how much gear for 30" tires?

[HaleYes]

if i were to swap in an axle from a four cyl. xj with the 4.10's, could i use my big-joint axles? i know the older xj's had smaller u-joints, and my 97 ha the bigger ones....

Yes.

 

[XJBlackPDX]

With 31" tires, would expect the fuel economy to go up going from 3.55 to 4.10?

 

[C85D4x4]

I ran 30's, 4.10's and with an AW4 for a long time. I loved it. I miss it. I'm seriously thinking of dropping back down.


If you're 100% sure you'll go with 31's; My recommendation is to wait to regear until then. Then, after you have the 31's, regear to 4.56's.

X2

I was on 32s with stack gears for a long time............

Then a year ago I went with 4.56 with my 32s..............amazing

But if I were to to it over, I would go with 4.88

 

[Ivan]

Come on, it's not a fuel pig. A stock one gets similar gas mileage to a sedan.

Looks like a lot of people get ridiculous gears so they can crawl in overdrive, put on roof racks, etc... then get 12mpg and say "XJ is a fuel pig anyway, buy an econobox!"

Sounds like stuff Hummer owners would say!!

True for a stock rig, but we're not talking stock here when other gear ratios are involved... We are at the point of modifying our rigs by adding larger tires, bumpers, roof racks,lifts, etc. All this adds up to more weight, a higher stance profile, scews even more with the already aerodynamic profile of a brick the XJ has; which then begins to play with ones gas mileage in a negative way....

So yeah, I admit I created a fuel pig and I'm willing to pay the fuel price for the added capabilities I added to my XJ. It just cracks me up to hear all the whining that results from those that modify their rig and expect the same gas mileage as stock....what did they expect?

 

[Darky]

Actually, with 4.56 and 31" tires going 75, you are at about 3500 rpm, which I would say is worse. It seems to me that you lose OD with the AW4 in that combo.

You'd be in OD more with that combo. My friend ran 4.56s with his 89 on 31s and didn't do nay worse than mine on gas mileage. I was running 30's with stock 3.55s. Still am actually...even sitting at nearly 6" of lift now, I'm still averaging 17-19 mpg depending on how much highway driving I do and if I'm running 75 or 65. At 65, I get just over 20.
Hale's real world on a 98 would be the same as on a 91. Below that you lose the HO, but even then mileage numbers will be similar and you might want to go a little deeper on gears than you would with the HO to make up for the little bit of power.

 

[winkosmosis]

There's no rule that says you must have heavy steel bumpers and a roof basket. If you do add a basket, you don't have to keep it on for street driving. Unless it's required for posing at the mall I guess.

If you regear to keep the stock speed:rpm ratio, bigger tires alone won't change highway mileage except with the little extra drag they add!

 

[Darky]

Come on, it's not a fuel pig. A stock one gets similar gas mileage to a sedan.

Looks like a lot of people get ridiculous gears so they can crawl in overdrive, put on roof racks, etc... then get 12mpg and say "XJ is a fuel pig anyway, buy an econobox!"

Sounds like stuff Hummer owners would say!!

Looks like people just enjoy being to still climb hills in overdrive, tow on occasion and carry a few friends without having to run in 3rd.

Similar mileage to a sedan?? Don't know what sedans you're driving, but they suck! Real world mileage on a new Impala is about 30ish on the highway (doing 80), dropping to about 25ish in the city (driving it like the rental it was). Has your XJ ever gotten above 20 in the city? Even stock, I got 22mpg highway, tops. 17-19 city. Which coincidentally lines up nicely with the rated figures for it from the EPA. My mom's old 92 Lumina Euro sedan got 34mpg highway (following her UHaul at 55mph). She usually averaged around 28-32 highway though. My Ford Focus (which I drive floored to try and take full advantage of all 103hp...) averages 28-31 mixed driving.

Stuff Hummer owners would say is, "Damn, 10 mpg with a 30 gallon tank and $5 gas friggin sucks!"

