Will downsizing tires give me better responsiveness?

OC TJ

OC TJ

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Jan 24, 2019
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Southern California
Hi all,

I recently purchased a 99 Wrangler and have been having the best time with it. My question is pretty general, please help so i know which direction to go.

My Jeep has a set of 285 70 R17 tires that the previous owner has installed with a 4 in lift, plenty of tread left on it. Being a four cylinder the engine feels very weak and struggles on hills/highway.
My question is will downsizing to say a 235 75 R17 give it a noticeable change in responsiveness? My goal is to put less stress on the engine and transmission as possible so that it will last as long as possible.
Downsizing I would go to a 2 in lift as well.
If it matters the TJ has a manual transmission.

Thanks in advance.
 
Two options: Smaller tires or re-gear. Re-gearing will make a huge difference although it will never be a powerhouse. Personally, 31's are the tallest I would go on stock gearing. And even then, I feel it is under-geared.
 
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As others have mentioned, the 4 cylinder will always feel like it's lacking something.

I had the 2.5 for 6 years before moving up to the 4.0. I ran 31's and it still suffered on the highway/hills (hills mostly).

The only way you can improve the pep is by regearing to a taller ratio.
 
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And will these large tires shorten the working life of this engine/transmission because of it being worked too hard? Just wondering because I am looking long term.
 
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To @OC TJ:

Your four banger 5-speed most likely has 4.10:1 axle gear ratios and came from the factory with either 29" or 30" diameter tires. Your current tires have a 32.7" diameter, substantially larger, which means that you are severely undergeared for those tires. It is no wonder your jeep can't get out of its own way.

You can do two things to restore performance: (1) re-gear to 4.56 or 4.88 gears - $$$$$; or (2) buy smaller diameter tires -$$.

Since you are new to jeeping I would suggest smaller tires for now. You can spend the big bucks later. 31" tires would be my choice for a 4 cylinder 5-speed jeep with 4.10 gears.
 
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ShredAZ said:
As others have mentioned, the 4 cylinder will always feel like it's lacking something.

I had the 2.5 for 6 years before moving up to the 4.0. I ran 31's and it still suffered on the highway/hills (hills mostly).

The only way you can improve the pep is by regearing to a taller lower ratio.
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Lower gearing is what you want, and I am sure you meant.
 
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OC TJ said:
And will these large tires shorten the working life of this engine/transmission because of it being worked too hard? Just wondering because I am looking long term.
Click to expand...
Not at all. You'll just find yourself cruising in 4th gear on the freeway to keep up with traffic. My RPMs would constantly hang around 3000 - 3500. My Jeep was stolen with 190k on it, but it still ran like a top.

The largest diameter tire I ran was 32". There was a negligible difference between 31" and 32". The 2.5 runs perfectly fine offroad especially in 4lo.

If I were you, I'd keep the tires and start saving for a regear. A proper regear will restore the stock feeling. Just remember - even a stock 2.5l on the freeway lacks.
 
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OC TJ said:
OK from your experience did the large tires and 4 cylinder cause any issues with steep hills off road?
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Mine came with 31's. Even with the more powerful 2.4 and lower geared NSG370 it was lacking. Now with 4.88's it does great. Still no monster but significantly better.
 
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If the tires are new(they have date codes) and the lift is done right with good parts, then you are in a wonderful position to be able to regear. I would add a aussielocker up front, and regear to lower(numerically higher) gearing. Cost would be the same as purchasing 4 new 33x12.50-15 or shorter tires to improve performance.

I just bought a 2002 4.0l 5 speed manual, perfect running condition, and it has 3.08 gears. I added 31x10.50-15 tires and it's horrible, 5th gear is unusable. So your 4 banger can't be that bad.....

The best performance mod you can do to a TJ is regear. You could sell your 4:10 gears to someone with a 6 cylinder in order to recoup some of the expense of regearing, and you are on the right side of the carrier(no need to purchase/replace carriers).

Might as well add a diff locker or a limited slip while you are in there, and do it once.
 
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OC TJ said:
And will these large tires shorten the working life of this engine/transmission because of it being worked too hard? Just wondering because I am looking long term.
Click to expand...
The short answer is yes and this is why.
A couple weeks ago my machinist explained it to me like this..

With "too large" tires and improper gearing, the 4 cylinder constantly struggles and strains. This constant strain on the motor means combustion temperatures are higher than normal. Don't confuse this with engine coolant temperature which can always show to be normal.
These higher than normal combustion temps over time can cause a 'tuliped valve' which actually causes the valve to recess somewhat into the valve seat. This can cause a random misfire and is only fixed by replacing the tuliped valve with new.
 
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It's a regear in reverse ...same end result too.

I drove a jeep recently with 31" tires vs my 33" tires , and it would scoot .
 
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