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This is probably best viewed as a general question, however specifically I'm talking about a 2004 Jeep Wrangler Unlimited with an automatic transmission.

I have gone from stock 29 inch tires up to 33 inch tires without regearing. The tires are in fact a few pounds lighter than stock, upgraded breaks manage them well, and getting up to and maintaining speed does not seem to be an issue (relative to stock). Other than correcting the speedometer, I don't have any personal reasons why I'd want to regear.

Will this cause significantly reduced life of the drivetrain?

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  • Is your vehicle a manual or automatic?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Apr 5, 2016 at 20:20
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    Having done the same thing on a Toyota Land Cruiser I would say that the bigger rocks and deeper mud the new tires will let you get into will be more of a strain on your drive train.
    – JasonJ
    Commented Apr 5, 2016 at 20:37

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Your automatic transmission is your weakest link

The wear will primarily be in your automatic transmission.

When starting from a stop your first gear is now considerably taller. When you first start to roll it takes the rear wheels longer to get the inner hub to the RPM's that full 1st gear engagement occurs because of the larger diameter of the rear wheels. This additional time to fully engage is like fanning a clutch on a manual transmission for a longer period of time. Due to the longer engagement and the transmission 'slipping' for a longer period of time the components associated with that act wear at an accelerated rate.

You can compensate for this situation a bit with a manual be allowing the clutch to fully engage a bit earlier but it's still not good for a manual clutch to have to endure the increased slipping and will, as well, experience accelerated wear due to the larger diameter tires.

To answer your question, the additional wear is on your automatic transmission. Re-gearing your rear end to match so the automatic transmission can fully engage earlier is the proactive solution to prevent the destruction of it.

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It really depends on what your stock gear ratio is. You mentioned you have a 2004 LJ (Wrangler Unlimited), which means it probably came with a 3.73 axle ratio unless you have the Rubicon, in which case it came with 4.10 gears. There should be a small metal tag on the rear differential that's stamped with your gear ratio, but it's not uncommon for that to be missing.

Assuming your Jeep is not a Rubicon, and is running 3.73 gears, then you will be putting some additional strain your automatic transmission, but it won't be significant. That transmission (42rle) was offered in TJ models with gears as low as 3.07 in both 4 and 6 cylinder models, which has the same "tall" first gear effect. Running 33" tires will increase the wear on your automatic transmission, but you're not pushing it outside of it's design specifications.

You should probably consider getting a tuner to correct the speedometer/odometer readings and adjust the shift patterns for your tire size/gear ratio combination. For example:

http://superchips.com/products/f5-jeep-flashpaq-3874-3874/

If you have the Rubicon, which came stock with 4.10 axle ratio, then you're in even better shape. The Rubicon came equipped with that axle ratio specifically to handle larger tires.

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  • Seems you have the 'model specific' expertise on the Jeeps which is awesome. Again, not votes on this? Good info. +1
    – DucatiKiller
    Commented Apr 23, 2016 at 22:02

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