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Discussion Starter · #1 ·
Hello,

I'm restoring an 83' CJ7 w/ 258 i6 & T176 transmission (4 speed NO overdrive). I'm pretty much keeping everything stock except for a Rough Country 2.5" lift and 33"x12.5"x15 tires & 15x10 rims/wheels.

The majority of my driving will be on the road and sometimes on the highway. I may do a trail ride every so often.

With that being said, what gears would be ideal w/ 33" tires? I want to be able to do at least 75mph on the highway. Currently, I have the stock 2.73 gears in it now. I have not been able to test drive it yet because it is all in pieces at the moment. I just want to plan for the future.

Thanks,
 

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4.10's will be the best choice for 33" tires and the 258.

75 mph in a CJ is not safe in my mind. Yes, they will do it, but when it comes to evasive action and or hard stops, you could get in trouble real fast.
 

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X2 what @CrustySprings said about driving 75 mph. I doubt the engineers at AMC intended it to be driven that fast.

However, if you had front and rear sway bars, good brakes, and some highway-oriented tires, i'd pick 3.73:1 for ring and pinions, the 258 seems to be happier cruising below 2500 rpm and still has good torque down there.
 

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Go here http://www.grimmjeeper.com/ . Plug in the info for the tires that originally came on your Jeep and its gearing. Then use your current tires use various gear ratios that replicate the same RPMs at speed.

Using a GPS I determined that my speedometer was 10% slower with 31s which made my 3.73 gears effectively 3.36. Applying the same logic 33s should make it 20% slower and around 2.98. That's not scientific like the link above, but it seems I read here that each inch increase in tire size changes the gear ratio by -5%. That would be much closer to your present 2.73s. 3.56 gears may be what you need.

I'd go the cheaper route and get 31s instead of new gears.
 

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Discussion Starter · #5 ·
Go here http://www.grimmjeeper.com/ . Plug in the info for the tires that originally came on your Jeep and its gearing. Then use your current tires use various gear ratios that replicate the same RPMs at speed.

Using a GPS I determined that my speedometer was 10% slower with 31s which made my 3.73 gears effectively 3.36. Applying the same logic 33s should make it 20% slower and around 2.98. That's not scientific like the link above, but it seems I read here that each inch increase in tire size changes the gear ratio by -5%. That would be much closer to your present 2.73s. 3.56 gears may be what you need.

I'd go the cheaper route and get 31s instead of new gears.
I have another CJ7 (1980 narrow track) w/ 31" A/T Cooper Adventurer and it rides fine at about 55 - 60mph. I wouldn't want to go any faster than that because I'm still working on some steering issues.
 

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75 mph in a CJ is not safe in my mind. Yes, they will do it, but when it comes to evasive action and or hard stops, you could get in trouble real fast.
When it comes to evasive action in a CJ I have one simple rule; DON'T DO IT. Wear a seat belt, square up, brake firmly, and hit it. They just aren't happy with sudden changes in direction and you are better protected if the wheels stay on the ground.

Sent from my iPhone using Tapatalk
 

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X2 what @CrustySprings said about driving 75 mph. I doubt the engineers at AMC intended it to be driven that fast.

However, if you had front and rear sway bars, good brakes, and some highway-oriented tires, i'd pick 3.73:1 for ring and pinions, the 258 seems to be happier cruising below 2500 rpm and still has good torque down there.
Agreed, I have a similar setup as you except I have 32" tires with 4.10 and it is not worthy of sustained highway use. 3.73 for the win!!! Mine feels like it is going to throw a rod trying to maintain highway speeds.
 

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3.73:1 is a good choice.

If you are doing 75mph, please let me know so I can choose a different road and not be in front of you.

33" tyres, if M/T or off road biased tyres (they are cool) will double your stopping distance in the wet. Just saying, cool tyres are for cruising slow with lots of space in front of you.
 

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OP your T-176 is likely a T-177 with a 3.82 first gear which would make a 3.73 gear set with 33's more tolerable (than a true T-176 with 3.52 first gear) since, as you say, the Jeep will be mainly street with an occasion trail ride. You can verify if your trans is a T-177 be looking at the metal tag fastened to the right front of the transmission case. The tag number should be 5359389.
 
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