Speed, Gearing, and getting outrun by grandma. - JeepForum.com

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post #1 of 9 Old 03-13-2003, 11:43 PM Thread Starter
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Speed, Gearing, and getting outrun by grandma.

Ok, as a preface, I'm pretty new to all this (my first jeep, i've had it right at 2 months, and i'm finally getting into modifying it), making it entirely possible that I say something that makes little to no sense. That said, I want to regear my jeep.

It's an 81 CJ-7 with a 258 in it, 5 speed, holley carb, probably 4 1/2 inches of total lift running 33"x12.5 BFG MT's, flowmasters, air filter, and other small performance upgrades.. (upgraded ignition too as soon as i get around to it)

It runs great, sounds great, but.. it is slow, real slow. I got schooled by a minivan earlier today at a stop light. Alright, so i wasn't trying to race, but that just illustrated to me how slow my jeep is. Overdrive is worthless, I drive on the interstate in 4th at 2000/2100 rpm for 100 miles at a time, and even get better fuel economy than were i trying to drive in 5th. And i don't drop down to 50mph everytime a hill comes up.

I know the problem comes from the fact that it's still got stock gearing, but beyond that, I don't know much. How involved is the replacement, etc ? I want to do it all myself, something about paying someone to work on my jeep just doesn't strike me right.

So, the questions I have, after all that:
1) How can i confirm what gearing is in it?
2) What should my target cruisng rpm be ?
3) What gearing should I be changing to, what's everyone else using?
4) How involved is the swap, and what would the final cost be?

Thanks for all the help in advance, and sorry about the long post.

Last edited by SirYoplait; 03-13-2003 at 11:50 PM.
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post #2 of 9 Old 03-14-2003, 04:45 AM
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I am new to Jeeps, I am building my first one, but if you will take a Ford persons advice...

Gear Ratio - jack up the rear, the whole rear.
Mark your drive shaft position on the axle housing.
Mark your tire for top position.
Turn the tire around for a full turn while counting the number of times the drive shaft turns.
If the driveshaft turn four times to the tires once, your gearing is approx. 4 to 1, get it...

My thought on target rpm is fourth gear, 2600 rpms, 60 mph.
I will be running 4.56s, 35" tire, and my calculations come up with 2680 rpm in fourth at 60 mph.

Gearing is based on tire size vs rpm...
Find out your existing gear ratio and tire size, i.e. 32", 35".
If the tires are not the originals and the gearing is, find out the original tire size.
Decide what tire size you want to go to, 3s or 35s maybe.
Divide the old tire size into the new tire size and multiply the answer by you existing gear ratio.
What will that give you, the new gear ratio you need to run to increase the performance of your Jeep.
I.e. - I was running 4.10 gears with 33 in tires, I want to run 37s, run the formula - New tire / Old tire = ratio of change ( 37 / 33 = 1.12). Take the ratio of change (1.12) and multi the existing gear ratio (4.10 x 1.12 = 4.592) So I should be running 4.59s, no such thing so I can go with 4.56s or go higher with the gearing to get a little more power from the engine.
When you do run the numbers and find out your gearing remember that you had an under power condition before so go a little higher in the gearing to get a little more power feeling.
As you can see the process isn't hard just take a little time and decide on the changes you want and plug them in.
Also, there are sites that have calculators to determine all of this just by plugging in some numbers. Where ??? Sorry not sure.

Work/Cost of re-gearing - To re-gear you will need to disassembly both differentials put on the gears and install the pinions. If lucky the diff won't give you too much trouble coming out and the pinion/diff shims will be the same as the old shims.
Mt thoughts would be if you don't have much experience with this type of work leave it to someone that has, it will pay off in the long run. The cost should run approx. $750 for both diffs, including gears and new bearing/seal.

Sorry for the long response, I hope you can understand my thoughts, I tend to ramble some.
Good Luck,


Do it right the first time !
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post #3 of 9 Old 03-14-2003, 09:08 AM
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I PM'd you.


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post #4 of 9 Old 03-14-2003, 03:59 PM Thread Starter
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As far as the rpm's go, my Cj screams like it's going to blow up at about 2600-2800, is that normal ? I try and keep it under 2200, that's where it *feels* like i should shift under normal usage. Obviously some situations are different, but usually that's the case. (again, original 258 w/ a T5)

Right now I'm considering the 4.10 or 4.56 gears. I'm thinking I'll go with 4.10's, just because I don't plan on having 35's on this rig any time soon, and i still need it to be able to go at least 75 mph on the highway. Any thoughts ?

And thanks for your PM Elliot, it helped greatly in getting a better idea of what i was looking to do, especially since we have almost the same setup. What do you think of the 4.10 gear idea?
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post #5 of 9 Old 03-14-2003, 08:38 PM
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I am going to change the gearing in my CJ5 to 4:10. I currently have 3:31's with 33 inch tires. Just FYI.
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post #6 of 9 Old 03-14-2003, 11:42 PM
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I would definately go with 4.10 or 4.56 with 33's. Like I said, I have 4.56 now and it runs great. Fifth gear does good up hills but still slows down a little on steep hills. Fifth gear on the T5's is a 0.86 ratio. I really prefer the 4.56 in mine because of my setup, and ours is so similar. Its hard to say what my speed is now with the larger tires and gears, but according to others my rpms at 65 are around 2200. The 258 is not screaming, but it has the power to get up hills on the highway and actually pass other cars. And to be honest, I do not want to go any faster in it.

I really would like some more power out of my 258. I plan on adding the mopar MPI system and a 4.0 head within the next few years.

Best of luck to you and just ask if you have any more questions.


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post #7 of 9 Old 05-17-2011, 02:19 AM
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I would definately go with 4.10 or 4.56 with 33's. Like I said, I have 4.56 now and it runs great.
Does this apply to 3speed transmisson too?
I have 3 speed transmission, AMC232 with 33" tires, I dont have a tachometer so I dont know my rpm.
D33 and D40 axles, stock pinions.

My max speed is probably somewhere around 70-75mph.....Im looking for something more towards off road use. One more thing, these ratios will directly affect MPG, as they change the rpm/speed..correct?
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post #8 of 9 Old 05-17-2011, 10:46 AM
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The top gear in all non-overdrive transmissions is 1:1, so the 3 or 4 speed will be the same as the 5 speed in 4th, as the OP is describing he cannot go into 5th because of the gearing being too high with his tires. So if you compare his description of running in 4th to you in 3rd with your 3 spd, everything else is equal.


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post #9 of 9 Old 05-17-2011, 12:07 PM
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