The higher number of gear makes it a lower gear rear end. A 4:10 is a low gear, but a 2:76 is a high gear. Sounds pretty dumb, but that is how it works. Its easier to picture the speed at a given rpm. With 4:10 gears at 2,000 rpm, the vehicle speed will be pretty low. But with 2:76 gears at the same 2,000 rpm, the vehicle speed is much higher.
I had a 2:76 gear 8 1/4 in a 54 Dodge pickup for a while. The problem with that high of a gear is that you put your foot into the gas pedal more to get it to crawl away from stop signs. That gear with tall 15" tires sure kills off the stop power. I actually gained about 2 mpg when I swapped in a 3:23 geared 8 3/4 into the truck. I didn't have to step into the gas so hard to get the truck to move.
The OD with the lower gear is the best of both worlds, you get it rolling easier with less throttle input the lower gear, than once at a steady cruising speed on the highway, the OD drops the rpm. Those small block Mopars are very efficient at 1800 -2000 rpm at the chosen mph. Gear the rear end to turn those 1800-2000 rpms at the speed you expect to be driving at the most. Much lower rpm and it works too hard pulling hills of any height and will kill the mpg. Its very hard to find a cam that will produce the torque you need to pull the hills at those low rpms.
It does not work harder at lower RPM, it does the exact same amount of work as long as it is able to maintain speed without downshifting. Back when everyone ran carbs and it was harder to atomize fuel properly at very low RPM and high load an engine could get worse MPG if the RPM were too low because the fuel simply wasn't able to burn fast enough before the piston was at BDC and the ex valve opened, modern engine/trans combos really bring the RPM down at high speeds and have picked up quite a bit of MPG by doing so. At low RPM and high load is when an engine should get best MPG if everything is ideal.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
