Another darned gear question

[53guy]

Ok, kill me now, but I've got some very pointed questions for those of you with 35's, either 4.88's or 5.13's, and the auto tranny. I've been crunching numbers on the gear calculator all day and have been reading posts from people who don't even exist on here any more and still haven't come to the conclusion that I've been wanting. I want to know, for street use, majority highway between 70-80mph, which gears will yeild the best mpg with 35's? I've read that 5.13's will put me right in the power band for best mpg instead of 4.88's and i've also read that 4.88's will be lower rpm's (roughly 200 rpms) and should yeild better mpg than 5.13's because of the lower rpm's. I also want to know what's the difference in 200 rpm vs. speed and acceleration. I've gone through and punched numbers for various speeds and no matter how I throw it, there only seems to be a 200 rpm diff between 4.88's and 5.13's. Will the better acceleration of the 5.13's be better for mileage or will the lower rpms of the 4.88's be better? I've gotten advice from many people saying conflicting things, such as I'd be happier with 5.13's and they are "perfect" for 35's or 4.88's will be better on gas, but I wish I'da gone with 5.13's, but no one explains why they'd have liked to go to 5.13's instead. I'm about to pull my hair out here for no reason other than the conflicting info and my indecision. Any help from those who know? And yes, I know, those with 33's have the same issue but between 4.56's and 4.88's. Go figure, its a 200 rpm battle there as well....

[j99xj]

I don't think 200 rpm will affect gas mileage to the point of concern. So if it were me I'd go with the 5.13s.

[BlueCuda]

If your using the D30 front axles 4.88 is as low as you can go. I have 4.56s with 33s and really wish I went with 4.88s. My old toyota had 5.29s and 33s :) but it only had 115hp so it was requiired.

[53guy]

Yeah, I know about the 33 debate as well and a lot of people wish they had went with 4.88's, that is why I'm trying to figure out WHY they are wishing they did that....was it for power reasons or was it for gas mileage? I no longer have a d30 anywhere near me and one will never be installed in my jeep again. I've since moved onto d44's front and rear and I can only suggest that to every one else!

[Big Red]

Since you have the auto tranny like I do in my 91' you have a great over drive gear of .705 I run 37" MTRs and love them with 5.43 in my 9" and 5.38s in my HP44. I geared the lowest I could with the HP44 and am happy I did that. The difference between 4.88s and 5.13 is not major for gas savings...heck running 35's and the height required to run it and wheel gives you Jeep a lot of wind resistence going down the road as it is. I had 35's with 4.56 gears HP30/44 and didn't like it especially going up to Tahoe and climing the grades and trying to keep the power up. My tranny would be hunting for gears. I say go with the lowest gear for 4 wheeling, because you already are leaning that way big time with 35's. I'm not saying you cannot get decent MPG with 35's, but the height of your rig, rolling resistence of 35's with a aggresive to semi aggresive wheeling tire etc doesn't help MPG anyways, so why not go with a gear that will help you offroad and deal with the slight MPG loss if any onroad.

If I were you I'd go 5.23 or even 5.38s with 35s. Any lower gearing then this (higher # gears) and you are sacrificing too much strength, 5.38s are nice, I personally would just run these almost on principle alone being that I was held up with 4.88s being the lowest for my stock axles so now that you have 44's why not dip into the lower gears, 5.13s would not be bad, but I say go a little deeper....then again is this dd, I see you like many of us wheeling your built XJ initially and then going extreme enough where you are looking at getting a cheap dd that is good on gas.

I got a 98' 2dr XJ 5 speed dd, but now fight inchitis and wanting to build that one up to, it's a disease I tell you. I'm trying to justify putting in my bro's 30/9" with 4.88 gears and locked f/r with 31's in my dd since he is going the full width HP44/9" like me and is going to sell his axles to me or someone else, I guess I could sneek in 33's and be o.k. right??? :-)

With 37s I'm at 3K RPMS at 80 MPH which is fine with me. I love the nice overdrive gear in the AW4. You can run a 4.0L all day at 2,500 - 3K RPMs and that's in its power band where you want to be.

I liked your fender trimming and "SS" badge job 53guy.


Troy

[Beej]

I'm inclined to say that I really don't think its going to make a heap of difference to your gas mileage between 4.88's or 5.13's. When people say they wished they'd gone to 5.13's they are likely considering the advantage of the lower gears offroad, not streetability.

Your rpm's on the highway are only one factor. Wind resistance on a lifted vehicle will be a much bigger one. Weight from the added tire size and larger axles is another, etc. My point is that if you want increased fuel efficiency in your XJ, its probably not going to come about via regearing.

I'm guessing that the difference between the two gear sizes might only yield a difference of 1 mile per gallon, generously.

Factors that will make a bigger difference are your power to weight ratio, decreasing wind resistance and decreasing rolling resistance. Since all of those factors are presumably going to be held constant, I don't think much difference would be seen by the different gears.

All that said, I would just go with the lower gears (5.13's) since they will be preferable offroad, and it sounds like that's where your XJ is built to perform.

Just a couple of cents for consideration...

[53guy]

Yeah, I'm pretty sure that as long as I don't get worse mileage than I'm getting now with 4.56's, I'll be satisfied. If it brings my mpg up any, I'll be really excited then. I've been running 35's and 4.56's for the past year and a half and they have been tolerable, but because of a recent accident I had to replace my front axle, leaving me open to regear. I think that I will go with 5.13's and be happy with that as I can't see 200 rpm really hurting me much. I think with the 5.13's, I'll have better mechanical advantage over the wind resistance and weight of the tires than I do now and I'll be pleased with that. I do think, however, 1 mpg is a large amount when you're talking the difference between 12 and 13 mpg. It might not add up right away, but over time it will either cost me or save me a few bucks.

[53guy]

I liked your fender trimming and "SS" badge job 53guy.


Troy


Why thank you!! I've had a few questions about the SS badge's, but never any compliments on them! I think they look decent and make it stand out a tad better than Sport did.