Lets say there is a stock rubicon running 32" tires and 4.10 gears. If this Jeep were to put on 35" tires (like you would like to do) the mileage under similar conditions will go down as the new tires are heavier and harder to turn with the same gearing. The tires will also travel further per rotation of the axle shaft which takes more energy as well. Plain and simple that is where you will lose the MPG that you have.

Now, same rig running 35" tires, if you regear to 4.88s this will put you closer to the stock power and MPG as it will compensate for the extra weight the engine now has to turn as well as the longer travel per axle rotation. You may not get better than stock or even right at stock MPG but you will get better MPG than you are with the current gears.

MY EXPERIENCE:

'98 XJ with the 4.0. Stock gears were 3.21. This was fine with the 28" stock tires. MPG was 19-21 city (5 speed). After going to 33X12.50 tires the MPG dropped to 10-12 MPG city. I regeared to 4.56s (actually the correct gearing for 35s but I wanted more torque) and the MPG went back up to 16-18.

With all that said I just don't understand how its possible to go with larger tires and regear and not see an improvement with the MPG unless the gearing was either not low enough or it was too low (as in my case but I STILL gained back some MPG)

 

My stock Rubi with 4.10 gears, 32" wheels, stock muffler, stock program, etc. got about 18.5 MPG average - not including off-road excursions which dropped it a lot. Now that I have 5.13 gears, 35" wheels, aftermarket air filter, Magnaflow exhaust, Hypertech Max Energy, lifted, etc. I get about 17 and some change - again not including off-road which is a lot worse. Is it the gears? Maybe. Tires? Maybe. I know the gears and tires went on about the same time and the mileage went down. I've been using "hypermile" techniques and can best get about 18 MPG - about same as stock and driving normal.

Bottom line is there are too many variables to say if one change makes a big difference one way or the other. The exception would be anyone who makes accurate measurments of their mileage (miles traveled divided by gallons used over an extended period of varying weather conditions) with one configuration then again using the same exact enviornment after a specific mod - well, I doubt many have really done that. Most observations, including mine, are done after several mods and in varying conditions. Way too hard to say exactly "you'll get xxx MPG improvement with yyy mod".

BTW - I firmly believe this is how most aftermarket manufactures get their incredible MPG figures - pick the worst possible conditions for the initial measurements and pick the best possible environment for their wonder product comparison. Like comparing apples and Chevys.

My 2 cents.

 

What to do....what to do....

I guess I'll regear to 4.88 and hope for the best when I do. I'd like to avoid a further drop, but I'd also like to maintain highway speed uphill. The Jeep feels like it chokes on the inclines I see on the highways around here. I barely seem to maintain 60.

Oh and I already have my 35s and Clayton 3.5" Suspension lift for those who thought I didn't yet. Pic Below:

 

when you regear wouldnt it mess with you lifetime powertrain warranty? i have an 09 rubi and i want to lift it up and but 35's on it, but i think i'm just going to but 33's on it with spacers cause id like it to be covered under warranty for as long as possible, you know?

 

Im sure it does......but i dont give a crap...... Im sure thats not the first thing i did that messed with my lifetime powertrain warranty. I gave up on that warranty thing a month after i got my jeep.

corey d

 

If they wanted to void your warranty, the spacers and 33's would do it anyway, depending on the dealer. If you change the suspension and bigger tires than stock, it gives them a whole host of reasons to void the powertrain warranty. I took into account my bigger tires will ultimatly break an axle, etc. You have a 50/50 chance of them repairing, some do, some don't. If your really worried about it, talk to your dealer. Bringin them donuts every once in awhile doesn't hurt either, establish a good relationship with the service manager and his guys, it'll go a long ways. Couple times a month,$5 worth of donuts could save you $500 in repairs.

 

My stock Rubi ato got 18 mpg. Put 35's and it went down to 14. Put 5.13's in and it went back up to 18 mpg. No complaints. Oh yeah - the power is much better. Offroad with the 5.13's and with the 4:1 TC in low range power is never an issue.

The consensus seems to be that with 35's 5.13's work well for the auto and 4.88's for the stick (due to differeng overdrive ratios - .69 vs .84).

When I put 37's on I'll probably wish I had 5.38's although I have concerns about the smaller pinion size. Although that was my concern originally when I went with the 5.13's and I haven't heard of any problems. Apparently the JK's D44's have a beefier pinion than prior 44's.

The only downside to regearing is he cost. As to the concern about the higher rpm from regearing - the 3.8 likes it - it doesn't like to lug. I'm at 2600 at 75 on the freeway and would prefer to be at 2700-2750.

 

I love my 5.13s, but for how I drive on the interstate it kills gas. I drove 80-85+ on a recent trip and got about 11mpg. It is all how you drive. The jeep is a brick anyway so I doubt any other ratio would do better. In town my MPG went up from the gear change. I don't put a whole lot of miles on the clock so a little extra at the pump would well be worth the extra go.

 

I dont think that would work for me. I dont think there are any cops that work in the service dept. where i got my jeep.

corey d

 

Well, the sevice guys at my dealership are in the same FAntastic shape that our local PD are, so they can be bribed with just about anything edible and unhealthy. Vegetable trays are a sure way to get your entire factory warranty voided even if your still stock.