 

[winkosmosis]

I didn't think those midsize sedans got such good gas mileage. They must have improved them since the 90s. A V6 altima still gets in the mid 20s though.

I don't understand the appeal of climbing hills in overdrive. So what if it downshifts? I'd rather have much better gas mileage than have the strange pleasure of having a tranny that never downshifts. No matter how many friends you have, it won't affect the gear on a flat highway...

 

[Darky]

There's no rule that says you must have heavy steel bumpers and a roof basket. If you do add a basket, you don't have to keep it on for street driving. Unless it's required for posing at the mall I guess.

If you regear to keep the stock speed:rpm ratio, bigger tires alone won't change highway mileage except with the little extra drag they add!

Yes, yes bigger tires alone will hurt your mileage. Weigh a stock 225/75 against a 33x12.50. Weight hurts mileage. Compare the skinny 7-8" wide stock tread against the 12.5" wide tread. Drag hurts mileage. Check rolling resistance for a set of stock tires vs a set of tall, wide, mud terrains. Rolling resistance hurts mileage. Ask Toyota why the Prius wears fairly small wheels and has such skinny tires. Also ask them about the special casing and tread design meant to minimize rolling resistance. There's more to making decisions on tire/gear than just doing the math. Tell ya what, when I finally get new tires for my Jeep, I'll tell you what kind of difference they make in my mpg. I plan on going with either 4.56:1 or 4.88:1 gears to go with the 33s I want. I have stock bumpers, stock roof rack, the only armor at all is a belly skid and front end skid once I finish upgrading my steering. None of the extra variables you're spitting out.

 

[Darky]

I didn't think those midsize sedans got such good gas mileage. They must have improved them since the 90s. A V6 altima still gets in the mid 20s though.

I don't understand the appeal of climbing hills in overdrive. So what if it downshifts? I'd rather have much better gas mileage than have the strange pleasure of having a tranny that never downshifts. No matter how many friends you have, it won't affect the gear on a flat highway...

Friends weigh something don't they? Not all driving is done at steady speeds on flat ground. In fact very little is. If you gear specifically for those rare moments, you're not going to be happy during 80% of driving. My mom's Lumina was from 92. It averaged 30ish highway, 25 city. Better than any XJ. It had the 3.1 V6 so it wasn't a four banger either. For me, the appeal is climbing those hills without losing a bunch of speed. Out here, there's two hills that you have to climb if you go down to Palm Desert for the mall or Sam's club, whatever, that even in 3rd my Jeep will lose 10-20 mph. And I'm only on 30s.

 

[Darky]

But for the OP, I would say wait til you're ready for the bigger tires and then buy gears. Or even gear for the bigger tires now while you still have the smaller tires. If going to 33s you'll probably want at least 4.56s. For 31s I'd recommend 4.10s. Friend of mine did the 31s with 4.56s but he never went above 60 and went bigger anyways on his tires.

 

[winkosmosis]

Looking forward to seeing your numbers.

I covered the drag from bigger tires, but I don't think it should be THAT significant. A Corvette gets 30mpg on the highway even with very wide tires. There is a lot more to rolling resistance than size. Air pressure is probably a lot more important. Correct me if I'm wrong, but at the same pressure two tires of different size should have very similar rolling resistance, because the contact patch and deformation of the tire is the same. If anything a larger diameter tire should have less rolling resistance.


And yeah, mud terrains do add a lot of rolling resistance, but IMO most people are better off with all terrains. A lot of mall crawlers around here go with MTRs for the aggressive look even though they spend most of their time on the highway.

As far as the weight of the tires, that only matters when you're accelerating.

 

[winkosmosis]

Hmm.. maybe you should get skinnier friends

 

[Darky]

Hey, 3 100lb friends or one 300 pounder its still 300 lbs....


Unless its one fat chick and two skinny people in a Geo Tracker. That makes a huge impact, but on handling not mpg...

