Tires, Gears and RPMs

[TJK]

First of all, let me just say a huge THANK YOU to Chris and all the members that have made this forum the incredible resource that it is. I have learned more in these last few months of research through this forum than anywhere else hands down and now I can say that I am a proud new owner of a beautiful TJ of my own.

I know these topics have been posted about a lot. (I think I've read em all by now hah) but I hoped I could get some input on what tires to put on my TJ now that I'll be replacing them.

Some background: It's a 97 Sport; 4.0 litre; 32RH 3-speed automatic; Gears are 3.07 (I know, I know); Dana 35 rear. VIN reads the stock tires were 225-75R15 which is what matches the spare (though the door jamb states 205....Yes the door jamb VIN matches the engine stamp VIN and it's definitely a Sport not an SE...weird...maybe this is a topic of its own...) But pretty much everything is as close to stock as they come.

I don't plan on any intense wheeling. The roughest terrain I'd hit would be Forest Service Roads up in the BC mountains.

I'd like to put bigger--and all terrain--tires on it (currently sitting on 235) I've read it's possible to go up to 31's on stock suspension but I don't want to lug my engine. I've read some differing opinions on healthy RPM range on here.

What I want to know is, what's the ideal tire size for my gearing and set up?

Any advice?

 

[Fishtaco]

Dont sweat it! 3.07,s are not as bad with 31,s as everyone without 3.07 would lead you to believe!

 

[JMT]

http://www.grimmjeeper.com/gears.html
On this site you can compare your current gears and rpm’s with other gear ratios. The 4.0 likes to be over 2500rpm at 70mph.

 

[TJK]

So I've used the Gear Ratio calculator and it puts me at 2329 RPM at 70mph in 3rd gear on 31" tires, in contrast 30" sit at 2407 RPM while 235 tires sit right at 2500 RPM. I just don't know if the bigger tires are worth the loss for the engine or if that RPM range is close enough to make it worthwhile...

Apologies if this thought process is super green, first time car owner over here

 

[Fishtaco]

Here's one of your countrymen driving 33,s on 3.07,s, he does have a 5 speed though so might make a difference.

Also some TJ,s showed 205 215 and 225 as tire size on the plate, mine does anyway and apparently the Rubicons showed 31.s?

Take it up some big hills and see how it runs with the 235,s you have on it now and you may be unhappy with how it drives even without the 31 tire change you want or you may be fine with it?

I am running 29,s and to me its a perfect size unless you go 2" or above lift then it looks odd.

 

[TheBoogieman]

I wouldn't waste the money for a regear for your intended use and 31" tires. I had 3.07s with a 32rh and never had any issues with it. It's still better than a 4 cylinder with 4.10s and 31s. I also swapped in some 4 cylinder Jeep axles with 4.10s on another Jeep that had 3.07s and 33s. Much cheaper than a regear.

p.s. 31s won't lug the engine.

 

[97' 4 Popper]

If you want to run 30s or 31s the recommendation for that tire size and your transmission is a axle gear ratio of 3.73. With 3.07 gears it would be liveable but not getting as much power as it could. I would consider trying to run with your 28's for a while if the terrain you will be on is relatively flat or forgiving. Then go to 30's or 31's and see how you feel. I once had 32.5s with the 32rh on 3.21s it was a slug.

Also definitely compare rpms on the jeep calculator/charts and see how you feel.

 

[TJK]

The 235's on it now feel peppy enough. Gotta take it up so

Here's one of your countrymen driving 33,s on 3.07,s, he does have a 5 speed though so might make a difference.

Also some TJ,s showed 205 215 and 225 as tire size on the plate, mine does anyway and apparently the Rubicons showed 31.s?

Take it up some big hills and see how it runs with the 235,s you have on it now and you may be unhappy with how it drives even without the 31 tire change you want or you may be fine with it?
I am running 29,s and to me its a perfect size unless you go 2" or above lift then it looks odd.

Thanks fish that's sound advice. The 235's on it now feel pretty peppy. I'll take it out to some steeper hills and really try to push it before I commit on a tire size. I guess my instinct is to just go bigger, I assume the added traction and clearance are clear benefits, though I don't really know if they'll be necessary...

 

[TJK]

If you want to run 30s or 31s the recommendation for that tire size and your transmission is a axle gear ratio of 3.73. With 3.07 gears it would be liveable but not getting as much power as it could. I would consider trying to run with your 28's for a while if the terrain you will be on is relatively flat or forgiving. Then go to 30's or 31's and see how you feel. I once had 32.5s with the 32rh on 3.21s it was a slug.

Also definitely compare rpms on the jeep calculator/charts and see how you feel.

 

[TJK]

 

[Fishtaco]

The 235's on it now feel peppy enough. Gotta take it up so

Thanks fish that's sound advice. The 235's on it now feel pretty peppy. I'll take it out to some steeper hills and really try to push it before I commit on a tire size. I guess my instinct is to just go bigger, I assume the added traction and clearance are clear benefits, though I don't really know if they'll be necessary...

