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30's and 3.55

jls095

NAXJA Forum User
Location
MA
Looking to upgrade to 30x9.5x15 over 225/75/15 since I just installed my lift. I'm not too concerned about gas mileage, but what's the performance impact?

Some of my highway driving is on hills and it would be a huge PITA to have downshifting constantly. Additionally I don't really want to throw too much load on the engine when accelerating, not what I'm looking to do.

I know that it's really only ~1.3 inches of height, but I've read some stories of people on here and other forums with 31s having notably decreased performance.
 
You will be fine. The only time I noticed any decreased performance with 31's was going through Colorado on the way to Moab.

JIM.
 
I was working in Honesdale PA when I changed tire size from 225X70X15 to 30X9.5X15 and the impact was immidate. Driving the hills between Honesdale and Scranton, the AW4 would hunt comtinuosly for the correct gear goinng between 4 OD down to 3 then back to 4 then to 4 OD then back down to 3. After awhile, I just accept it and selected 3 when it down shifted. Of course, using the cruise control amplified the perceived problem. Also, slowing or stopping going down the hills required a bit more leg effort. I also lost some gas mileage,...more when driving the hilly country not so much on flat highways. By-the-way, my XJ hates 3,000+ RPM,...or maybe it is the other way I am not sure but the driving with the gear lever in third causes the gas guage make a 'Bee-line for the "E" marker'.

I learned to live with it. Many on this forum say the combination is perfect. To me, it was/is doable but I hate it. Good or bad depends on your driving style, roadway you drive on, load the XJ is carrying and how sensetive your butt dyno is.
 
I was getting 16-17mpg with 31's & 3.55's. Never really noticed it having to downshift much to go up hills on the highway. It definitely has to downshift more now that I have 33's with the stock 3.55's. It's not so bad that I want to switch back though...yet. I'd say you'd be perfectly fine with 30's and 3.55's
 
that is the combo i have now, i think it would be perfect with 3.73 gears though, but those are not worth the trouble to put in or find...
i have been on 30s with 3.55 gears for 5 years now... looks like i need another year for the tires to wear out still.
 
Go 4.88's if possible.

JIM.

I was leaning strongly towards 4.88 but got a deal on built axles w 4.56

I'm not going past 33" tires and it's a dd so it should be cool I think.

But 4.88 would be more fun for shure and had I bought new that's prolly where my $ wouldv'e went
 
I noticed a difference when I put 235/75 duratracs on my 99. I won't go any bigger until I regear, but I also live at altitude (5000 feet), and I'm in the mountains frequently.
 
Mine has a 31"/3.55 combo; it hasn't been bad at all for me performance-wise, not enough to bother me anyways. I have honestly still gotten over 21mpg (mostly highway of course) with mine too. Around town, it's more like 14 or 15 now.
 
I hated 3.55's and 30's. My mileage went from a steady 19mpg down to 14-15. Power loss was noticeable. The engine was constantly bogging and it sucked on the freeway unless you really had your speed up.

I hated it so much that i dropped a grand to get 4.56's. It's a slippery slope!
 
I hated 3.55's and 30's. My mileage went from a steady 19mpg down to 14-15. Power loss was noticeable. The engine was constantly bogging and it sucked on the freeway unless you really had your speed up.

I hated it so much that i dropped a grand to get 4.56's. It's a slippery slope!

OH! Come-on now! You are too sensetive. What, with the XJ and a 4.0 engine?
:nono:
 
I put 30x9.50s BFGs from 225x75 Mastercrafts about 4 years ago.('00, 4.0, AW-4) 'Didn't notice any mileage change.(about 19.5 before and after) I did notice the transmission hunting a lot(enough to bother me).
About 2 years ago, I had a rear pinion bearing fail. Since it had to come apart, and I had a "low" D-30 truetrac in the garage, I went ahead and re-geared both axles to 3.73.
Transmission doesn't hunt any more(It's now exactly like stock - including the speedo reading) I'm really happy with it.


Having said that: It's a good combo IF that's as big as you're going to go. As expensive as gears are, if you have to re-gear and there's ANY possibility you're going to put bigger tires on, put at least 4.10s in.
 
I hated 3.55's and 30's. My mileage went from a steady 19mpg down to 14-15. Power loss was noticeable. The engine was constantly bogging and it sucked on the freeway unless you really had your speed up.

I hated it so much that i dropped a grand to get 4.56's. It's a slippery slope!
I went from 225/75 to 235/75 to 30x9.5 MTs and saw a very small drop in performace with mpgs going from 20 with 225 or 235s down to 19 with 30s. I'd run it and be happy. This was in hilly territory, and going down big hills, I could put it in 3 and coast down hill and not get above 55.
 
