Bigger Tires Speedometer change

[jafo]

Ok going with 33's. How much and how can I adjust the speedometer to read correctly?

 

[Lumpy Grits]

What R&P ratio are you runing? You might regret 33's.
A GPS will tell you exactly the difference.
Most smart phone have a GPS app.
What T'case are you run'n?
The D/300 is easy to change the drive gear in it. The D/20 is a PITA.
LG

 

[jafo]

What R&P ratio are you runing? You might regret 33's.
A GPS will tell you exactly the difference.
Most smart phone have a GPS app.
What T'case are you run'n?
The D/300 is easy to change the drive gear in it. The D/20 is a PITA.
LG

Oh you know I got the Dana 20 PITA Transfer Case . I wouldn't have anything easy.

I haven't figured my gear ratio yet. I know it won't tell you what gear ratio it is but when running at 55mph, if the tach is accurate, it's spinning at around 2500 rpm,

 

[Lumpy Grits]

You had better find out what your R&P is before you go to 33's.
That 3 speed may not like it much and your engine will be lugging.
What size tires are you running now?
What letter code is stamped on the rear axle diff housing, on the right side near the diff cover?

LG

 

[jafo]

You had better find out what your R&P is before you go to 33's.
That 3 speed may not like it much and your engine will be lugging.
What size tires are you running now?
What letter code is stamped on the rear axle diff housing, on the right side near the diff cover?

LG

The 33's are already on the rims sitting in the garage.

It's really hard to see. But it looks like a TY?

 

[jafo]

The 33's are already on the rims sitting in the garage.

It's really hard to see. But it looks like a TY?

OK, the TY was on the drivers side back of the diff housing.

I scraped the passenger side and found a huge "A" stamped on that side.

So which one am I looking for?

 

[jafo]

I googled the codes and it appears if it is the "A" stamp I'm going by it is a 3.54 ratio open diff.

The tire size It has on it now is a LT 235/75 R15

 

[Lumpy Grits]

What dia are the tires you have on now?
FWIW: I only run about 25psi in my 33's on the street.
Run about 12-14 psi off-road and 9-10 in the sand.
Mine are on 8" rims.
If you see only one 'A' then yes, it's and 'open' 3:54. If it was a double stamped 'A' then you would have the trac-lock LSD.
3:54 ain't a really bad choice-Give it a go..........

LG

 

[jafo]

What dia are the tires you have on now?
FWIW: I only run about 25psi in my 33's on the street.
Run about 12-14 psi off-road and 9-10 in the sand.
Mine are on 8" rims.
If you see only one 'A' then yes, it's and 'open' 3:54. If it was a double stamped 'A' then you would have the trac-lock LSD.
3:54 ain't a really bad choice-Give it a go..........

LG

Lesson learned here. I'll be more careful on tire sizes next time.

I looked up some charts on tire size to gear ratio and it looks like I'm just below what is recommended as highway use. I think it said something like 2300 rpm at 65mph with this tire and gear ratio combo. That of course is with a 4 speed Transmission .

The set up I just got is 33x12.5x15 tires on 10" wide rims. The outside diameter of the tire is 103.5" inflated at 35lbs.

 

[jafo]

Oh and these tires and wheels are not on the jeep yet. I'm still working through deciding on what lift kit I'm going to install. So it still has the "stock" rims and tires on it now.

 

[CJ]

Might want to look at ....

-->> how determine your gear ratio

 

[jafo]

Might want to look at ....

-->> how determine your gear ratio

Was going to do this tomorrow. Got a date with some gear oil and a jeep.

 

[jafo]

Might want to look at ....

-->> how determine your gear ratio

Excellent info page!

 

[Lumpy Grits]

"The outside diameter of the tire is 103.5" inflated at 35lbs."
LOL- measure again.....

Before you run those. I would make sure the hub nut on your OEM 2-piece rear axles is tight(250 ft/lbs torque). Or look into some Moser 1 piece axles.

LG

 

[Torxhead]

The way I see it Jafo, you probably do have a 3.54 rear gear ratio. With those new tires it might work out pretty good on the highway also. Since you have a 3 speed tranny it will be pretty tough on the clutch though as you will have to slip it more to get rolling. So you may want to consider a 4 speed with the lower first gear to improve taking off in first plus a lower crawl ratio. Those large tires can be pretty hard on the two piece axles also. Just Empty Every Pocket.

 

[jafo]

"The outside diameter of the tire is 103.5" inflated at 35lbs."
LOL- measure again.....

Before you run those. I would make sure the hub nut on your OEM 2-piece rear axles is tight(250 ft/lbs torque). Or look into some Moser 1 piece axles.

LG

Current tires on the jeep are 28.9". I cheated and googled it.

I have the one piece axles on the list.

 

[jafo]

The way I see it Jafo, you probably do have a 3.54 rear gear ratio. With those new tires it might work out pretty good on the highway also. Since you have a 3 speed tranny it will be pretty tough on the clutch though as you will have to slip it more to get rolling. So you may want to consider a 4 speed with the lower first gear to improve taking off in first plus a lower crawl ratio. Those large tires can be pretty hard on the two piece axles also. Just Empty Every Pocket.

I figured it would be harder on the clutch. It's not going off road until the one piece axles are in also.

 

[Lumpy Grits]

If you confirm the hub nut torque and take it easy on the 'skinny' pedal. You'll be fine for a spell.
I would save the one piece axle deal till you decide what R&P you want and what type of diff carrier you will go to. Then do it all at one time.
LG

 

[jafo]

If you confirm the hub nut torque and take it easy on the 'skinny' pedal. You'll be fine for a spell.
I would save the one piece axle deal till you decide what R&P you want and what type of diff carrier you will go to. Then do it all at one time.
LG

Front ration has.to match rear ratio right?

 

[Lumpy Grits]

Front ration has.to match rear ratio right?

Yes-all axles must have the same R&P ratio. Also, you want the same size tires.
LG