The easiest way to improve axle ratio is by tire size. Tire diameter is linearly proportional to road speed for a fixed RPM.
If you double the tire size, you'll double the speed for that RPM. Or, conversely, you'll only need 1/2 the RPM's for the same linear speed down the highway.
Now, of course, doubling the tire size is not practical. But if you take the stock 39" tire and add say 1/3 to that you would have 39 + 13 = 52". So a 52" tire with the stock differential gearing will increase your maximum speed by 1/3. Let's say the maximum stock speed is 54 mph on 39" tires. Then with 52" tires that would be 54 + 18 = 72 mph.
The trade off is torque & brakes. larger tires have more rotational inertia plus a larger radius to the contact patch on the ground. They are therefore harder to start/stop. And kinetic energy goes as speed squared. So going faster by 1/3 is going to make the K.E. ~ (4/3 V)^2 = 16/9 V^2 = 1.78 times the original kinetic energy.
Therefore, going 72 mph will take, at minimum, 78% more braking energy than at 54 mph. Something has to give. What gives? Distance. It will take MUCH further to stop the same truck, not even including the larger rotational inertia of the tires. (78% further at maximum braking efficiency).
That's why it's deceptive to say "Oh, 55 to 70 isn't that big of a deal." Well, yes it is. It might only be 15 mph difference, but it makes a 78% difference in kinetic energy that the brakes must dissipate.
So either way you go, with larger tires or different gears in the differentials, you will be over-driving the brakes if you are pushing 70 mph.
A "modest" tire-size change is probably the easiest way to improve your road speed. Somewhere in the middle like 395's or 14R20's should be just about right.
The 4.90 gear sets being sold are for Jeeps & other lightweight 4x4's running Rockwell axles. These lighter vehicles are easier for upgrading brakes. They will work in the Deuce but why?
Don't reinvent the wheel. There's a reason why semi trucks & large buses have engine brakes, exhaust brakes, and transmission retarders. The Deuce will never fly down the road like a Greyhound bus.
