on a 1986 f250 super cab with a 6.9 liter diesel and a manual transmission with a 4.10 rear end how high a gear ratio can you go to improve economy trucks no longer being used for heavy hauling ( changing rear end ratio)

 

I can't say that I'm an expert on it, but a good set of 3.31's or 3.55s would be ideal for that truck just to run around on the highway with. I can imagine those 4.10s the truck is a screamer at highway speeds.

 

A 3.08 is available for the Dana 60. Don't know about the Dana 70 and the Ford/Sterling rear. Close to 20mpg is possible.

 

What transmission do you have now? I'd recommend a ZF5 swap if you don't have overdrive. Should drop maybe 750 RPM off your cruise RPM. If you never use the truck for anything truck-ish then get something else to drive every day.

 

I had the same problem but instead of changing gear ratios I put on some bigger tires. worked out great for me, just something to consider.