rear axle gear ratio

racer4 wrote:
Get the 4.10

The rpm will be 1550 at 62 mph with the 3.73. I suspect the trans will downshift to 5th at 62 frequently, or all the time with 3.73.

The peak torque rpm is 1700, that is when the engine is the most efficient.
The rpm will be 1700 at 62 mph with the 4.10

It use to be with the old mechanical pump diesels that the engine's max efficiency (brake specific fuel consumption) corresponded very closely to the the rpm where the engine produced the maximum torque. This was a mathematical truth. Under full throttle operation the engine would inject a specific volume of fuel per engine revolution. So the rpm where the maximum torque was produced corresponded to the rpm where the engine made the most cylinder pressure with a power stroke shot of fuel. With common rail fuel injection the amount of fuel injected per revolution under full throttle operation is determined by computer software and can vary depending on rpm. Some diesel engines are designed to have an absolutely flat torque curve over a very wide operating range....this does not mean that the engine's brake specific fuel consumption is constant over that rpm range.

Additionally, just because an engine is running at the rpm of its max brake specific fuel consumption does not mean that the truck will get its best fuel economy at that rpm. For instance to pull a typical 5th wheel down the road at 60 mph takes about 100 crankshaft hp. If the diesel engine doing the work is capable of making a maximum of 900 lb ft of torque at 1800 rpm under full throttle it is capable of producing 308 hp at that rpm. Obviously it does not have to run at 1800 rpm to accomplish its task. It takes far less power to turn the engine, alternator, fan, power steering pump, A/C pump, transmission, etc 1300 rpm than it takes to run all that equipment at 1800 rpm. Therefore if the engine is capable of making the required 100 hp at 1300 rpm it will normally get significantly better fuel economy by gearing up and slowing the engine down.