Gear Ratio

bmcgc

TJ Enthusiast
Joined
Dec 13, 2023
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423
Location
Georgia
I am scratching my head trying to figure out gear ratio and tire diameter.

My 99 TJ has a 3.07 gear and came with 30-9.5-15 tires that are about 29.5" tall.

Some TJ's came with the same 3.07 gears and 205-75-15 tires that are about 27.1" tall.

The 3.07 and 32RH are a good match on the road, not so good offroad.

Increasing tire size for more ground clearance is going to decrease the effectiveness of the 3.07 gears offroad and at highway speed.

What is the happy medium?
 
I am not changing the gears. Too expensive and not worth it for the type of driving we do.
 
I am not changing the gears. Too expensive and not worth it for the type of driving we do.

If you are dead set against a gear change but want the most power then stick with the stock size tire. I don't think anyone here can tell you the tipping point of too much tire vs just right that's more of a personal opinion that requires butt in a seat.
 
I am scratching my head trying to figure out gear ratio and tire diameter.

My 99 TJ has a 3.07 gear and came with 30-9.5-15 tires that are about 29.5" tall.

Some TJ's came with the same 3.07 gears and 205-75-15 tires that are about 27.1" tall.

The 3.07 and 32RH are a good match on the road, not so good offroad.

Increasing tire size for more ground clearance is going to decrease the effectiveness of the 3.07 gears offroad and at highway speed.

What is the happy medium?

With an AT your gear ratio is really only beneficial for onroad driving. Gear properly for that and you will be good to go.
 
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I thought the tires you decided on were around 29"? It won't be great, but it won't be like 33's on 3.07s or 35's on 3.73s.

255-70-15 at 29" is the current plan. My factory steps arrive tomorrow. Once they are installed, I will see how my misses handles getting in and out. If I can sneak a 30.5" tall 265-75-15, that will be what I go with. it's about the same height as a 31-10.5 with a 1" narrower tread.
 
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255-70-15 at 29" is the current plan. My factory steps arrive tomorrow. Once they are installed, I will see how my misses handles getting in and out. If I can sneak a 30.5" tall 265-75-15, that will be what I go with. it's about the same height as a 31-10.5 with a 1" narrower tread.

Those would be fine with 3.07 gears for the intended use.
 
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Jeep keeps the speedo accurate with the speedo gear or programing, depending on what you have.

My question is what the actual effective gear ratio is when factoring in tire diameter.

I have seen a couple online calculators, but I can't make the math work out on them.

If I start with a 205-75 and bump up to a 31-10.5, the online calculator says I have an effective final gear ratio of 2.72.

If I start with a 30- 9.5 and bump up to a 31-10.5, the online calculator says I have an effective final gear ratio of 3.00.

It can't be both. A TJ could have come with a 205-75 or a 30-9.5, with the same 3.07 gears and the same 1:1 3rd gear ratio.
 
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If I start with a 31- 9.5 and bump up to a 31-10.5

31" is the diameter of both tires. There is no rolling circumstance difference of those tires.

If I start with a 205-75 and bump up to a 31-10.5

205/75/15 is a 27" tire. The percentage difference between 27 and 31 (30.5 actual) will effect the gear ratio/leverage a similar percentage. Let's say that's approximately 10% difference between those tires. Going from 27 to 31 would effectively make your gear ratio feel like 10% higher. From 31 to 27 would be the opposite.
 
Typo, I meant 30-9.5 not 31-9.5.

My point is that when determining the actual final ratio when factoring in tire size, what is the starting point?

TJ's came with several different tire sizes with different diameters, all with the same gear ratio and transmission ratio.

Probably doesn't matter, but I am curious.
 
What is the happy medium?

any of these should fit the bill:

20240221_130258.jpg


20240221_130435.jpg


20240221_130518.jpg
 
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My factory steps arrive tomorrow. Once they are installed, I will see how my misses handles getting in and out. If I can sneak a 30.5" tall 265-75-15, that will be what I go with.

Since you're not regearing, it is what it is.
I would go with the upsize. They're only that size once, then they start to wear smaller. ;)