Here I go again with fuel economy

First the amount of knowledge on this web site is amazing, thank you all for your words of wisdom. As a newbee, I feel quite respected. It would apear that I have a carborated 258, with a t18 and a dana 300 transfer case with a differential "N" 3.54 trak loc. I am now settled on keeping every thing but looking at changing the rear diff and getting larger diameter tires to improve hightway gas milage (decrease RPMs if possible). which differential (rear) would work best? And I don't speed

 

If you go bigger tires and lower(numerically) gears, your jeep will probably get worse mileage because you will be out of the proper zone for the engine. There are charts online that you can find the best fuel mileage or best power with regards to gearing. In fact, there is usually one of those charts in the 4wd.com catalog I get all the time.

With 3.54 gears, I wouldn't go too big on tire size.

If you want better mileage make sure your Jeep is tuned up, aired up, and lightened up as much as you can. I haven't checked my mileage yet after doing my fuel injection upgrade, but I'm sure it's better than the leaky, out of tune carb I had.

High5

 

Larger tires are going to be a heavier and you probably wont gain to much mileage. I think your 258 6cyl with larger tires and higher gearing is going to bog the engine and you will be on the gas more to compensate. I did the larger tire for mileage thing on my DD. But I have extra torque to spare- it is a 04 dodge ram 2500 CTD w/ 4:10 gearing. And I dont think I gained to much, if any mileage. Maybe if you go tall and skinny on the tire then it would work. What tires you thinkin'?

 

Decreasing RPM does not automaticly mean better mileage, infact it can hurt it. You need X amount of HP to go down the road at Y speed. If you are turning less RPM you could infact be out of the powerband and need to use more throttle.

Going to higher( numerically lower) gears helps when you have something like 5.38s or 4.88s on some motors because then they are wound up doing 65mph, 3.54 you motor should be loping along at a reasonable RPM. Larger tires I don't think will help at all, I have never seen a situation where larger tires got better mileage, normally the reverse is true.
Remember these Jeeps are slightly chiseled bricks going down the road, fuel economy was never the intent when they were built, heck it still to some extent is not now.

 

I noticed you wrote "rear diff" - note that you have to change BOTH the front and the rear differentials. If you change just the rear, you'll have to remove the front driveshaft and never use 4WD again, ever.

How tall are your tires now? What mileage do you get now? How many miles per year do you drive?

 

I agree with all of the above, I had a 74 Cherokee with a 258 and 3.54 gears, I had LT235/75R15 tires on it. Around town it seemed like it had enough power to maybe handle a larger tire but out on the open road on a long steep grade I could tell that a tire much larger would probably pull the rpms down just enough to force a shift down out of direct to prevent a lugging condition.

Personally I would not run anything larger than LT235/75R15, You might get away with 30*9.50 but I would get 2 used ones on extra rims, Freewheel the front and try them before spending big bucks for tires you may be unhappy with.

Its better for the engine to run more rpm at a leaner mixture than washing the cylinder walls down with fuel you dont need mildly lugging it constantly.

My 2 cents worth, Hope it helps you make a good choice.

 

As usual, I agree with Brian. My only addition is, get a vacuum gauge and use it as if it's your savior. My rules of thumb: (in an area with no flat roads) ... 2" minimum, 5" preferred, 10" WOW. On flat roads, 10-15", you should be getting all the mpg you can realistically hope for. Adjust driving habits to the machine at hand.