Over heating on bigger tires

I took my first long freeway trip on the new 31x10.5 tires today (old ones were stock size) and the temperature idiot light came on. I have never had overheating problems before. I did not regear the rear end when I went to 31" tires and the freeway had a lot of uphill stretches. I would think that taller tires would raise the final gear ratio and make the engine work less but i noticed that around 65 mph (indicated on speedo with no recalibration) the transmition was was down shifting and hunting for the right gear, especially on hills. Does this sound like a gear ratio thing or am i just trying to blame a new problem on the latest change that i made.

sdigs said:

I did not regear the rear end when I went to 31" tires and the freeway had a lot of uphill stretches.

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This is your answer right here - plus (probably) a cooling system in need of attention. Although revs may be lower, you're making the engine do more work to reach a given speed if the gearing doesn't match the tyres.

Does this sound like a gear ratio thing or am i just trying to blame a new problem on the latest change that i made.

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No, you're on the right track. But you probably want to at least acid flush your cooling system, change the coolant, and check for any leaks as well as regearing. I just recently went from 3.55s with 31" tyres to 4.10s, and noticed a good 10 to 15degF drop in coolant temperatures while running - plus less tranny heat and about a 15% mileage improvement.

Yes, it will hunt for gears more with 31's vs. stock. You just have to learn to accomodate for that, and start giving it gas sooner before you get to the hill

I've noticed the same thing, as I just went to 31's from stock about a month ago. Havent had any overheating problems after switching though. I used to work 35 miles away from my house, all highway miles. Did great in the 90* weather, but tomorrow will be the first good test. 100* and a 70 mile highway commute to college.

Thats why i like having the tow package with 3.73s it gives you a lil more leeway.

Generally, the 4.0 did not come with 3.73 gears in the Cherokee, it did in the WJ, I don't know about the ZJ. The tow package did not include lower gearing for the XJ that I've ever read. There are 3.73 geared axles out there, what their origin is, I don't know.

Fred

Use 3rd on hilly sections of road & make sure you have a trans. cooler. JIM.

shouldn't have too much trouble on only 31s...do you do a lot of mud. if so make sure the back of the radiator is clean. i replaced my t-stat, rad. fluid, etc, before figuring out my only problem was the back of the radiator being covered in mud. and im on 35s, have driven 70+miles a couple times on stock gearing a couple times and never had a problem

Definitely look closer at your cooling system, but a by-product of too tall of gearing is increased tranny slippage which causes significant heat buildup, which is also the auto tranny's worst enemy.
Believe it or don't........31's with a 4.0 and an auto need about a 4:25 gear ratio, which isn't available for XJ axles, so most folks go with 4:10's, which are borderline if you have HD bumpers and carry added cargo weight. 4:56's are your best bet, which will also be OK with 32's, but jumping to 33's will leave you with more 3-4 tranny hunting for the right gear, wishing that you'd installed 4:88's, which are just about perfect. And folks that may try and convince you that stock gearing is just fine with 33's-35's are in denial and should seek mechanical help.

Not exactly what you asked about, so I'm not gonna charge you extra.................

I have 33's on my truck with the tow package with stock 3.73 gears. There was never any temperture problems or odd shift problems with mine, after I increased tire size. So really you must have already have had those issues before you put those tires on. You just really never noticed before. Now I should be running 4.11 or 4.56 gears.
If you're driving at 65mph and the tack reads around 2200-2400 rpms your gears are close. Too low of a gear your performance and torque is good, but you loss on gas milage. Too high of of gear to get better gas mileage can be bad on the transmission by over working it, leeding to a shorter life.

My idiot brother has 3:07's,31's and a 5spd, No complaints except for no 5th, I haven't driven it though

i have 33s stock gearing and i drive my car 100 miles plus at least twice a week. no tire related overheat problems. the only thing that has ever caused a overheating problem in my experience is a malfunction in the cooling system not large tires.

If the trans is hunting for a gear its probably comming slipping out of converter lockup mode as well which causes mass heat which will head for the cooling system via the fluid.

it's not "hunting" so much as dropping down a gear, like when you floor it and get the passing gear. It is a 94 4dr 4.0L HO with auto tranny. I have no Tach so no idea what engine RPM is. I am going to put it on stands this weekend and spin the wheels to get an idea what the rear end ratio is. Unless anyone can tell me where I might find a sticker with the gear ratio on the vehicle. I already have an appointment to get the cooling system checked out.

I ran 33's on 3.07 for a brief period 6mo.I never had an overheating problem but I'm not convinced it wasn't a major contributing factor to grenading the 1st 4.0 at 120,000 they should last longer then that. Lugging the motor is never a good thing If only I'd listened.:doh: