Help figuring out gear ratio

[SouthernTJ2000]

So I recently bought a '00 Sport (4.0, 5spd). It has a 4" suspension lift and running 33's, stock Dana 30/35. Previous owner said it has 3.73 gears, however after comparing what they said about other things I've seen/worked on since taking ownership, I have my doubts. So through some digging here I found grimmjepper.com and entered my parameters. If I place the ratio at 3.73, I'm typically just shy of the proper RPM/Speed that GJ spits out. So I tried 3.07 and its even further off (my RPM' are higher). I'm utilizing a GPS app for my speed. So I decided to jack the passenger side rear wheel up, leaving the driver side rear wheel on the ground, and give the passenger wheel two full revolutions while counting driveshaft revolutions. I get just shy of 3 full revolutions. So now I'm confused as all get out. Will someone verify I have entered the correct parameters into GJ, shown below. If I have, WTF is going on? I guess my only way to verify the actual gear ratio, at this point, is pull the covers and count? Thanks.

 

[P man]

Just pull the covers..too much swapping goes on to trust anything. Plus your diff could benefit from some fresh oil

 

[SouthernTJ2000]

Just pull the covers..too much swapping goes on to trust anything. Plus your diff could benefit from some fresh oil

Its on the short list of things to do. I just keep getting side tracked on other things...lol. Guess I need to move it to the top of the list and get it done.

 

[TheBoogieman]

It has a 4" suspension lift and running 33's, stock Dana 30/35. Previous owner said it has 3.73 gears.

Do you think the PO lied because he was embarrassed that he spent money to regear a Dana 35?

 

[machoheadgames]

Your tires are not actually 33”. Put in 31.5-32.0” and I bet it’ll be spot on. And you need to be using a gps app of some sort, not the dash speed reading just in case the dash is off.

 

[Rickyd]

Did you enter 33" as the height? Your tires are going to be 32-32.5" in reality.

 

[SouthernTJ2000]

Your tires are not actually 33”. Put in 31.5-32.0” and I bet it’ll be spot on. And you need to be using a gps app of some sort, not the dash speed reading just in case the dash is off.

Straight from my post..."I'm utilizing a GPS app for my speed."

I've got that covered.

 

[deadbeat son]

Your tires are not actually 33”. Put in 31.5-32.0” and I bet it’ll be spot on. And you need to be using a gps app of some sort, not the dash speed reading just in case the dash is off.

Did you enter 33" as the height? Your tires are going to be 32-32.5" in reality.

These posts are correct. Measure from the ground to the center of your hub, multiply by 2, and enter that as your tire diameter. As an alternative, look up the specs of the tires on Tire Rack, the manufacturer's website, or something similar, and use their revolutions per mile figure.

 

[N757KF]

Your tires are not actually 33”. Put in 31.5-32.0” and I bet it’ll be spot on. And you need to be using a gps app of some sort, not the dash speed reading just in case the dash is off.

^^^
This.
With vehicle on the ground, measure from center of wheel to the ground , multiply this number by two. This gives your actual rolling diameter which you need for Grim Jeeper.

 

[machoheadgames]

Straight from my post..."I'm utilizing a GPS app for my speed."

I've got that covered.

I skimmed real quick to see if you said GPS before I posted and didn’t see that. Missed it.

 

[SouthernTJ2000]

I skimmed real quick to see if you said GPS before I posted and didn’t see that. Missed it.

All good, I do the same shit on a daily bases. Over the years, I've tried to include as much information when asking a question as I can.

 

[MightyJoe]

What numbers are you specifically seeing vs the calculator?

 

[Ir0nH3@d]

Measure from the ground to the center of your hub, multiply by 2, and enter that as your tire diameter.

With vehicle on the ground, measure from center of wheel to the ground , multiply this number by two. This gives your actual rolling diameter which you need for Grim Jeeper.

wouldnt tire PSI affect this number? I have always measured from side to side instead of top to bottom or ground to center of hub to eliminate the PSI factor.

 

[SouthernTJ2000]

What numbers are you specifically seeing vs the calculator?

I'll have to jot them down. However when I enter the gear ratio as 3.73 with the above parameters, I'm coming up just shy of the calculated rpm. example:

60MPH Calculated RPM, 3rd gear - 3,167
60MPH RPM (dash/actual), 3rd gear - 2,9XX (maybe 3,0XX)

Obviously with the 3.07 ratio entered, I'm even further off. I.E. my RPM on the dash is much higher than the calculated for gear/speed/rpm. I'll jot down some gear/speed/rpm readings on my way home this afternoon.

 

[N757KF]

Use whatever tire pressure you run at and measure from hub center to ground….

 

[Ir0nH3@d]

those numbers would be more accurate with 3.55 gears or larger than 33" tires, but 3.55 gears were only an option in 97

 

[TheBoogieman]

Over the years, I've tried to include as much information when asking a question as I can.

Yet, your Jeep profile doesn't mention any mods.

 

[deadbeat son]

wouldnt tire PSI affect this number? I have always measured from side to side instead of top to bottom or ground to center of hub to eliminate the PSI factor.

Yes the PSI does affect it, and that's why you need to measure from the bottom and not side to side.

A bit of an exaggeration, but imagine a completely flattened tire. The distance from ground to the center of hub will be roughly the same as that of the wheel alone, while the diameter across the tire may be roughly the same as when inflated. When you drive, the smaller radius of the flattened side is what is affecting the distance traveled per rotation, and not the larger side-to-side dimension.

 

[MightyJoe]

I'll have to jot them down. However when I enter the gear ratio as 3.73 with the above parameters, I'm coming up just shy of the calculated rpm. example:

60MPH Calculated RPM, 3rd gear - 3,167
60MPH RPM (dash/actual), 3rd gear - 2,9XX (maybe 3,0XX)

Obviously with the 3.07 ratio entered, I'm even further off. I.E. my RPM on the dash is much higher than the calculated for gear/speed/rpm. I'll jot down some gear/speed/rpm readings on my way home this afternoon.

Gotcha, I use Crawlpedia instead of Grimjeeper (Loaded with malware and bad certificates) but a smaller tire as suggested would make your RPMS go up (e.g. 3266 with a 32" tire). A 35" tire would bring you back to 2986.

As someone else mentioned it seems like a 3.55 gear ratio aligns more closely with the rpms you are seeing at 3015.

 

[OldBuzzard]

Yes the PSI does affect it, and that's why you need to measure from the bottom and not side to side.

A bit of an exaggeration, but imagine a completely flattened tire. The distance from ground to the center of hub will be roughly the same as that of the wheel alone, while the diameter across the tire may be roughly the same as when inflated. When you drive, the smaller radius of the flattened side is what is affecting the distance traveled per rotation, and not the larger side-to-side dimension.

It's the circumference of the tire, where the rubber meets the road, which gives the distance traveled per rotation. That doesn't change, even if the tire is flat.

You had it right farther up, with suggesting the manufacturer's revolutions per mile figure.