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Decisions, Decisions, what about roadability?

R

rstarch345

NAXJA Forum User
Location
Where ever I can wheel
OK guys, time for me to ask an EXPERIENCE type of question.

2000 XJ, 3" spring lift, SYE and a crap load of other mods.

I am currently running 3.55 gears with stock tires when running on the street and 31x10.50 tires when wheelin. I swap back and forth as necessary.


I stay with stock for economy and towing. When wheelin, I have not run into a situation where the 3.55 gears were a problem with 31x10.50 tires.

Presently I’m changing the front axle to a new one that already has 4.10 gears. Now, the optimum gear ratio for the 31x10.50's are 3.90 gears and could still go with 32’s if necessary with the same gears.

I could also go with the 4.10 gears and with 31 inch tires and my mileage will suffer but will have abundant power for towing.

This is the question; If you are using 31 or 32 inch tires with 4.10 gears, how bad has your street performance been? Keep in mind, I wheel about 10 hours a month. I tow about 8 hours a month. The rest of the time is the street.

I’m trying to decide if to stay with 3.55 gears and keep switching tires, balance everything out and change front & rear gears to 3.90 or up just the rear gears to 4.10 and see what the penalty is.


I am also aware about the speedo differences when mucking with gears and tire size.

Now here this. FORGET ABOUT COST of the swap. I don’t want to hear anything about cost as the gear cost is NOT an issue.

For those who don’t read posts, FORGET ABOUT COST.
 
Have you actually found 3.90 gears for both axles? Just go to 4.10s for the 31s, calculations that indicate a certain gear as optimal do not take the added weight of the tires and wind resistance into account.
 
Yucca-Man said:
Have you actually found 3.90 gears for both axles? Just go to 4.10s for the 31s, calculations that indicate a certain gear as optimal do not take the added weight of the tires and wind resistance into account.
Click to expand...
Thanks Yucca-Man. I respect your opinion. I've read a couple hundred of your posts.

With all I've read (and this isn't the only site) I'm still concerned about 4.10's on the street. And for all that are reading, I have considered weight, tire size and weight/resistance and airodynamics.
 
I ran 4.10s on the street with tires that small, and the mileage difference was not an issue with 31s,

go 4.10 in the rear for several reasons
you still never have to worry about changing tires every weekend,
you'll appreciate the additional torque
you could run 33s if you chose, though towing would suffer.
it will be out of the shop quicker
its a much more readily available ratio if something should break.
 
I'm on 31s now and hope to gear to 4.56 by the end of the summer.

YMMV.
 
rstarch345 said:
Thank you. I did read that thread, but it did not answer real world results with street performance.
Andy 'Yes', I have access to 3.90 gears.
Click to expand...


Hmmm......who is your supplier of 3.90 gears for a short pinion 30?

Curious.

Why do you believe your mileage will suffer with 4.10's? I run 3100 RPMs for hours at a time with a 4.6 liter stroker and 35" tires. I get darned near 17 mpg........my wifes 1999 gets 20 with 27" tires an auto and 3.55's.

I bet your mileage improves with 4.10's.......
 
CRASH said:
Hmmm......who is your supplier of 3.90 gears for a short pinion 30?
Click to expand...
I didn't say what I was replacing my stock D30 with. And the gears would be custom out of Canada. I'm owed a favor.


CRASH said:
Why do you believe your mileage will suffer with 4.10's? I run 3100 RPMs for hours at a time with a 4.6 liter stroker and 35" tires. I get darned near 17 mpg........my wifes 1999 gets 20 with 27" tires an auto and 3.55's.
Click to expand...
Two different engines. I presently get 22 MPG running 2100 RPM (less abusive on the engine). 16 to 17 when towing. And, I congratulate you on your MPG with the stroker.

P.S. Some of the MPG comes from using a Blazer air dam up front which I take off when wheelin.
 
Last edited:
just regear the rear to 4.10 and all will be well, even with your stock tires it will be fine
 
You seem to assume that good milege is somehow connected with low RPM's, why is that?

CRASH

rstarch345 said:
OK guys, time for me to ask an EXPERIENCE type of question.

2000 XJ, 3" spring lift, SYE and a crap load of other mods.

I am currently running 3.55 gears with stock tires when running on the street and 31x10.50 tires when wheelin. I swap back and forth as necessary.


I stay with stock for economy and towing. When wheelin, I have not run into a situation where the 3.55 gears were a problem with 31x10.50 tires.