I bought new tires 6 months ago and was under the impression that 31,s did not fit without a lift but have since found out they would have fit anyway if I knew they fitted I would have gone 31,s but given I already spent I am not going to scrap brand new tires for the sake of an inch higher! My Falken AT3w,s are actually a bit bigger than my wranglers were for some reason? 235/75R/15 Falkens measure 29" and 31,s are not actually 31" anyway they are smaller so less than an inch of lift over the 235,s.
The Falken wildpeak AT3W,s are a great tire and fairly aggressive looking for an All Terrain but if I went back 6 months I would have definitely bought the Wilpeaks in a 31 not 29!
When I enquired about 31,s my tire fitter said I would loose the warranty because 235 was the biggest tire he was allowed to fit on the TJ by his dealer book (big chain store)
If you go 31,s I don't think you would notice that much difference to the 235,s with the 3.07,s? I have 3.07,s and cant for the life of me see anything wrong with the gearing but I don't have too many hills where I live.

 

[Fishtaco]

So OK I see a TJ parked at work today and it had 31,s X 10.5 and personally it looked a bit "Not happening" with its 2" lift", tires were not matching either, but the tire size on the ecco 7.5 inch just wasn't doing it for me! they looked narrower than the 29,s I have?
I would suggest looking at a few TJ,s on 31,s because I am now thinking wheels should be at least 8" not stock 7.5" and 31,s need to be wider than 10.5 if you are going for aesthetics 32x 12.5 seems the go?

 

[97' 4 Popper]

So OK I see a TJ parked at work today and it had 31,s X 10.5 and personally it looked a bit "Not happening" with its 2" lift", tires were not matching either, but the tire size on the ecco 7.5 inch just wasn't doing it for me! they looked narrower than the 29,s I have?
I would suggest looking at a few TJ,s on 31,s because I am now thinking wheels should be at least 8" not stock 7.5" and 31,s need to be wider than 10.5 if you are going for aesthetics 32x 12.5 seems the go?

Maybe the lift threw you off, with a 2 inch lift yea 32s would look better from an aesthetics point of view but would suck without a regear. Just thinking about cosmetics a 1.25 inch front lift with rear stock and/or a 1.25 inch body lift would probably be the best for 31s. 15x8s is what I'm running with my 31x10.5s, backspacing is 3.75, they stick out quite a bit but that's what I wanted. I believe 15x7s work fine even with a backspacing of 5.5.

 

[Cornbread]

I doubt you notice much difference going to 31's. I ran 31's for a week when I bought my JK wheels/tires before reskinning to 35's and I didn't notice a power loss. I'm going on year 5 of running 35's on your same transmission/gear ratio and still do not "hate my life". I have been around the 4 bangers and mine still feels like a powerhouse compared to the 4 cylinder, trust me. The biggest annoyance for me is that I'm constantly riding the brake on trails (BTW I run 4low for 90% of where I wheel). We don't have rock crawling here but have technical forest trails. I want to regear mostly to have consistency and precision on the trails, not that I'm lacking power. With the higher gear ratio, engine braking on steep downhill descents is not slow enough for my liking.

 

[TJK]

So ya I’m definitely not planning on regearing, I just want the most power without dinky little tires. Not planning on a lift or even new wheels at the moment either. I’m leaning toward 30’s at this point. The 31’s would be nice but again, I just don’t know if it’s worth the power loss or the risk of rubbing.

 

[BugoutJeep]

Get quality brakes no matter which size you go. I ran the OEM sized tire for awhile and my braking was still the worst of any modernish vehicle I've owned. Got the Black magic pads and centric premium rotors and it handles my 33"' KM3s no problem.

It does matter a little bit about which tires you pick. I had 29s that were bias ply mud tires, if you want an extreme example and though awesome off-road, I couldn't do anything on the highway. And I'd lose about a 1/4" of tread every highway trip as the tires would try round off the flat spots from sitting overnight.

For generic easy off-roading, I've always preferred 31s to the smaller stuff and don't mind it compared to the bigger tires, but I've ran a bunch of different tires, experimenting before doing a regear. The looks really throw me off on the OEM sized tires though most 31s aren't 31" in diameter anyway. My preference are skinny and tall tires and I've blown several side walls doing nothing difficult, so I do try to get a tire with a good side-wall. My TJ came equipped with 33x12.5s, which I liked the look and the Jeep performed pretty well with them on the road, but the tire seemed to float off-road, though mostly bald at that point.

I'm geared for 33s now, but I'm still thinking of getting a summer tire that's a 31 for a bit more towing and mostly for just running around town in. But 31 vs 30 I'd just go for a good quality tire and I doubt you'll tell much difference.