good ratio for mpg

That is where running a ratio that has the truck in the next gear down from the top while loaded and then having the top gear for empty works very well for some people. Then targeting a ratio that fits with speed and RPM of the engine under load and the next gear down from the top is ideal.

 

Im currently running 3.90 rear ends ratio with a 10 speed and 295/75r22.5's in my company truck and turn just under 65 at 1600. Looking at getting my own truck and going to spec 3.55 gears and run an 18 with 11r22.5's. Should put me right about the same rpm at 65 in 17th and 1350 in 18th at 65. Im running out in the northwest and upwards of 105,000 when loaded with my tankers.

 

Pretty close.... 11R22.5 (496 RPM) and 3.55....

 

http://spicerparts.com/calculators/transmission-ratio-rpm-calculator

This is what Ive been using to play with gear ratios.

 

That works. The mathematics is the same. I like doing the spreadsheet thing to see how it looks across an RPM range and how the different gears compare to each other. Gives a better picture for me to make a decision.

 

Guys you also need to look at the specific tq curve for your motor and power rating.
If you're planning on a downspeeding drivetrain you'll shoot to be at the bottom of the Tq curve at cruise speed.

Like the Volvo XE runs 1150@65mph with 2.64s and 22.5LP rubber.
That motor is flat tq down to 1050rpm.

The proper gear at cruise will depend on where your tq falls off and gearing for rpm just above that ..

 

In my case Im driving a '13 pete 386 with a 450/1650 ISX and am looking at a '15 pete 389 with a 550/1850 ISX which will probably get reprogramed to a 600/2050.