Spare tire size

I just got a new to me '98 4.0 AW4 NP242. I plan on going 3" lift with 31's. I really like how the stock spare tire fits in the cargo area. Would it be ok to use the stock tire as a spare. It would only be installed on the front if needed. Would this hurt the D30 if driven like this @ 55mph for 100 miles.
TIA

-AJ-

AnarchyJet said:

I just got a new to me '98 4.0 AW4 NP242. I plan on going 3" lift with 31's. I really like how the stock spare tire fits in the cargo area. Would it be ok to use the stock tire as a spare. It would only be installed on the front if needed. Would this hurt the D30 if driven like this @ 55mph for 100 miles.
TIA

-AJ-

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I'm highly inclined to say Yes. Especially since there's a fairly large disparity in size (the OEM tyre is somewhere around 28", and you're going 31's. That's just diameter, not circumference - which is a bigger problem.)

You're likely to overheat the gears in your differential at the very least (side gears/spider gears, since they provide the differential action,) simply because the two wheels will be turning at a significantly different rate, at high speed, for a long time.

Hell, I'd be loath to run it under power even for a couple of miles. Check your phone book for somewhere that sells used tyres, and get a spare 31" from them with at least 25% tread or so - you'll thank me for it the first time you have to use it.

Never run two different size tires on an axle in 4wd.

Do what 5-90 said and get a 31" spare.

joe_peters said:

Never run two different size tires on an axle in 4wd.

Do what 5-90 said and get a 31" spare.

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I think he was talking about running it on an axle not under power at the time, but the vehicle itself was still under power. That's why he specified front axle.

Still, the overuse of the differential gearset will probably result in wrecking the temper of the gears due to overheating, which is why I specifically described that as a probable failure. There are probably more possibilities, but that's the first one that came to mind.

5-90 said:

I think he was talking about running it on an axle not under power at the time, but the vehicle itself was still under power. That's why he specified front axle.

Still, the overuse of the differential gearset will probably result in wrecking the temper of the gears due to overheating, which is why I specifically described that as a probable failure. There are probably more possibilities, but that's the first one that came to mind.

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I fully agree with you on this. Just wanted the new poster to be clear on the issue of 4wd with odd size tires.

PS--isn't 100 miles exceeding the recommended limit on a doughnut?

joe_peters said:

I fully agree with you on this. Just wanted the new poster to be clear on the issue of 4wd with odd size tires.

PS--isn't 100 miles exceeding the recommended limit on a doughnut?

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Donut? Yeah - I think they're rated 50/50 (50mph/50 miles.)

I don't think he was talking about a donut tho - more like going to get 31's and using the "best spare" - best condition out of the four old tyres gets mounted as a spare. Still a bad idea - powered axle or no.

5-90 said:

Donut? Yeah - I think they're rated 50/50 (50mph/50 miles.)

I don't think he was talking about a donut tho - more like going to get 31's and using the "best spare" - best condition out of the four old tyres gets mounted as a spare. Still a bad idea - powered axle or no.

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That's what I was thinking. I think I will PM him just to verify.

It is a full size spare, and the tire is brand new. I figured it wouldn't work, but just thought I ask.

what about the difference of a goodyear 225/75/15 a/t spare used with a BFG 235/75/15 MT? or do i seriously have to buy a new bfg mt for a spare?

The difference between a 235/75/15 and a 225/75/15 is about 1.85" in overall circumference. Not a big deal for temporary use.

MaxxXJ said:

what about the difference of a goodyear 225/75/15 a/t spare used with a BFG 235/75/15 MT? or do i seriously have to buy a new bfg mt for a spare?

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It doesn't have to be the very same tyre, just the same size. The difference between the 225 and 235 is nominal (1.85" in circumference runs to about 2/3" in diameter,) so you're nowhere near as badly off - considering the disparity in size between a new tyre and one with most of the tread worn down, you're about there anyhow using a new spare on the same axle with an old tyre.