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Gearing/lack of power or speed question

ljobbins

NAXJA Forum User
NAXJA Member
Location
SD County
88’ ax15

I just put 33” tires on and swapped an 8.25 rear axle with 4.56 gears and an lsd.
I now have great power and get up but top out at 55mph via a gps. No front driveshaft as the front is still at 3.07 gearing. I expected a loss in mpg jumping to 33” tires but not a loss in top speed. It doesn’t seem as though anything is creating drag other than bigger wider tires. The engine doesn’t feel like it struggles at all, just tops out and whines. I don’t have a tach but I would guess I’m in the 3000 range on the freeway. Is this strictly a gearing issue in the rear axle or am I lacking or losing power in the engine?

Oh and my first two full tanks of gas measured 12-13mpg with 80 percent freeway and 20 percent street without much in the way of hills.

I’m building an 8.8 rear and want to make sure I get the right gears for it.
 
When you say it tops out and whines, are you sure your clutch isn't worn and struggling to maintain friction against the increased weight, resistance of the larger tires?

I ask because with that gearing and tire size, 3000rpm should be around 65mph, and quite doable.
 
Well that was my first guess, but it’s a brand new clutch. It’s a fresh swapped ax15 from my ba10 and a brand new flywheel and clutch and clutch components.

Everything I’ve read and heard and the gear ratio chart seems like 65 mph should be easily doable. But I just got off a long flat highway and fully floored, I was doing 58mph with people flying past me.
 
I looked back at your previous posts and it seems like you have been looking power. Some general reasons why are low compression, leaky valves, stretched timing chain, clogged catalytic converter, inappropriate fuel injectors.... there are lots of reasons that all need to be run down. Did you ever correct your first loss of power issue. Don't minimise the effect of larger tires on wind resistance, it's not just the tires the truck is a lot higher as well.
 
Got power back previously but I can’t remember what the problem was. The timing chain is good, injectors are good, plugs wires and the usual are all good, and new cat and muffler. My only guess engine wise would be low compression due to worn rings. I’ll check that Monday or Tuesday when I can get a compression tester. But I thought I would check what you guys thought in the meantime. I had good compression last year when I checked though.
 
I am running 35's on 4.56 gears. I did 118 and was still climbing. The gearing is not a problem. Bog at top end can be caused by many things such as a clogged CAT, a distributor that is indexed incorrectly, low fuel flow/pressure, bad MAP, even an O2 can have an indirect effect.

My bet would be clogged injectors or carbon. Run a can of BG44K through the fuel and see results. Worst case, you cleaned every thing out. I run a can every 10k miles.
 
I'm running close to same setup but still the stock 3.07s and bog down doing 80 going uphill or with a head wind wind
Running 33"s, on 3.07 gear, is going to be a very unsatisfying experience.
Your really need 4.56 gears, which brings the cruising RPM back to the near stock gearing.
At 65-MPH, my XJ, on 33"s and 4.56s, runs at about 2400-RPM.
 
Running 33"s, on 3.07 gear, is going to be a very unsatisfying experience.
Your really need 4.56 gears, which brings the cruising RPM back to the near stock gearing.
At 65-MPH, my XJ, on 33"s and 4.56s, runs at about 2400-RPM.

I plan on going to 3.73, which is the closest to where it should be to keep the speedo correct without changing the speedo gear. I'm only going to 3.73 to keep road manners and MPG up. Right now when I am doing 80 my RPMs are at 2000k. My speedo says 72. lol
 
I plan on going to 3.73, which is the closest to where it should be to keep the speedo correct without changing the speedo gear. I'm only going to 3.73 to keep road manners and MPG up. Right now when I am doing 80 my RPMs are at 2000k. My speedo says 72. lol

a speedo gear is $30 and one bolt to replace. you want mpg? gear it deeper and replace the speedo. you are wasting your time with 3.73s. i ran 35s with 5.13s and got way better gas mileage letting the motor rev than i did back in my broke college days with stock gearing.

for the mean time, use 4th gear, dont bother with 5th. youll have more throttle on tap, and wont notice a hit in mpg.
 
im running 4.11s on 33x10.50s and have no issues with speed or power. I would deff run some sort of cleaner through it and see if it helps. why such deep gearing on a 5 speed? I geared mine for 35s. If I remember correctly 3.73s will put you close to factory specs with 33s. I hit 2500RPM at 70 in mine.
 
im running 4.11s on 33x10.50s and have no issues with speed or power. I would deff run some sort of cleaner through it and see if it helps. why such deep gearing on a 5 speed? I geared mine for 35s. If I remember correctly 3.73s will put you close to factory specs with 33s. I hit 2500RPM at 70 in mine.

first off... it wasnt a daily.

offroad, the deeper the better with a manual (IMO). more control and lower wheel speed for crawling. with tall gearing, i found myself letting the clutch slip a lot so that the vehicle wasnt taking off like a bat out of hell. the clutch was much happier with deep gearing.

on road... its nice to have the power on tap. minimal highway miles meant the RPMs werent an issue for me. but i would still prefer to be too deep than too shallow. turning high RPMs and actually being able to pull on a hill burns less fuel than putting the pedal to the floor and having the jeep struggle to chug along.
 
im running 4.11s on 33x10.50s and have no issues with speed or power. I would deff run some sort of cleaner through it and see if it helps. why such deep gearing on a 5 speed? I geared mine for 35s. If I remember correctly 3.73s will put you close to factory specs with 33s. I hit 2500RPM at 70 in mine.

This is doing 80 with stock gearing with 33X12.50r15

4jq71j.jpg
 
I really don't miss the 3.07 days. the 4.11s are good to me because I travel interstate everyday to and from work. if it was offroad only id have much higher gears
 
your gears are a tad deep for 33s but you get way more than 55 out of it
 
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