Okay, Here's the deal, I'm not sure how to directly factor in weight, rolling resistance, and wind resistance quite yet, so I figured I'd ask here, there's some pretty intelligent people whom use the forum.


The Stock Gear Ratio (3.54) on 265/55r16 (29") @ 70MPH is 2533RPMs
Upgraded Gear Ratio #1 (4.11) on 315/75r16(35") @ 70 MPH is 2347RPMs
Upgraded Gear Ratio #2 (4.56) on 315/75r16(35") @ 70MPH is 2704RPMs


I know from talking to Bill from GBR a while ago that weight and resistance plays part in gear ratio calculation, but I'm not quite sure how to do it. I don't want the truck underpowered as it's the 4.0 and already is and I don't want the truck to over rev on the freeway. It's going to have weight, the usual: armor, bumpers, roof rack, and other random things, should I go with the 4.56's with 35's? I might call Bill and ask his opinion on this.


Dusty, when you talked to Bill when you bought your diffs what size tire did you use to set up the ratio?


Thanks Again,
Dane.

 

Talk to Bill about the 4.14

That will most likely be the ticket for your setup...

Unless you will primarily use it for serious wheeling... then the 4.56 will probably be a better option...

 

Why use math?

http://www.ashcroft-transmissions.co...atio_calc.html

 

ZG, I used that calculator... That's how I got the rpms But I'm looking to see if 200 rpm's over stock at cruising speeds is to much for the engine, even though I'll only be at 65mph, because anything over 65 kills mpgs for me on long trips. I'm considering do the swap to Toyota Land Cruiser FZJ80 Axles and going to 35's, I think I found a set at my junkyard for 300$ for both... I don't think Bill will want to talk to me about converting to Toyota axles either.


What are you RPM's at 65 MPH?
Is +150 too much more?

 

I did a little math.. the 315(35) is 19.1 percent over stock in tire size, with the reduction of a 4.11 I lose roughly 7% of power, and if I use 4.56 I'll be overpowered by 1%. I think 1% in gear ratio shouldn't effect my RPM's at cruising speed, or am I wrong? I can be over .9% or 150 Rpms?


EDIT: If I was to find a TRUE 35" tire, I would be near perfect, the calculator I was looking at was showing a 315/75's at 34.5, and just .5'' over would put me at roughly .7% (+/-.2) under. I figured this out I guess

Thanks.

 

of-course you are going to replace you axle shafts as well?

 

If he's running FZJ80 axles he won't really need to, they're so much stronger than Rover stuff.

 

4:43 w 33s @70mph 2750ish
dunno w 34s yet. I have not driven with, yet

 

Is that with GPS speed or in-truck speed Dusty?

 

gps, in a school zone while talking on the phone.....the speedo tells a different story..