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Seem to recall running across a thread at one point (here?) that correlated the choice of axle ratio to peak torque (or peak HP?) RPM on the engine's power curve for setting up the best fuel economy at a preferred driving speed. ie. If you figured on wanting your best economy while driving at 70 mph, you'd setup your axle gear ratio (& tire size) to set your engine RPM at 70 MPH to = peak torque or something? I might otherwise think it would've been lowest specific fuel consumption/HP point, but I dont think I've seen anyone actually reference that figure; maybe its not figured on the dynos?

Searched through the forums a bit and couldnt quite find it, so does anybody know what I'm referring to enough to give me a refresher? I've got a 6.2L N/A engine (J-code I think), THM-400 Tranny, and LT265/75R16 tires (don't know what axle ratio I've got yet). And I if I'm going anywhere, I'm driving 60-70, otherwise 40-60 around town.

So I guess what I need: Which point on the 6.2L's power curve am I trying to match RPMs to? (and what is that RPM figure?), and what axle ratio would get me there with the THM400 and 265/75R16 tires?
 

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http://www.dieselplace.com/forum/showthread.php?t=67104

Think this may be the thread you were referring to.


It depends on the specific engine as to whether or not it is most efficient at its torque peak; most have the lowest consumption below their torque peak. But since diesels make their torque peak at very low rpms, I think that gearing to run 70 mph at peak torque is a good compromise between maximum economy and not lugging the engine. With the 6.2 Diesel, you are looking at 240-250 peak lb-ft at 2000 rpm. Problem is there are no axle ratios for the 14-bolt any higher than what you probably already have, the 3.73. You would have to do an overdrive tranny or change axles. You could do the 700R4 conversion. The power output of this engine is so low that with any higher rear gears it will really be a dud. With the .70 top gear and the 3.73, you will be turning about 1943 rpm. This would probably be the best setup for mpg. Again, this is for empty truck maximum economy. If you will be doing any towing, you should leave it the way it is.
 
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