RPMs aren’t lining up with GrimJeeper gearing calculator

[Bbrayden85]

So according to grimm jeeper my jeep should hit 60 at about 2260 rpm with 4.11 gears on 31” tires. I just replaced the speedometer gear so my speed is acurate (i tested it with a gps speedometer) and its hitting 60 at about 26-2700 rpm. This is all in 5th gear by the way.

 

[Bbrayden85]

So according to grimm jeeper my jeep should hit 60 at about 2260 rpm with 4.11 gears on 31” tires. I just replaced the speedometer gear so my speed is acurate (i tested it with a gps speedometer) and its hitting 60 at about 26-2700 rpm. This is all in 5th gear by the way.

I did turn my tire twice and the pinion rotated 4 and almost 1/4 times.

 

[Chris]

Well yeah, you have to account for a small margin of error with any calculator.

 

[jjvw]

Well yeah, you have to account for a small margin of error with any calculator.

There is no error in the calculator. He entered the wrong tire size.

 

[jjvw]

So according to grimm jeeper my jeep should hit 60 at about 2260 rpm with 4.11 gears on 31” tires. I just replaced the speedometer gear so my speed is acurate (i tested it with a gps speedometer) and its hitting 60 at about 26-2700 rpm. This is all in 5th gear by the way.

With the Jeep on flat ground, set a tape measure next to the axle tube to the ground. Where the tube touches the tape is the hub height or the rolling radius of the tire. Double that number and enter it into the calculator.

Anyone who tells you to look up anything from the manufacturer to enter into the calculator is wasting your time.

 

[Claytone]

Get a tape measure and see what your tire size really is. Go from there.

jjvw beat me to it...

 

[jjvw]

Get a tape measure and see what your tire size really is. Go from there.

Is your way different from the correct way?

 

[Bbrayden85]

There is no error in the calculator. He entered the wrong tire size.

No. 31” tire right where it says tire size in inches.

 

[Bbrayden85]

It says its running at the rpm it ahould be with the stock tire size

 

[jjvw]

No. 31” tire right where it says tire size in inches.

For the purposes of the calculator, that is the wrong tire size.

 

[never monday]

No. 31” tire right where it says tire size in inches.

The actual diameter of the tire will be different than the advertised size. Your RPM increase with tire wear.

 

[never monday]

For the purposes of the calculator, that is the wrong tire size.

Unless he put in the wrong trans info

 

[jjvw]

The actual diameter of the tire will be different than the advertised size. Your RPM increase with tire wear.

The rpm will also increase with tire pressure.

 

[Bbrayden85]

Get a tape measure and see what your tire size really is. Go from there.

jjvw beat me to it...

Yea figures. Its a 31” tire but only about 30” actually

 

[mrblaine]

Well yeah, you have to account for a small margin of error with any calculator.

That math has no margin of error. It is simple and basic as 1 x 5 = 5. There is only one variable and that is the entered tire size which most get incorrect. If you want to know the tire size, keep changing until the RPM matches the specified MPH in the correct gear and that tells you your tire size actual, not what's on the sidewall.

 

[Bbrayden85]

Unless he put in the wrong trans info

Nope. Everything is correct.

 

[jjvw]

Unless he put in the wrong trans info

Perhaps. In which case, the calculator still isn't wrong.

 

[mrblaine]

It says its running at the rpm it ahould be with the stock tire size

Then that is your tire size all other things being correct.

 

[mrblaine]

No. 31” tire right where it says tire size in inches.

Your tire size is not what it says on the sidewall. Your tire size is the distance from the center of the hub to the ground multiplied by 2 to get the rolling diameter.

 

[jjvw]

That math has no margin of error. It is simple and basic as 1 x 5 = 5. There is only one variable and that is the entered tire size which most get incorrect. If you want to know the tire size, keep changing until the RPM matches the specified MPH in the correct gear and that tells you your tire size actual, not what's on the sidewall.

This is the most correct method. Use gps to get an accurate speed. The resulting tire size will be very close to the one arrived at from finding the rolling radius and doubling it.