Help me understand re-gearing

[UKTJ]

Haha, I’ll trade you my Dana 44 in exchange for the millions of cheap parts you have access to in the states . It is a funny one though. I spoke to a Jeep dealer last year (I was looking at a JL) and he said that a few times a year they convert the Jeep factories to produce the RHD versions in little sprints before going back to LHD. Because of the tight windows, they keep it to a handful of configurations (mainly higher end) so they can maximise output. Hence why our TJs here tend to be better kitted out. It could be a load of bs but that’s what I was told.

There may be some truth to it. My wife has a 2017 Grand Cherokee and it is very well specified, it was fantastic value when we bought it about 12 months old.

I think some of the parts issues we suffer for our TJs are less so for JK and JL owners, there seem to be a number of companies who deal with mods for those two models, but they don't do anything for TJs.

 

[TJim]

Hey! I'm coming back to this thread to ask one more question.

So let's take for example my TJ (2.4L 4-cylinder, NV1500). It came stock with 215/75r15 (27.7") with 4.10 gear ratio.

Playing a little with the grimmjeeper calculator I came up with this results.

4.10 Gears:
1) Running 215/75r15 tires: 2889 rpms at 70 mph.
2) Running 31" tires: 2582 rpms at 70 mph.
3) Running 33" tires: 2425 rpms at 70 mph.

4.56 Gears:
1) Running 31" tires: 2872 rpms at 70 mph.
2) Running 33" tires: 2698 rpms at 70 mph.

4.88 Gears:
1) Running 31" tires: 3073 rpms at 70 mph.
2) Running 33" tires: 2887 rpms at 70 mph.

5.13 Gears:
1) Running 33" tires: 3035 rpms at 70 mph.

SO...
On paper, it seems that by running 31" tires with 4.56 Gears, or 33" tires 4.88 Gears would be the closer choices as my stock setup.

My question is which would be the difference between 215/75r15-4.10 vs 31"-4.56 vs 33"-4.88 performance wise?
Is the top speed of 33"-4.88 < 31"-4.56 < 215/75r15-4.10?
Is the mpg 33"-4.88 < 31"-4.56 < 215/75r15-4.10?

Are there any other differences? (Am I talking mainly about on road performance, but any input would be helpful!)

 

[freedom_in_4low]

My question is which would be the difference between 215/75r15-4.10 vs 31"-4.56 vs 33"-4.88 performance wise?
Is the top speed of 33"-4.88 < 31"-4.56 < 215/75r15-4.10?
Is the mpg 33"-4.88 < 31"-4.56 < 215/75r15-4.10?

Are there any other differences? (Am I talking mainly about on road performance, but any input would be helpful!)

generally, performance and fuel economy will both decrease as the tire size increases, because of the additional weight and rolling resistance of the tire. This may be reflected in a corresponding reduction in top speed, but I'm not sure how significant it would be since that point the aerodynamics are the dominant resistance and aren't being significantly changed with the tires.

 

[Jerry Bransford]

Hey! I'm coming back to this thread to ask one more question.

So let's take for example my TJ (2.4L 4-cylinder, NV1500). It came stock with 215/75r15 (27.7") with 4.10 gear ratio.

Playing a little with the grimmjeeper calculator I came up with this results.

4.10 Gears:
1) Running 215/75r15 tires: 2889 rpms at 70 mph.
2) Running 31" tires: 2582 rpms at 70 mph.
3) Running 33" tires: 2425 rpms at 70 mph.

4.56 Gears:
1) Running 31" tires: 2872 rpms at 70 mph.
2) Running 33" tires: 2698 rpms at 70 mph.

4.88 Gears:
1) Running 31" tires: 3073 rpms at 70 mph.
2) Running 33" tires: 2887 rpms at 70 mph.

5.13 Gears:
1) Running 33" tires: 3035 rpms at 70 mph.

SO...
On paper, it seems that by running 31" tires with 4.56 Gears, or 33" tires 4.88 Gears would be the closer choices as my stock setup.

My question is which would be the difference between 215/75r15-4.10 vs 31"-4.56 vs 33"-4.88 performance wise?
Is the top speed of 33"-4.88 < 31"-4.56 < 215/75r15-4.10?
Is the mpg 33"-4.88 < 31"-4.56 < 215/75r15-4.10?

Are there any other differences? (Am I talking mainly about on road performance, but any input would be helpful!)

Understand that you would lose performance if you regeared so your engine rpms were the same with 33" tires as they were with 30" tires. Bigger tires require more rpms to maintain the same performance. Which is why I recommended 4.88 for you.

 

[carssuckjeepsrule]

Hey! I'm coming back to this thread to ask one more question.

So let's take for example my TJ (2.4L 4-cylinder, NV1500). It came stock with 215/75r15 (27.7") with 4.10 gear ratio.

Playing a little with the grimmjeeper calculator I came up with this results.

4.10 Gears:
1) Running 215/75r15 tires: 2889 rpms at 70 mph.
2) Running 31" tires: 2582 rpms at 70 mph.
3) Running 33" tires: 2425 rpms at 70 mph.

