I am looking to get the best fuel mileage out of my truck I can. I know I need to get the gears/tire size right for the peak engine performance. I never tow and it's a daily driver. I'm sure one of you will know how to calculate that good stuff, I have no clue. Anyone mind helping me out?

Here's what we are working with:

1989 Ramcharger with a 1990 4x4 W250 suspension.
1993 Cummins 12v non-intercooled
518 transmission 3 speed w/ OD
1990 W250 Dana 61/71 combo 3.07 gears.
16 inch rims.

Am I missing anything?

 

Mike,
Sounds like a pretty cool rig!

I'm no expert, but I have discoved that these engines like to rev. Got a tach?

I think 1900-2000rpm is the sweet spot.

I had an '83 RC in college. Fun truck!

 

I have a 1991 with the 3.07's and I swapped the 727 for a 618, I still get around 17 to 18mpg which is what I got before the swap.
I agree the best mileage is in the 1900-2000 rpm range.

 

You're missing tire size......very important factor.

 

Not sure if this is in reference to my post but I have 285 75 16. Same size as before the swap.

 

Actually, tire size is what I'm trying to solve for...I just don't know enough to work the equation backwards..

When I buy new tires, what size do I want for peak performance?

 

Is the ramcharger gonna be lifted?

with overdrive and 3.07's i'm not sure i"d go taller than stock but let's face it.....285/75/16 looks nice on stock diesel suspension and stock tires look small.

I think you're gonna find yourself not using Overdrive very often with the 3.07 rears So I'd just go with 285/75/16

 

I have LT245/75/16's on it now and they look small. I wasn't going to lift it any more than the 250 suspension already did.

http://www.cumminsforum.com/gallery/.../ppuser/102573

 

Yea that's cuz they are hardly 31" tall.

 

That forum tells me [and probably a few other non-members over there]:
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A TALL SKINNY tire will usually get better fuel mileage than a wide lower profile version. The other variable is tread design which slightly alters the rolling resistance.

 

I hear you on liking the taller tires for aesthetics. I run the 285s on my junk too. But, a narrower tire will make a remarkable difference in fuel economy. You might look closely at the 255/85/16 size tires. They are tall but skinnier than the 285s are.

 

Problem is the only tread pattern they offer in 255/85/16 is M/T and he may not want the noise of M/T's

 

I use this calculator to figure out what happens when I change tire sizes

http://www.miata.net/garage/tirecalc.html

And I like this one to figure out what those different sizes will do to rpms at given speeds.

http://www.ringpinion.com/Calc_RPM.aspx

 

I would run 265/75/16 or 235/85/16 for mileage (same height).

The bigger the tire, the more rotating mass too.

My "tall, skinny" tires, aren't doing so good for mileage; 255/85/16s M/T's and 62 lbs each.

 

Yup. I went to h2's (315-70-17) from stock 2nd gen steelies (315-75-16). I could almost juggle the h2's whereas the steel rim/lot's of rubber combo was super heavy! The reduced rotating mass made a noticiable difference, imo. truck felt like I swapped in 4.10's. If I were in the original posters spot, I'd go with 235-85's or 255 85's for best mileage.