Tons of good information here. Thanks a lot for the input. I guess it's one of those things that varies with the person.
 
Looking to upgrade to 30x9.5x15 over 225/75/15 since I just installed my lift. I'm not too concerned about gas mileage, but what's the performance impact?

Some of my highway driving is on hills and it would be a huge PITA to have downshifting constantly. Additionally I don't really want to throw too much load on the engine when accelerating, not what I'm looking to do.

I know that it's really only ~1.3 inches of height, but I've read some stories of people on here and other forums with 31s having notably decreased performance.

Transmission? Gears?

These were RENIX rigs, but I've driven with 30's and 31's - and much of my driving was on-road, with plenty of freeway and mountain time.

With the AW4 and stock 3.55 gearing, throw on 30's or 31's and forget about it - I honestly think that either of those larger sizes actually work out /better/ than the OEM tyre size (of about 28", as I recall.)

If you have a manual (BA-10 or AX-15) and still have the stock 3.07 gearing - which, y'ask me, was a mismatch and mistake! - then the fix until you regear is simple. Stop using fifth gear. Trust me on this one - my 88 perked up considerably and mileage improved noticeably when I stopped using fifth gear (both with the OEM BA-10/5 and replacement AX-15.) The improvement was noticeable on the OEM tyre size, 30" tyres, and 31" tyres.

If you go larger than 31" with any setup, start thinking about regearing. Throw axles from a 6-242 auto under, and you go from 3.07 (a horrible idea) to 3.55 (which will complement 32" or 33" tyres, unless you get silly.) 4.10 gearing can be found under some four-cylinder rigs, and you can also turn up 3.73 under some 6-242 (AW4 with tow package, I think,) and there /may/ have been some 4.55 out there (four-cylinder with tow package, I think. Early years only. Don't hold me to it.)

Swapping an axle in good condition outright is easier than regearing, just inspect it will and install new seals and wheel bearings out of hand.

If you change rear axle models (the D35 and ChryCo 8.25 were common, the D44 was available in 6-242/AW4/tow package as an uncommon option through about 1988/1989,) then make sure to grab the rear driveshaft as well - the pinions are different lengths. Therefore, the driveshaft length changed (the D35 has the shortest pinon length from the tube centreline, the D44 has the longest, as I recall.) The splined output shaft should be the same in all cases. You /may/ need to adapt the handbrake cable as well - I think it's the same until it splits to the wheels, and grabbing the cable assembly (or ordering it new) shouldn't go amiss.

Good luck!
 
Transmission? Gears?

These were RENIX rigs, but I've driven with 30's and 31's - and much of my driving was on-road, with plenty of freeway and mountain time.

With the AW4 and stock 3.55 gearing, throw on 30's or 31's and forget about it - I honestly think that either of those larger sizes actually work out /better/ than the OEM tyre size (of about 28", as I recall.)

If you have a manual (BA-10 or AX-15) and still have the stock 3.07 gearing - which, y'ask me, was a mismatch and mistake! - then the fix until you regear is simple. Stop using fifth gear. Trust me on this one - my 88 perked up considerably and mileage improved noticeably when I stopped using fifth gear (both with the OEM BA-10/5 and replacement AX-15.) The improvement was noticeable on the OEM tyre size, 30" tyres, and 31" tyres.

If you go larger than 31" with any setup, start thinking about regearing. Throw axles from a 6-242 auto under, and you go from 3.07 (a horrible idea) to 3.55 (which will complement 32" or 33" tyres, unless you get silly.) 4.10 gearing can be found under some four-cylinder rigs, and you can also turn up 3.73 under some 6-242 (AW4 with tow package, I think,) and there /may/ have been some 4.55 out there (four-cylinder with tow package, I think. Early years only. Don't hold me to it.)

Swapping an axle in good condition outright is easier than regearing, just inspect it will and install new seals and wheel bearings out of hand.

If you change rear axle models (the D35 and ChryCo 8.25 were common, the D44 was available in 6-242/AW4/tow package as an uncommon option through about 1988/1989,) then make sure to grab the rear driveshaft as well - the pinions are different lengths. Therefore, the driveshaft length changed (the D35 has the shortest pinon length from the tube centreline, the D44 has the longest, as I recall.) The splined output shaft should be the same in all cases. You /may/ need to adapt the handbrake cable as well - I think it's the same until it splits to the wheels, and grabbing the cable assembly (or ordering it new) shouldn't go amiss.

Good luck!


Should have mentioned it earlier...2000 with AW-4 and 3.55's. Right now it's got a 2" lift. Why do you think 30 or 31's work out better than 225/75/15's (28's)?
 
Little follow up...purchased 5 perfect Ravines with new 30's on them for $400 on Craigslist today. Won't have them on until I get home for Thanksgiving!
 
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