Another gear ratio / tire size thread

[75sv1]

Mine would be what terrain I'd be running. I am on 4.10 and 32's. I was on 31's. I do some driving in WV, so mountains. The 31's were good. I also did an exhaust upgrade. I do have a build going on with 3.73s. I am planning 30.5's for it.  

[derf]

I would suggest doing a side-by-side comparison of the ratios with a calculator that tells you everything you need to know:

 

http://www.grimmjeeper.com/gears.html

 

Plug in your information and compare the ratios side by side.

 

The best bet for the most accurate results is to find the "revolutions per mile" for your tire.  The P-metric numbers are rarely correct.  And the advertised outside diameter is also almost always wrong.  But the revolutions per mile tends to be pretty accurate.  I usually go to tirerack.com and look up the exact tire I'm running.  In the specs they have a column for revolutions per mile.  Plug that in and play with the axle ratios to see how they run.

[Eagle]

18 hours ago, JustEmptyEveryPocket said:

I've been playing around with mine (4.10 gears + AX-15 + 31" tires) and found that it LIKES to run at ~3,000 rpm. Sure it can pull much lower than that, but my fuel mileage gets better as I keep RPM higher. Now a days I shift up at 2k rpm and shift down at ~3.5k or so. Lots of the county roads I drive are @ 55mph and I just leave it in 4th. So far I have gained 1-2mpg on the average tank driving this way. Plus its WAY more fun! 

 

 

I have posted before that the 4.0L engine's basic design goes back to a 232 cubic inch I-6 that AMC introduced in 1964. As a carbureted engine, that engine eventually saw iterations as a 199 c.i.d, 232 c.i.d, and 258 c.i.d. (which was the 4.2L used in the square headlight Wrangler YJ). Back in the 1960s and 1970s overdrive was an expensive option that virtually nobody bought, so top gear was almost always a 1:1 ratio. And the typical rear axle for AMC was 3.08 -- so they generally worked out to 24 MPH per 1,000 RPM.

 

That meant 60 MPH was 2500 RPM, and 3,000 RPM was 72 MPH. So we routinely cruised at 3,000 RPM and the engines didn't mind it at all. My brother had a 1970 Gremlin with the 232 that went over 300,000 miles without any internal work.

 

Quote

It has to be good for the engine (clean it out or something?) by revving to 4K on a regular basis, right?

 

Back in the day, this was referred to as a "Mexican tune-up."

[JustEmptyEveryPocket]

@Eagle The info above is the exact reason I started playing around with RPM ranges. I used to think that low RPM was better for more torque and improved MPG. However, the 4.0L seems to enjoy the opposite. So thanks for posting it up!