Gear ratio question

I have a 75 cj5 with a I6,t-14 and a dana 20, 3:73 gear, my question is I drive it on the street 95% of the time and the engine seems to be screaming at 70 mph, has anyone run a 3:08 gear before in set up? would it be to high of a gear? the gear ratio calc says it should be at about 2600 rpm at cruise. Thanks

 

JMO - I think that would be a mistake. How tall are your tires? Even with short 28" tires, you're turning under 3000 at 65. In my truck, with 3.54s and 29" tires, I can drive well over the legal speed limit (75 is about the fastest I've driven it) and the engine speed seems very comfortable.

If you are used to driving a passenger car, a Jeep has only a single sheet of metal between the engine and driver. Plus they make a lot more gear and tire noise than a passenger car. If the engine noise bothers you, I'd suggest you insulate the floors (carpets and padding). If you have a fixed fan, change it for one with a fan clutch or an electric fan. Run a quiet exhaust and have it exit at the rear corner.

3.73s are already highway gears IMO. When I had them on my '75 CJ-6, I wished I had gone with 4.27s. You will almost certainly have a higher top speed with a lower (higher numerical) axle ratio, like 4.10 or 4.27. Plus, I would guess that your 3.08 equipped Jeep would require downshifting at even moderate hills becasue you'll be well below the power band. And the T-14 2nd gear is really low, and you'll have to slow down, so you don't want to downshift if you don't have to.

Sorry for the negative response.

 

I agree with Tim. It is very easy to go too high( numerically low) in gearing and if you take the motor out of its powerband going down the hiway you actually will make it harder on the motor and burn way more gas just to stay at 70. These Jeeps are not like modern cars with lots of sound deadening, heck take a car pull out all the sound deadening they put into them today and see how loud everything really is.

 

I don't think that was a negative responce.... I agree with it totally. I have 4.27 gears in one jeep and 4.56 gears in the other. Loud at highway speed... YUP very lound. Sounds like something is going to give up. My kid regularily drives it at 65. I am afraid to drive it that fast... not because the motor is turning too fast, because it is a short wheelbase jeep and dangerous at those speeds.

You may want to consider going to a larger tire tho. You didn't post what size tire you have. I run 33's with the 4.27s and 35's with the 4.56s. You can probably buy a set of tires for what it would cost to have it regeared. Unless you plan on gearing it yourself with used parts.

 

The highest I would go is the 3.54 IMHO. And from 3.73 to 3.54 is not really worth the change. The formula for calcualting RPM at a certain MPH in the high gear on the trans (1:1) is the following:

RPM = MPH x diff gear ratio x (336/tire diameter in inches)

For an OD as final gear it is:

RPM = MPH x (diff gear ratio x OD%) x (336/tire diameter in inches)

If the goal is to reduce RPM or save fuel, you could install an Overdrive.

 

Thanks for the input. i have 28" tires, i guess i never thought about the insulation and sound deadning in a car. I just bought a tach and I'm turning just over 3000 rpm at 65, i guess I'll leave things as they are. Thanks abunch. This is my first jeep I guess I have alot to get used to lol. thanks again

 

Toss some 30s or 31s on it.
Borrow a set if you can to see/feel the difference