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Dead Parrot · 05-20-2019, 05:59 PM

Power to move the truck at 65 should be about the same in either 3rd(1:1) or OD(.7:1). Aerodynamics don't care about rpms or gear ratios. Only savings is the difference in engine friction between 2700 and 2000rpm.

Depending on the mechanical timing curve, you are probably losing some advance between 2700 and 2000. Same for vacuum advance at the lower vacuum due to lower RPM. Might be worth seeing if you can adjust the timing curve for the mechanical without causing pinging.

If that carb has any type of part throttle mix adjustment, might see about adjusting it to make up for the lower vacuum with the OD.

To remove one possible variable, might do a 100 mile trip in 3rd and repeat the same trip in OD just in case the new transmission reads different then the old one on speed & miles traveled.

05-20-2019, 05:59 PM	#14
Dead Parrot Registered User Join Date: Aug 2012 Location: Oklahoma City, OK Posts: 2,496	Re: MPG and Manifold Vacuum Power to move the truck at 65 should be about the same in either 3rd(1:1) or OD(.7:1). Aerodynamics don't care about rpms or gear ratios. Only savings is the difference in engine friction between 2700 and 2000rpm. Depending on the mechanical timing curve, you are probably losing some advance between 2700 and 2000. Same for vacuum advance at the lower vacuum due to lower RPM. Might be worth seeing if you can adjust the timing curve for the mechanical without causing pinging. If that carb has any type of part throttle mix adjustment, might see about adjusting it to make up for the lower vacuum with the OD. To remove one possible variable, might do a 100 mile trip in 3rd and repeat the same trip in OD just in case the new transmission reads different then the old one on speed & miles traveled.