I'm going through a similar quandry with my '93 Ramcharger 4x4. Just got it locally, really nice condition, about 150,000 on the clock, has the 3.55 gears and 30" tires.

Discovered the overdrive isn't engaging, and my highway rpm has been a nice 2400-2600 rpm, and I've been getting ~ 15 mpg - not too bad really. But that's with the tranny in 3rd, not overdrive. When I get the od functional my 65 mph cruise rpm will be 1650-1850 rpm, and I can't help but think those rpms will pull the 318 out of it's sweet spot, and likely my mileage will go down.

I've always felt the small blocks 'enjoy' a bit more rpm, and I'm thinking seriously about replacing the 3.55s with 4.56's.

With 4.56s and a 31" tire, 65 = 2250 and 75 = 2600, and that sounds pretty good to me.

I can't help but think the 4.56's would be a better gear ratio, especially considering I do a lot of around town driving and the gears should help get the truck around lots easier.

What rpm range do y'all think a Magnum 318's 'sweet spot' is?