I've increased my mpg from 17mpg to 19mpg. Which sounds about right since my speedo is off, I'm running 31x10.5
 

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Discussion Starter · #4 ·
A tire size calculator will show that you had 29.3" tall tires and are now running about 31.1" tall tires.

(Odometer miles) x 31.1 / 29.3 = actual miles

That gives you about 170.6 miles.

So new mileage is just a hair under 16.


To be more accurate, start off with a full tank, use the GPS to track your route as you burn off half a tank or so, go back and fill up the tank to see how much you burned off. Get the distance traveled according to the GPS and divide that by the fuel you used. That will eliminate the speedometer accuracy issues completely.

Thank you much! My Procal should be here soon, after I reclaibrate the speedo I'll do the gps deal to check it. Thanks for the advice!

So just for future referance and for anybody else who would like to check thiers, to figure the differance you just take the odo, multiply new tire size, then divide by old tire size to get the actual miles. Then divide by gallons to get the actual mpg. Does that sound right?
 

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If you above equation is right(which I won't doubt it is) then I've increased my mpg from 17mpg to 19mpg. Which sounds about right since my speedo is off, I'm running 31x10.5
It's close. Given that speedometers are not always 100% accurate I always do the GPS method to figure my mileage. I've had cars off the showroom floor that were as much as 10% off (reading higher speed than I was actually going). Old mechanical speedometers were the worst because you had to get the closest match between two gears and you could never get them exactly accurate. But then again tire diameter is not entirely fixed. Two tires from two manufacturers that have the same size can be off by half an inch or more from each other. Even tire pressure can change the effective diameter of the tire.

That equation should get you in the ballpark. A GPS is better.


BTW, the equation to get about the right tire diameter is pretty simple.

((SW/25.4)x(Aspect/50))+rim = diameter

A tire size is (SW)/(Aspect)R(Rim),

So a 255/70R17 tire gives you

SW = 255 (Section Width, or the width of the tires, in milimeters)
Aspect = 70 (Aspect ratio, or how tall the sidewall is compared to the tire width)
Rim = 17 (diameter of the rim in inches)

((255/25.4)x(70/50))+17 => (10.04 x 1.4) + 17 = 31.056 = 31.1
 

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Thank you much! My Procal should be here soon, after I reclaibrate the speedo I'll do the gps deal to check it. Thanks for the advice!

So just for future referance and for anybody else who would like to check thiers, to figure the differance you just take the odo, multiply new tire size, then divide by old tire size to get the actual miles. Then divide by gallons to get the actual mpg. Does that sound right?
Yep. The key is to multiply the miles by the new tire size and divide by the old tire size to get close to the actual miles driven. Once you have a more accurate estimate of miles driven the rest of the math works like it always has.

Same principle applies when considering what gears you want to run in your axle to compensate for the tire size change. Current gear ratio x new tire size / old tire size = new gear ratio. Then round up to whatever is actually available since you'll want to compensate for heavier tires.
 

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Discussion Starter · #7 ·
Sweet! So when I go to 35" tires (assuming they are actually 35", wich most arent) with my 3.73 gears, I'd actually want to regear to a 4.21 or higher (numericaly). Cool! Who knew math would actually be helpfull one day....I should have paid more attention in school lol
 

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derf said:
It's close. Given that speedometers are not always 100% accurate I always do the GPS method to figure my mileage. I've had cars off the showroom floor that were as much as 10% off (reading higher speed than I was actually going). Old mechanical speedometers were the worst because you had to get the closest match between two gears and you could never get them exactly accurate. But then again tire diameter is not entirely fixed. Two tires from two manufacturers that have the same size can be off by half an inch or more from each other. Even tire pressure can change the effective diameter of the tire.

That equation should get you in the ballpark. A GPS is better.

BTW, the equation to get about the right tire diameter is pretty simple.

((SW/25.4)x(Aspect/50))+rim = diameter

A tire size is (SW)/(Aspect)R(Rim),

So a 255/70R17 tire gives you

SW = 255 (Section Width, or the width of the tires, in milimeters)
Aspect = 70 (Aspect ratio, or how tall the sidewall is compared to the tire width)
Rim = 17 (diameter of the rim in inches)

((255/25.4)x(70/50))+17 => (10.04 x 1.4) + 17 = 31.056 = 31.1
Ha I'm not the only one into math! Yea I've been too busy with other projects to actually look into this stuff and was happy with my 17mpg. Been doing equations and algorithms for software development with a buddy. But this is very helpful.
My tires are 31" but being Dtracs it specifies they're 30.8 actual size so that's what I used. And if it gets me in the ballpark of 19-20 mpg there no complains here haha
 

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Sweet! So when I go to 35" tires (assuming they are actually 35", wich most arent) with my 3.73 gears, I'd actually want to regear to a 4.21 or higher (numericaly). Cool! Who knew math would actually be helpfull one day....I should have paid more attention in school lol
Yeah, I studied math in college and all I can do with it is help people with arithmetic on websites. :D
 
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