Off Roading Forums banner
1 - 10 of 10 Posts
G

·
Discussion Starter · #1 ·
I have a 1994 YJ Sahara with the 4.0L. It has a 4" Superlift and 33" BFG AT's. I took my Jeep for a little highway driving and noticed that in 5th gear going uphill was very weak, almost no power (almost had to shift into 4th) This wasn't a steep hill either.. I believe the diff gears are stock. I guess the question is do large tires give you less power at the high end? or should I be looking at the engine as the problem?? I'm going to change plugs and wires tomorrow, just wanted to see if there was anything else I can look for?

Steve

Steve

1994 YJ Sahara
 
G

·
Discussion Starter · #2 ·
When you put larger tires on it changes the final drive ratio. You will feel a power lose whenever you put bigger tires on. To get to power back you will have to change the gears in both the front and rear diff.

Tim Springer
1980 CJ7
 

·
Registered
Joined
·
6,394 Posts
/wwwthreads_images/icons/tongue.gif Technically, Steve, what you would do is get the turns/per/mile of the old tires....the turns/per/mile of the new ones, and multiply each one by the existing axle ratios. What that will give you is RPM at 60 MPH (Old) and RPM at 60 MPH (New). Then you can solve for the new ratio as you try to get back to the original RPM at 60 MPH. Remember, all the engine knows is RPM @ 60...it doesn't know how you got there.....be it tires or gears.....but in any case it wants to be back to that RPM again. /wwwthreads_images/icons/crazy.gifYour engine is telling you that you owe him a gear change./wwwthreads_images/icons/tongue.gif

CJDave
 
G

·
Discussion Starter · #4 ·
/wwwthreads_images/icons/smile.gif Just Empty Every Pocket /wwwthreads_images/icons/smile.gif
How true it is... but seeing how my Jeep is the only thing I actually enjoy spending money on, I will end up getting the diff gears swapped. I knew that the bigger tires would mean some power loss, but I had no idea how much. /wwwthreads_images/icons/frown.gif Where can I find out what a stock '94 YJ Sahara consists of? (diff gears, axles, wheelbase, and other misc. tech data)? I believe my Jeep has D30 in the front, D35C in the rear... Did you figure out turns/mile by measuring the circumference of the tire in feet and dividing by 5280 (the number of feet in a mile)? is this the best way to calculate the new gear ratio?

Oh yeah, one last question, if I wait a few weeks, is there a chance of hurting anything by driving like this?

Steve

1994 YJ Sahara
 
G

·
Discussion Starter · #5 ·
Chances are you won't do any damage, unless you are really having to slip your clutch to get going. I believe there should be a tag on the diffs, it will have a letter, number, or both (not sure on YJs). Your dealer can tell you what gears you have with that code, or, I think there is a place on Off-Road.com that tells you what the codes are.

Brad (from the 4 Wheeling center of the universe, 4 corners USA)
 

·
Registered
Joined
·
6,394 Posts
/wwwthreads_images/icons/tongue.gif The most accurate way to measure is to run the Jeep...park on good surface....measure from the top edge of the rim to the ground....then subtract 1/2 the rim outside dia....then double that number for diamenter....then multiply by 3.1416 to get inches of ROLLING circumference.....then divide by 12....then divide 5280 by that number to get turns per mile. OR...go to a tire shop and look in the book./wwwthreads_images/icons/crazy.gif

CJDave
 
G

·
Discussion Starter · #7 ·
hey if you run a lot of street and no off road try around 4.27 if you wheel maybe 4.56 or deeper, you can get away with ratios that sound really deep because you have an overdrive tranny to compensate during highway driving.

 
G

·
Discussion Starter · #8 ·
I believe the formula for figuring axle ratio needed is, 336 x tranny final drive ratio, x gear ratio in axles, divided by tire diameter equals rpm.
336 x 1 x 4.10 divided by 31 = 2500 rpm (just an example) If you know what rpm you were running with the stock tires, and what your final drive ratio is on your auto, .73 or .69 if you have OD, and work the numbers. If you need help on that e-mail me. [email protected] Steve

Mopar360yj
87yj+20,74CHR.030360,SOA,35BFGATs,46RH,241DHD,
D60-44-4.10
 
G

·
Discussion Starter · #10 ·
I had the same problem with my 94 wrangler. It and most 4.0's have 3.73 gear ratio. I changed to 4.10 and it drives like new. If your going to stay with 33's and do a lot of highway driving I think that ratio is best.

P.S.
Buy good gears like Dana Spicer. I've seen cheeper stuff that are full of mfg flaws.

Good Luck
Jason

 
1 - 10 of 10 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top