3.70 vs 3.90, what is the difference?

That's really a loaded question.

You're talking about two identical trucks what is the horsepower/ torque? Where are you traveling? Are you deep in the mountains or you completely on flat land? Are you pulling legal loads or are you pulling a hundred and fifty thousand pounds?

The short answer is that the 390s will give you a lower gear for pulling out, it will give you better pulling power on the Hills, but the 370s will give you a faster top speed and a lower RPM at whatever speed you Cruise.

Honestly it's best to look at whatever engine is in it and find out what the optimum RPM for cruising is and then pick your rears for whatever speed you're going to cruise.

That will give you the best fuel mileage and driveability.

 

Don't forget tire size mathers unless both trucks have the same size tires

 

The difference is almost nill. If the tires and transmission are the same……

If your gonna run all highway the 3.70 would be slightly better shorter heavier the 3.90 would be better….

 

Top speed would be faster with 3.70.

Top of the hill? They'd be pretty much even. Pulling power comes from the engine.

 

Top speed will only be about 5 mph different.

 

They will both pull a hill at exactly the same speed. Even a truck with 4.11s and truck with 3.55s everything else being the same will pull a hill the same speed. The truck with 3.55 will be 1/2 or a full gear lower but pull it the same speed. The only advantage you have with the taller number rear is start off is easier

 

I smoked a lot of weed in high school but the physics on that statement don’t sound correct…….

 

Simple. To put the same amount of torque to the ground with a 3.55 as a 3.90 you'd need to put the transmission in a lower gear. Speed is lost in the transmission but is made up for in the rear end.

Rear end gearing isn't the magical solution to rocketing up a hill like everyone thinks.

 

So did i but everything else the same they'll top the hill at the same speed. Schlumberger had 425 isx 12s. The newer ones were geared to run 1600 at 65 and the older ones ran 1900 at 65. Didn't matter what truck i was in they both would pull the same mph on a hill. The longer legged one was just a gear lower