so, I'm running 33x12.5 on my truck, and I want to improve mpg at highway speeds. would swapping from factory 3.92 to something higher in numerical value like 4.88's be the way to go? or lower in numerical value like 3.55 or even lower? mind you this is on the 3.9 v6. also, after the swap in any direction, what would happen if I went to taller or shorter tires?

thank you for your input

 

for highway and mpg you would typically go with a lower "numerical" gear ratio.

 

As SLT said, lower "numerical" is easiest on mileage- in theory. Many guys around here will tell you that after switching from 3.XX gears to 4.56's with larger tires, the noticed an increase in response and economy. Theoretically though, higher gears (lower number) is better for MPG. Example: 3.55's would mean a more direct relation to your final drive or overdrive speed, thus lowering your RPMs. The lower gears (higher number) allows for less strain to get the vehicle moving, which can increase MPG, as long as extended highway time isn't needed.

Lower gears= Less strain at low to mid speed range, but more RPMs at top end.
High gears= More strain at low to mid, but less RPMs at top end.

So, it really depends on your driving habits and location. Hopefully that wasn't too confusing.

Edit: Similarly.... Bigger tires = more mass to rotate/less RPM
Smaller tires = less mass to rotate/more RPM

 

no, it's good info!! but with say 3.55 and 33" tires would the gears be more easily worn out from the strain of turning those tires? or am I over thinking it? because I tend to have to commute on the highway for work a lot, and I usually only pull a trailer with quads or have pipe and fabrication material in the bed. I just don't want to swap to something that would have a higher probability of failing on me is all.

thank you for you input

 

33s and 3.55s is slow with a v8 (not mine). I can't imagine what it's like with a v6. Also, numerically higher gears will put much less stress on your transmission, which is likely a fairly light a500. You really want a balance between having a high enough ratio to actually get the truck moving, and a low enough ratio to not be spinning to the moon once you are up to speed. Essentially, you want to change tire size and gear ratio at the same ratio to retain stock performance and economy. Of course, heavier tires kill economy no matter what you do.

What is your mpg currently? Bigger tires and mpg don't really mix. Especially big wide ones. Also, what is your average highway speed? 55, or 75? Somewhere around 3.92s or 4.11s is really probably ideal for your situation.

 

I have stock 3.92's I'm getting around 15-17 at 63-ish ... in town surprisingly good, all with my rebuilt A904 with tf1 shift kit also I have a modified keg, v8 throttle body, ram air intake, free flowing exhaust and I primarily use 91 octane. What it comes down to is if changing gears would help get into the 20's with larger tires, then I will do it. or even swapping rear ends with the preferred ratio.

 

Get rid of the lift and tires if you want more MPG.
Straight up.

That alone will kill MPG, not gears, lift and tires.

17 is about the highest you can go with 33's on a dog V6 truck.

 

^ This. I don't think you can get significantly better gas mileage than that with 33s and 4.5 inches of lift.

 

That is pretty good mileage for the type of truck it is. You may not experience any increase in economy over what you already have. If I had your truck, and was changing gears, I would probably find a happy medium and run 4.11's, as WontAcceptThis mentioned.

 

I appreciate the responses, if I have reached max potential then cool