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Old 01-06-2016, 10:47 AM   #11 (permalink)
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Originally Posted by t vago View Post
Fair question.

Top gear on the dead 42RLE had a ratio of 0.69. Top gear on the replacement NSG370 has a ratio of 0.84. This represents about a 21.7% increase in propshaft rotation speed, for a given vehicle speed. If I were to have left the original 3.636 rear end in place, that should have resulted in a definite increase in fuel consumption.

The FDR was 3.636, while it is now 2.87. This represents about a 21.1% decrease in required pinion input speed, for a given vehicle speed.

These two changes almost cancel each other out, and the net decrease in engine speed (about 3.9%) does not account for the 10.1% decrease in fuel consumption.

Put it another way - Engine speed at 60 MPH was about 1850. Now, it's about 1780. There is no way that this change in engine speed (~3.9%), by itself, would have resulted in the measured decrease in fuel consumption.

Because I forgot? I only regarded the rear end swap as a necessary adjustment to counter the larger top gear ratio of the installed NSG370, nothing more. I figured that the 3.9% drop in engine speed that I calculated back in April of last year, would have given me a very modest 2% decrease in fuel consumption. This is based on the rule of thumb that a percent change in fuel consumption will be about half the percent change in the rear end ratio. The 2% change would have been rather hard to measure as it is just above noise.

You do bring up an interesting idea - What if I had left the original 3.636 rear end in place? Engine speed would have been about 2250 RPM at 60 MPH, which would have resulted in a net increase in engine speed of... 21.7% from when the 4-speed was still installed. I would have expected a 10.9% increase in fuel consumption just from swapping in the NSG370, if I were to have treated the engine speed increase as a result of swapping in a (nonexistent) 4.43 rear end.
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JRMichler (01-06-2016), t vago (01-06-2016)