Rubicon Owners Forum banner

1 - 7 of 7 Posts

·
Premium Member
Joined
·
4,159 Posts
Discussion Starter #1
Ok we all know bigger tires, without regearing reduces power and gas consumption.
My question however is, do tires affect the calculation of Crawl ratio

IE: [first gear] multiplied by [transfer ratio] multiplied by [axle gear ratio]
5 speed Rubicon: 4.02x4x4.11=66

I am just wondering if there is a modification to this calculation that shows the reduction of a 4wheeler's effectiveness with bigger tires or if you will always say 66 no matter how big or how small the tires are

Psssst [Soon to be 78]
 

·
Registered
Joined
·
1,530 Posts
Tire size has no bearing on crawl ratio's.....
 
G

·
Guest
Joined
·
0 Posts
The crawl ratio should really be to the ground, but I think they call that final drive ratio???

With an auto you get some benefit from the coupling in the tranny too when going up hill.

I'm not sure how to figure it, maybe you go very slow and count revolutions on the drive shaft, and wheels and figure it somehow?

With an auto it seems near impossible to really tell. But with a stick, it does seem there would be a way to really exactly figure it out.
 
G

·
Guest
Joined
·
0 Posts
Ahh the old crawl ratio vs. actual Feet Per Minute (FPM) Travel. I discussed this at length with an old friend of mine. Here is what we came up with. You need to have microsoft Excel loaded on your computer to use the calculators.

Crawl Speed explained with calculator

Ron
 

·
Premium Member
Joined
·
3,745 Posts
Crawl ratio is, by definition, lowest gear x T/C reduction x differential ratio. It does not take tire size or torque multiplication of automatics into effect.

I have always understood final drive ratio to mean the diff gear ratio.

elff, I think you could calculate some other figure of merit which would include tire circumference to get a number that would equate inches of forward travel to revs in 1st gear, but it wouldn't mean too much. Smaller tires would compute to have the best number, but we know that we don't need no tiny tars climbing some of those rocks we attempt.

What I have to figure is what combination of gears and tires will give me somewhat higher (but not way higher) revs in OD on the highway than stock. My Rubi was slooooow when it was stock, and now with the 33s it is an absolute dog in OD. And going down to 3rd gear by cancelling the OD gives me 3000+ revs at 70mph. There's too much of a gap between the 1.00 3rd gear and the 0.69 OD in the auto. DC should have set up the OD around 0.80 for better highway performance.

(Edit) Damn. PBR Stitch posted his calculator page while I was typing this. I could use that to do my calculations. Thanks Ron.

Jerry
 

·
Premium Member
Joined
·
4,159 Posts
Discussion Starter #6
Thanks everyone, I knew I had the calculation correct, but was just curious if there was a muliplier that mod it for tire size. O well.

Jerry,
Check out this
Ratio Calculator Web Page

I would think 4.88s would get u what u need
4.56s would probably be real close to how it as on 31s and since u thought that was sluggish, I would recommend 4.88s
 

·
Premium Member
Joined
·
3,342 Posts
It'll be 4.88's for me. I ran 4.56's with 33's and having that extra power and crawl was nice. Not to mention kept the mileage down. :D
 
1 - 7 of 7 Posts
Top