2K at 60MPH

Again, just simple math. A larger tire has greater circumference. That means it covers a greater distance on the road in one revolution than a smaller tire. Therefore, and all other things assumed to be equal (axle/transmission ratio, travel speed), RPM is lower.

but lower RPM isn’t always ideal because it can lead to engine lugging, which is bad! In simple terms, that’s when torque demand exceeds torque output. In extreme cases it’ll slow and then stall the motor. But if you floor it in too tall a gear and the truck slowly accelerates, that’s still lugging it.

 

I have a 2.5” front and 1” back lift. It seems I get less than you but I guess you’re just leveled and I’m lifted a bit more? So maybe that’s why…

 

I see, thanks for the explanation. I understand a little better now.
I try not to lugg my engine lol

 

Driving style and terrain affect our mpgs dramatically (as opposed to my previous vehicle a 2004 Tahoe Z71 that got ~14 (13 to 15.5) regardless of hooliganism or old man slow driving.)

 

I am not level. I have 1.5in of rake to haul things, similar to stock. I went up 1.5in overall as that was the minimum that the Icon RXT packs could be set at.

 

Ah that makes sense. I live in a hilly Nc town with a elevation of about 1400ft. The hills don’t help for sure.

that’s a nice kit man

 

The Seattle area has it's share of hills and passes.

 

At same rotation per minute, a smaller tire will do less distance than a larger one. So to keep the same speed on the same vehicle in the same conditions, smaller tires will require more engine RPM.

That being said, I’m sure there’s a plethora of factors that are coming into play (traction on the road surface, wind, aerodynamic of the vehicle - lifted is probably less efficient, so more RPM, tire inflation, tire lug pattern, type of gasoline, etc.). At this point, not sure it’s worth calculating.

 

I see this comment more than any, and with my truck it's just not true. People always tell me "keep your rpm's under 2k and you'll get way better fuel economy". I've tried that more times than I can count on my hands and toes and it makes 0 difference. I get the same fuel economy over the course of a full tank if I baby it, or if I give it the skinny pedal to get up to speed quickly. It's always the same.

What is true for me about your comment is terrain. Elevation, and wind direction/speed play a HUGE part in my mpg's. Also on the highway, anything over 65-70+ and this thing starts tanking. It really is a brick.

 

In reference to the chart, a gear ratio is the # of input shaft rotations to 1 rotation of the output shaft. I. In the case of a differential, the input is the pinion (which is directly connected to the driveshaft at 1:1) and the out put is the ring gear (which is directly connected to the axles at 1:1). So simplified, it is number of driveshaft rotations per 1 axle rotation (not tire rotation-changing tire circumference changes your effective final drive ratio, but not the axle ratio.)

A higher ratio means the input shaft must spin more times per 1 full rotation of the output, which means a higher RPM is required. This results in more mechanical torque multiplication, but lower efficiency. That's why you move so slowly in 4lo. The transfer case goes from 1.0 (or 1:1) in 4hi to 2.57:1 in 4lo. Lock it into first gear (to eliminate gear change in the transmission) and you will see you must keep the input (engine) spinning much faster to achieve 5 mph in 4lo then in 4hi.

 

After I switched to 33”s, my rpms dropped a bit when cruising on the highway, I’m happy.

 

Regarding tire size, let's say you have a 1:1 relationship between engine and tire rotation. Imagine you removed a tire, cut it across the tread and laid it out flat on the ground, so that the circumference is now a linear distance. That is the distance you are covering per 1 rotation of the axle. If you increase that distance by increasing tire circumference, you are now at something like 0.9:1 if you wanted to keep the same output RPM (tire rotation, or ground speed for the sake of argument.) So, because bigger tires cover more distance with each rotation, your engine speed must lower to keep ground speed constant.

 

It’s really simple math to calculate rpm at a given speed if you know your tire diameter and use the values in the gear ratio chart previously posted (or your custom diff ratio). If you don’t like doing math you can use this calculator:
https://www.crawlpedia.com/rpm_gear_calculator.htm

 

You guys have been so helpful. Thanks a lot!!

 

This excites me!!