I rediscovered "remondjp's" comment and thought it a good introduction to a subject we all lust after .... some more than others.remondjp wrote:You have a diesel engine, and it is a low-revving diesel engine (not like the VW 4-cyl diesels which will easily rev up to 4-5K rpm) and the maximum RPM is covered in other threads--from memory, somwhere just above 2K.
You're going 20 mph in 2nd gear??? Shocked I'm in second gear half-way across the intersection when starting out after the light changes (slows down the impatient people behind me as I double-clutch when the tranny fluid is cold).
First and 2nd gears in these trucks are positively tractor-like--3rd is where you finally get up some speed.
For nearly any given cubic displacement, diesels give better MPG than spark ignition engines running other fuels. We all know the BTU info so I won't go over that ground.
But what about optimal operating RPMs? It's against youthful / sporting nature to operate the engine with the idea of maximizing transmission output torque in mind. We tend to think about max engine horsepower and no further.
The SD22's output is generally accepted to be 65hp @ 4000 rpm / 105 ft/lbs torque @ 1900-2400 rpm (I've found more 'range' in the torque figure depending on application and country).
So for pulling power, the driver needs to think in terms of combining gears and rpm with the goal being maximum torque being applied to the driveshaft. In simple terms, you can run the engine at maximum torque in 4th (I'm citing 4th gear because the ratio is 1:1) and hold momentum on a grade or you run the engine at a higher rpm where it produces less torque BUT the torque multiplication from having downshifted to 3rd gear (1.4:1) results in a net increase in torque applied to the driveshaft ... the vehicle's speed increases (both examples assume full throttle). There are engine speed limitations of course!
With an empty vehicle on level ground and shifting for economy, diesels are usually at their best being operated at the low end of their torque band. Mitigating factors being a turbocharger and/or visible exhaust smoke, etc. Keep in mind that any visible exhaust smoke is wasted fuel.
Shifting strictly for economy (empty truck, low altitude, level ground):
1-2 @ 10mph
2-3 @ 17mph
3-4 @ 27mph
4-5 @ 35mph