I just gout a 79 Bronco, with some work I have it on the road now! Its way low geared. Really I think to low for a daily driver. 400, T18 and 4.11s with 31x10.50s. It seams like its spinning high rpms on the hwy, Even around town I wish there was more top end! Bigger tires are in the works. Was just wondering how big I should go, to get my Rpms down and my mpg up? I know there is a point that to big starts to cost me mpg?

 

I did some seaching around on this site because i know they mention the tires and rpms a lot and i found this thread(i know there are others though):

https://www.ford-trucks.com/forums/1...uck-today.html

When you click on the link, there are a lot of pages on there. Go to page 56, thats where i found the RPM info.

As for MPGs, im really not sure whats gonna help you there. Gettin the RPM down should help though i think. Hope this helps

 

Typical V8 should be turning between 2000 and 2500 rpm at highway cruising speeds. That leaves you a little room for passing power when you romp on it without having to constantly downshift. If you are a lot higher you have nowhere to go when you put the hammer down and you are using every drop of fuel to make power that you may or may not always need.

Take highway speed (60 MPH)
multiply by the axle gear ratio (3.55 in many Broncos but find out before doing the math)
multiply that number by 336 (this is a constant and MUST be done).
then divide by the actual tire diameter (NOT the nominal diameter... if you are running 31's the actual diameter is 29.25-29.50". Don't believe me, measure from the center of your hub directly to the pavement then double your measurement). THIS IS WHERE MANY WEB-BASED CALCULATORS FALL SHORT. If you want actual tire diameter for the tire you are running or tire you are considering running, the manufacturer's website will typically list this dimension in the tire info on the site.

The math looks like this:

60 x 3.55 x 336 / 29.5 = 2,426 rpm

If your transmission has overdrive, you should also multiply by that ratio as well.

I.E. 60 x 3.55 x 0.75 x 336 / 29.5 = 1,819 rpm

Yes OD drops cruising rpm way down which saves fuel.

Changing tire sizes, axle gearing, or transmission gearing will yield different results. Knowing the engine's optimal torque and HP output would be a good bit of info to have as well when determining which tires size and/or axle ratio to use.