Help with Gear ratios -
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post #1 of 10 Old 02-20-2013, 08:18 PM Thread Starter
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1978 CJ5 
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Help with Gear ratios

I have a 78 CJ5 with 258 engine, dana 20, T-18 trans,33" tires., I want to install ARB lockers and wondering about gear ratios. 410's 456's or 488's. The jeep is not a daily driver. I drive a short distance on the highway to get to the trail. No more than 45-50 mph. At 50mpr what would be my rpm's with these gear ratios? Is there anything I should be concerned about with any of these ratios?

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post #2 of 10 Old 02-21-2013, 04:15 AM
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Here's a link to a great gear ratio calculator

I ran 3.73s with 33s and a 360/t150/d20 combo. worked well for a dual purpose rig, although at times I could have used a slower crawl speed.

Have a few hundred miles with the same setup but 4.11 gears. haven't wheeled it, but its a huge difference on the road. I Wouldn't want to gear any lower with 33s if I was going to do much highway without o/d. If you're going to stick to the 2 lane then you can probably get by with 4.56s.

If you think you might want to go to 35s in the future, then I would def say go 4.56

77 CJ-7- AMC 360, Edelbrock cam, MC2100, Fenderwells, CJ T18/D20, Chevy D44/EB 9" rear, Trutrac f/r, 4.11, SOA, 1.25 lift shackles, 35x12.50 BFG KM2s, Shackle Reversal
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post #3 of 10 Old 02-22-2013, 01:59 PM
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If you have a t18 A! You don't want to go to a too numerically low ratio because then you'll have driveshaft problems because in granny 1st gear, all your torque is made at the output of the transfer case but then again you could get a fat driveshaft made , so it would just be more money. And if your not going over 50 mph ( i prefer to) then you 4.10's would be fine and lessen the need for a heavy duty driveshaft. 3.73's are good for keeping your engine happy at above 55mph. 4.10's and 4.56's obviously are better for crawling. At this point I'm just guessing but like jp360 said you might be able to get away with 4.56's for 50 mph or below.

1984 jeep cj7, 258 I6, rebuilt T176,rebuilt Dana 300, amc 20 , dana 30, 4.10's Trac-lok in rear , 4.10's detroit soft locker in dana 30, 2.5 inch bds suspension lift, 1 inch polyurethan body mount lift. Line ex. weber carburetor. factory tach. factory clock ,oem replacement speedomter cluster, and oil pressure gauge, factory volt gauge.
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post #4 of 10 Old 02-22-2013, 03:41 PM
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The driveshaft isn't subject to any more torque with 4.56s than it is with 2.73s or any other gear ratio. Axle gears don't multiply torque upstream of them, only downstream and even then it's probably less harmful as it's more controlled (lower engine RPM).
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post #5 of 10 Old 02-22-2013, 05:44 PM
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Any of those ratios will be fine for the speeds you are driving. The 4.88s will turn about 2700 RPM at 55 - your 258 will run at that speed indefinitely, unless there's something wrong with it. The Jeep will sound a little more relaxed on the highway with 4.10s, and this could matter if you ever choose to sell it. However, for the trail, the lower (higher numerical) gears will be incrementally better.

You'll have a pretty good crawl ratio regardless, with the T-18. If you want more reduction, you could modify your transfer case gears...

1) '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps. 2) '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR 3) '75 CJ-6, 304/T15, PS, BFG KM2s, soft top 4) '95 VW Golf Sport 2000cc 2D
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post #6 of 10 Old 02-25-2013, 08:16 PM Thread Starter
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Thank you everyone for responding, i am thing of 456's
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post #7 of 10 Old 02-25-2013, 08:52 PM
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There is more to gear ratios than just engine speed, the engine itself will have a sweet spot where torque and power come together and it will combat the amount of air and rolling resistance at a certain speed to deliver good economy and responsiveness. Allied with the right depth of gearing in your gearbox it will deliver optimum results.

If your axle ratio is too low the engine speed will be too low in top gear and it will bog. As you push the accelerator to go faster the vacuum will drop and more fuel will enter the engine, burning that precious fluid. Even though the rpms are low, it will act like a sailor on shore leave in a bar.

If too high it will just scream along, burning more fuel as the engine is turning faster.

If you think your optimum at 60mph is 2400 rpm the following is a useful chart of the sweet spot against axle ratios and tyre size. You could get away with 4.10s.

I would not go overboard on the ratios without investigating the number of teeth on both side of the ring gear. very high ratios can lead to issues with the limited contact.
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post #8 of 10 Old 02-26-2013, 08:55 PM Thread Starter
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If I am reading the chart correctly at 60MPH with a T-18, 456 and 33" tires my RPM would be 2554. Is that correct?
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post #9 of 10 Old 02-26-2013, 10:00 PM
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Originally Posted by Hartfamily View Post
If I am reading the chart correctly at 60MPH with a T-18, 456 and 33" tires my RPM would be 2554. Is that correct?
That is correct, except that the chart is for 55 mph. At 60 mph you would be turning around 2786 rpm.
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post #10 of 10 Old 02-27-2013, 10:34 AM
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Instead of limiting your data to 55mph, plug your information into this gear calculator. That chart is from the 80s and it shows.
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