31" or 30" tires?

Rfreter

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I have had my 1994 wrangler for 6 months and its time to replace the 31/10.5 good years that are dry rotted.
My yj is a soft top 4 cly. manual tranny. Has some trouble making it up the hill in 5th, 4th gear keeps us at about 50 mph.

question would I really notice a difference between 30/9.5r15 & 31/10.5r15 General grabbers ?

actual tire specs say both are true to size so were talking an 1 inch in diameter & width the 30 weighs 7 lbs less?
Considering here in MD we don't have anything close to do true off roading. I like the tight fit of the 31s, but if I am going to see some performance I can go smaller.

Thoughts?

Robert
 
Good question. All I can do is second your experience with the 31's. I just rebuilt my '92 2.5L with 31's and it's an old dog in 5th gear. The last few weeks I've just been driving it on grocery trips at 50mph. Yesterday I went of a 50 mile road trip and for the first time got it up to 60mph going down hill. Going up I had to shift into 4th. Smooth ride though and around town I'm estimating almost 19mpg.
 
Your tires are 4" larger than stock and the 30's you're contemplating will still be significantly worse performance than stock was. You're better off keeping the 31" size and regearing the axles to 4.88. Or, 33's and 5.13 if you have any desire to go to 33's.
 
Tire size and weight make a substantial difference in performance. I always, always pay attention to tire size and weight when I am shopping for tires.

That being said, what you really need to do is a re-gear for your axles. That's going to give you the performance you are currently lacking. Of course the 4 cylinder is anemic to begin with, so even re-geared, it's still never going to be a good highway vehicle by any stretch of the imagination.
 

Here is a good gear calculator. I went with 4.56 for my 2.5L with 31s and I'm happy. I live on the North Carolina coast so the closest I come to a hill is going over a bridge. I can run at 70 on the HWY at around 3,000 RPM.

If you need the information to setup the form the stock YJ 2.5L with a 5 speed came an AX5 transmission with a NP231 transfer case and 27 inch tires. I am happy with my choice but if I had it to do again I would go with 4.88 for a little more pep. None of the speedometer gear charts I've seen have a gear for 4.88s on 31s so your speedo might be off when you are done.
 

Here is a good gear calculator. I went with 4.56 for my 2.5L with 31s and I'm happy. I live on the North Carolina coast so the closest I come to a hill is going over a bridge. I can run at 70 on the HWY at around 3,000 RPM.

If you need the information to setup the form the stock YJ 2.5L with a 5 speed came an AX5 transmission with a NP231 transfer case and 27 inch tires. I am happy with my choice but if I had it to do again I would go with 4.88 for a little more pep. None of the speedometer gear charts I've seen have a gear for 4.88s on 31s so your speedo might be off when you are done.
For some reason the charts never show it but the math works out to be good enough that 43 tooth is proper for 31’s and 4.88. It’s a very similar setup to 5.13/33.

I’ve tested a bunch of gears and it seems that typically, I’ve found each speedo gear tooth to make a difference of about 3 mph. So typically you can end up right on the money, or 1 high or 2 mph low. By the time you reach 3 mph off you go to the next gear. So there’s pretty much always a gear you can use to be really close, unless of course you go so low that you’re off the chart (44+ tooth). At that point, it’s time to wire in a converter module to correct the electrical input the Speedo sees from the sensor, which can be calibrated to work perfectly regardless of what gear you install.

If running a 91 or earlier Jeep with a cable speedo, time to buy an electric or GPS speedo, or give up on being perfectly calibrated lol.