Corvette's were rated at 27mpg highway, but that's been dropped to 26 with the new standard. But a Corvette is a very different beast. Its highly aerodynamic with work done to direct airflow around it in such a way as to avoid hitting anything that would create excessive drag. Not to mention it sits a lot lower so probably 95% of the tire is covered by aerodynamic car. Whereas on a lifted XJ with big wide tires, a lot of that tire is sticking out - from the fenders, under the vheicle, etc - and exposed to passing air. A Corvette weighs about 3100lbs and puts out 400+ hp. A stock XJ weighs about 3500lbs and puts out 190 hp. A modified XJ can break 4000lbs easy. With the same power. What point am I makig there? The engine works a lot harder in an XJ than in a Corvette to maintain the same 75mph. The bigger the tire, the more it weighs. The more it weighs and the bigger it is, the higher its rotational inertia. Meaning it takes more to get it going and more to get it stopped. That's why its important to also upgrade brakes when getting bigger tires. Weight matters for acceleration, inclines, and even steady cruising. Try pushing a Bug up to running speed. Now, keep running behind it for 100 meters. Rest. Do the same with an Excursion. Extreme example, but it all makes a difference. Its a lot harder to keep that Excursion moving than it was the Bug.

But I think that really the most important matter here is what will the OP be doing with the vehicle? If he lives somewhere (like I do) with hills, chances are 4.10s and 33s won't make him happy. 4.10s and 31s most likely will. If he plans on wheeling it in mud mostly, maybe .10s will work for 33s, get the extra wheel speed. If its going to be hill climbs, rocks and trails, the extra torque and control offered by the lower gears will be greatly appreciated.

 

[RenegadeStang]

Well, currently my plans for my driving is this:

I spend a good 97% of my time on road and about 3% off road. To "kinda" find a middle ground for the two, I'm gonna be running 4.10 gears.

Most of my driving is around town, therefore, a lot of stop and go. Because of this, I'll be running 30x9.50's on the streets (either street tread or AT's).

For what lil' bit of off-roading I'll do, I have 33x10.50 MT's. Even though 4.10 isn't the greatest gearing for 33's, I'm hoping the narrow width and the low-range in the t-case will help make up for it just a tad.

Flame on. :gonnablow

 

[HaleYes]

Well, currently my plans for my driving is this:

I spend a good 97% of my time on road and about 3% off road. To "kinda" find a middle ground for the two, I'm gonna be running 4.10 gears.

Most of my driving is around town, therefore, a lot of stop and go. Because of this, I'll be running 30x9.50's on the streets (either street tread or AT's).

For what lil' bit of off-roading I'll do, I have 33x10.50 MT's. Even though 4.10 isn't the greatest gearing for 33's, I'm hoping the narrow width and the low-range in the t-case will help make up for it just a tad.

Flame on. :gonnablow

I think that's a good decision actually. I wish I could afford 2 sets of tires and do the same.

 

[RenegadeStang]

I think that's a good decision actually. I wish I could afford 2 sets of tires and do the same.

Well, the 33's and the 4.10's for my D30 were given to me (33's have around 30-40% tread). I just have to find me some 4.10's for my 8.25 and upgrade to some 30's (I've got 235/75's now).

 

[_MURDERED OUT_]

Im sure everyone will think im an idiot.... but i run 4.10 gears with 42" tires. :huh:
I mean going on everyones tire size vs gear ratio i would assume i would need like a 6.72 gear or some crap!! ha ha

 

[Darky]

Im sure everyone will think im an idiot.... but i run 4.10 gears with 42" tires. :huh:
I mean going on everyones tire size vs gear ratio i would assume i would need like a 6.72 gear or some crap!! ha ha

Well you also have a V8. Although I'd still expect lower gears for 42s...Most full-size guys even are running at least 4.56s with that size.

 

[XJBlackPDX]

BlackSport, are you trying to tell me that you think Jeeps are not aerodynamic. I may not be an engineer, but rockets are shaped like bricks too.