Presently I’m changing the front axle to a new one that already has 4.10 gears. Now, the optimum gear ratio for the 31x10.50's are 3.90 gears and could still go with 32’s if necessary with the same gears.

I could also go with the 4.10 gears and with 31 inch tires and my mileage will suffer but will have abundant power for towing.

This is the question; If you are using 31 or 32 inch tires with 4.10 gears, how bad has your street performance been? Keep in mind, I wheel about 10 hours a month. I tow about 8 hours a month. The rest of the time is the street.

I’m trying to decide if to stay with 3.55 gears and keep switching tires, balance everything out and change front & rear gears to 3.90 or up just the rear gears to 4.10 and see what the penalty is.


I am also aware about the speedo differences when mucking with gears and tire size.

Now here this. FORGET ABOUT COST of the swap. I don’t want to hear anything about cost as the gear cost is NOT an issue.

For those who don’t read posts, FORGET ABOUT COST.
Click to expand...
 
Impressive connection.

It took 3 years and minimum order of 1000 units to get TruHi9 gears sets made. To get one set of 3.90 gears made to your spec is a heck of a deal, and must have been a heck of a favor!

CRASH

rstarch345 said:
I didn't say what I was replacing my stock D30 with. And the gears would be custom out of Canada. I'm owed a favor.



Two different engines. I presently get 22 MPG running 2100 RPM (less abusive on the engine). 16 to 17 when towing. And, I congratulate you on your MPG with the stroker.

P.S. Some of the MPG comes from using a Blazer air dam up front which I take off when wheelin.
Click to expand...
 
I was always told that if you do any kind of wheeling and use anything over 225/75/15s you will thank the gods for 4.10s. How would (in the real world) 3.90s be any better then 4.10s?

Now I dont know much of anything but in the real world 4.10s will not hurt you at all unless your running 205/70s and do all highway driving. Am I wrong?...andy
 
et89xj said:
I was always told that if you do any kind of wheeling and use anything over 225/75/15s you will thank the gods for 4.10s. How would (in the real world) 3.90s be any better then 4.10s?

Now I dont know much of anything but in the real world 4.10s will not hurt you at all unless your running 205/70s and do all highway driving. Am I wrong?...andy
Click to expand...

Even with 205/70 and all highway use, the 4.10s would be useful, and dare I say, Fun.

I may be going with 30" or 31" this season... 3.55:1 will do for awhile, but I have 4.56:1 on deck. I have ran 31" with 4.56:1/4.0L/AW4 for a few thousand miles before... if anything, MPGs improved with a few more revs.

If money is no object, experimenting with different gear ratios would be interesting...possibly even fun... but the odd-sizes (3.73, 3.92) would have MUCH less resale potential than a set of 4.10 or 4.56.
 
I think in the first post he said that he is getting a front axle with 4.10s in it anyways. SO why waist time, money(I know not a big deal: BUT) and brain power doing anything else but 4.10s the rear axle? I just dont get it, but I'm still learning even after 4yrs working with XJs...andy
 
What are you towing, and what speed do you normally "cruise" at while towing?


Do you prefer to tow in OD or D?
 
RKBA said:
What are you towing, and what speed do you normally "cruise" at while towing?


Do you prefer to tow in OD or D?
Click to expand...
I tow either a 2700 lb boat or a 3400 lb horse trailer which includes two horses. I probably average 65 on flats and maybe 50 on inclines. When I'm on the pike I do OD with an ocassional drop to 3rd. In case you were wondering I have air springs in the rear. I would expect 4.10 to improve the towing.
 
rstarch345 said:
I tow either a 2700 lb boat or a 3400 lb horse trailer which includes two horses. I probably average 65 on flats and maybe 50 on inclines. When I'm on the pike I do OD with an ocassional drop to 3rd. In case you were wondering I have air springs in the rear. I would expect 4.10 to improve the towing.
Click to expand...

Looking at the calculations for your RPM/speed at the different ratios, I would select 4.10s for your towing set-up IF you are staying with 31s. Any larger tires and 4.56s should be on the list
 
A friend I wheel with and see on a regular basis has 31s with 4" lift and 4.56 gears. He drives it every where he goes, it is his DD. He never swaps to stock sized tires mind you, but he has no complaints about the setup. It climbs and wheels real good, does fine on the street and he gets better mileage than I do with 30's and 3.55s.

Stick with the 4.10s

P.S; I apologize for the resurrection of this 2 yr old thread. I paid no attention to the date when I clicked on it in the related threads pane...:)
 
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