4.56 Gears:
1) Running 31" tires: 2872 rpms at 70 mph.
2) Running 33" tires: 2698 rpms at 70 mph.

4.88 Gears:
1) Running 31" tires: 3073 rpms at 70 mph.
2) Running 33" tires: 2887 rpms at 70 mph.

5.13 Gears:
1) Running 33" tires: 3035 rpms at 70 mph.

SO...
On paper, it seems that by running 31" tires with 4.56 Gears, or 33" tires 4.88 Gears would be the closer choices as my stock setup.

My question is which would be the difference between 215/75r15-4.10 vs 31"-4.56 vs 33"-4.88 performance wise?
Is the top speed of 33"-4.88 < 31"-4.56 < 215/75r15-4.10?
Is the mpg 33"-4.88 < 31"-4.56 < 215/75r15-4.10?

Are there any other differences? (Am I talking mainly about on road performance, but any input would be helpful!)

Hey man!
I had a 2004 with the 2.4 but it was an automatic with the elusive 4.56 option (on 30x9.50s by the way). Miss that Jeep. Anyway, nice gears, but not quite enough. Go with the the sage advice of Jerry.... 4.88s. Also, I see all the 31s vs 33s comparisons but you can split the two with 32x11.50s which is what's on my current Jeep. Just a wee bit of rpm difference in either direction from 31s or 33s respectively. One last note and I'll shut up...the 33s can be had in 33x12.50s or 33x10.50s depending on brand. Little bit of weight saved (not much) and a tiny bit less resistance on road (again...not much) but every little bit helps the 2.4 squirrels. Anyway, skinny 33s or maybe 32s are something to consider. Sorry to add more calculations to your dilemma! ha ha

 

[TJim]

Understand that you would lose performance if you regeared so your engine rpms were the same with 33" tires as they were with 30" tires. Bigger tires require more rpms to maintain the same performance. Which is why I recommended 4.88 for you.

I am a little a bit concerned about cruising! thats all. I know TJs are not made for this reason. But because it is my only vehicle I try to combine everything. So:

Since on paper (using Grimmjeeper) with 4.88 gears running 31" tires I would have:

on 5th gear: 68 mph @ 3000 rpms.
If cruising at 75mph I would be around 3300 rpms.

Wouldn't be more ideal 4.56 running 31" tires?
on 5th gear: 73 mph @ 3000 rpms
Cruising at 75 mph @<3100 rpms

 

[TJim]

Hey man!
I had a 2004 with the 2.4 but it was an automatic with the elusive 4.56 option (on 30x9.50s by the way). Miss that Jeep. Anyway, nice gears, but not quite enough. Go with the the sage advice of Jerry.... 4.88s. Also, I see all the 31s vs 33s comparisons but you can split the two with 32x11.50s which is what's on my current Jeep. Just a wee bit of rpm difference in either direction from 31s or 33s respectively. One last note and I'll shut up...the 33s can be had in 33x12.50s or 33x10.50s depending on brand. Little bit of weight saved (not much) and a tiny bit less resistance on road (again...not much) but every little bit helps the 2.4 squirrels. Anyway, skinny 33s or maybe 32s are something to consider. Sorry to add more calculations to your dilemma! ha ha

Thanks for the input! I am installing 31x10.5r15 BF KO2s tomorrow though! 33x10.5 might be in the future though (likely not, because of the engine, we will see!). 33 x 12.50 I dont think that it's for my use at all!

 

[Jerry Bransford]

I am a little a bit concerned about cruising! thats all. I know TJs are not made for this reason. But because it is my only vehicle I try to combine everything. So:

Since on paper (using Grimmjeeper) with 4.88 gears running 31" tires I would have:

on 5th gear: 68 mph @ 3000 rpms.
If cruising at 75mph I would be around 3300 rpms.

Wouldn't be more ideal 4.56 running 31" tires?
on 5th gear: 73 mph @ 3000 rpms
Cruising at 75 mph @<3100 rpms

Your engine is perfectly happy at those rpms, don't confuse them for the much lower rpms car engines need with their typically small tires, low weight, and better aerodynamics. 4.88 is the most often recommended ratio for 33" tires, 5-speed transmission, and 33" tires. Personally, my ratio recommendations are always made with highway cruising rpms in mind.

Put another way, it's better to be slightly overgeared than to be undergeared. Not that I consider 4.88 to be slightly overgeared... you just don't want to be undergeared which is frustrating to have to live with.

 

[TJim]

Your engine is perfectly happy at those rpms, don't confuse them for the much lower rpms car engines need with their typically small tires, low weight, and better aerodynamics. 4.88 is the most often recommended ratio for 33" tires, 5-speed transmission, and 33" tires. Personally, my ratio recommendations are always made with highway cruising rpms in mind.

Put another way, it's better to be slightly overgeared than to be undergeared. Not that I consider 4.88 to be slightly overgeared... you just don't want to be undergeared which is frustrating to have to live with.

Thanks Jerry!

your input is really useful, like always!

I must recognize that for the last year that I am on this forum almost everyday, I have learned a lot from reading your answers to my questions or to other threads! Cheers for that!