So if I recall correctly, Eagles were available with 2.35, 2.73, 3.08, and 3.55 gears from the factory, depending on engine/transmission or if it was sold with the towing package or sold in the mountains.
I was told by a former AMC|Jeep technician (cruiser54 for those who know, RIP) that the AMC I6 engines (232, 258, 4.0) were all designed to run 1000 rpm for every 24 mph in high gear. So 1000 rpm at 24 mph, 2000 rpm at 48 mph, and 3000 rpm at 72 mph. Assuming a 205/75R15 (27.1” diameter) and a non lock up converter or a 1:1 transmission ratio, you’re looking at 2823 rpm at 72 mph with 3.08 gears and 3245 rpm at 72 mph for 3.55 gears. Either of those are good options for these cars. Both of which are also available in many Jeep XJ Cherokees, which have a near bolt in rear axle. See my rear axle thread for more on that.
When I told people that I was going to run 3.55 gears in my car (which is on 225/75R15s, or a 28.3” diameter), I was pretty much told not to do it because it would be way too much rpm for the car, even though the math says it should be decent. I went ahead and regeared the front axle to 3.55 and swapped in a Chrysler 8.25 with 3.55 gears. I previously had 2.73 gears. I wasn’t terribly concerned about highway rpm because I never drove faster than 60 mph in the past. Having done the swap, I actually started using the car as more of a daily driver because it’s so much more fun with the deeper gears. And it behaves so well on the highway. I have ran 75-80 mph several times lately and the car doesn’t mind it at all. I also did not notice a drop in mpg, in fact, mpg seemed to improve (I live in a rural area, so most of my driving is at 60 mph on county roads). The drivability improved greatly in town too because the car didn’t have to work as hard to get moving. My car is a 258 with an automatic. It now also has an NP231J, but it did not when I first changed gears.
In summary, don’t be afraid to give these cars some better gearing. The car is so much happier, the mpg improved, and it’s way more fun to drive.
Personally, I’d absolutely go with 3.55 gears again with the same size tire. With a smaller tire though, I think 3.08s would be good, especially if you spend a lot of time on the highway. If a lot of your driving is in town, don’t be afraid of 3.55s.
What are everyone else’s thoughts on gearing?
Topic: Gear Ratios (Read 1711 times)