Tural

some people have been talking about the gas mileage error that occurs with oversize tires. so, here's the calculation for the people that wanna benefit from it:
WARNING: you need to know basic geometry for this.
first... tire size... cherokee stock tire size according to my door sticker is 215/75 R 15...this means that the thickness of the tire in mm is 215mm. the sidewall is 75% of the width. converting it into inches 215/25.4=8.4645669in.
sidewall is above and under the rim means 150% (75% above +75% below). in other words 1.5 times the width... 8.4645669*1.5=12.69685in.
add the rim diameter to it 12.69685+15=27.69685in.
so the stock cherokee tire height is 27.69685in.
now the circumference of this tire is the ratio of 22/7 to the height (just a rule in math). so it's 27.69685*22/7=87.047244 inches. convert it to feet, that makes 87.047244/12=7.253937
one mile is 5280 feet so in order to travel one mile a tire needs to make 5280/7.253937=727.88059 ROTATIONS.
are you with me so far????
so, say your new tire is 31 inches tall. let's do the math for that... 31*22/7=97.42857 inches. so 97.42857/12=8.11904 feet.
now, if the current tire turns as many rotations as the stock tire it will make 727.88059*8.11904=5909.69717 feet. how many miles: 5909.69717/5280=1.11926 miles.
HENCE, YOUR MARGIN OF ERROR FOR 31 INCH TALL TIRES IS 1.11926 MILES PER EVERY ONE MILE ON THE SPEEDOMETER.
In other words, when your speedometer says you've driven 100 miles, you have actually driven 111.926 miles.

Tural

now, if you wanna get a gear reduction, here's what you do...
suppose your stock gearing is 3.55. And you move up to 4.10 gears.
just multiply your actual mileage for 100 miles on the speedo by your old gear number and divide it by the new gear number: 111.926*3.55/4.10=96.9116 miles.
So, if you have 31 inch tires and 4.10 gears on a stock 215/75R15 tire and 3.55 gear ratio axle, your actual mileage for every 100 miles on the speedo is 96.91 miles...
so, when you calculate your gas mileage remember, if your gas mileage happens to come to 15mpg, it's actually 15*.9691=14.5365MPG.
Enjoy....
Now go take some tylenol

The Black

Dude, You are making my head spin. so if I get a new speedo gear will that correct both problems of actual speed on the speedo and the miles stated on the odometer and trip meter?

Tural

YESSS. it will... but i would wait to see if you like your new tires on stock gearing or you need to regear your axles. that way you only have to do it once.

96Cherokee113

Wow, great information man! Works perfect for me since I'm getting 31 inch tires and you already did the math

Tural

if you run into a corner, just PM me and I'll give you the numbers for any other tire size. By the way, if you have 3.55 gears and you wanna put on 32 inch tires, 4.10 gears will absolutely correct your speedo and you won't need new speedometer gears.

The Black

Great info Tural, so I got 32" and will be going to 4.10 gears soonish. I have got a 36 tooth gear for this. Is it the correct one? Thanks.

Tural

i'm not sure on the hardware, i'm just a math wiz. if you have 3.55 now and the gears you're buying say 4.10, then that's it. a good way to check is to lift your rear end off the ground, chalk the driveshaft and the tire and see how many turns of the driveshaft makes a complete revolution of the tire.

Tural

currently, i'm researching the parasitic power loss due to taller tires and heavier tire mass. the number 3.627 keeps popping up for a ratio. i should have an update on that pretty soon.

Tural

hey, if anyone has any info on the parasitic power loss, please let me know.

superj

no info on that but there is a 4 cylinder cherokee here for 300 bucks so that means you can have both front and rear 4.10 axles for 300 bucks if you live near corpus christi texas. for all you guys with 3.55 axles and are running 31 inch tires.

Tural

wish i could come and get it.

84wagoneer

for all you people that dont want to waste your time doing it the long way, divide the new tire diameter by the factory tire diameter. 31 / 27.7 = 1.1191.....

you can also use this (very easy) formula to figure out what kind of gearing you need with your new tires to make your truck drive like its stock again!

if you divide your factory tire size by the new tire size it will give you a number, we will call it our multiplier.

27.7 / 31 = 0.8935.

take your multiplier and multiply it by your factory gearing (my case it is 3.55). 31's will turn my diff. gear ratio into 3.17.
(this does not help you figure out what gearing you NEED, just tells you what your gearing is at, compared to stock.)

to figure out your needed gearing, multiply your diff. gearing ratio by our speedometer multiplier (1.1191).

3.55 x 1.1191 = 3.97

so if you want your truck to accelerate and get mileage like it was stock, you would need 3.97 gears. good luck finding those, so most people get 4.10's.



Yay math!

Zab243

In other words... Wow

cherokee77

Dude your mathematics lost me. To many ditch days.