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I installed a TH700R4 between my fairly mild 454" and 3.08's right after I bought my '71 with the hope my 8 mpg fuel mileage would increase. Yes, my fuel mileage did increase but only to 9 mpg because my cruising rpm was right at the bottom of my 220/220 duration marine camshaft's operating range. The solution? I installed 3.70's to bring my cruising rpm higher. Before the swap my 60 mph cruising rpm was 2600 and after the swap it was 1800 rpm; a 30% reduction. The 3.70's brought my 60 mph cruising rpm up to 2200 which put it into the middle of my marine camshaft's operating range and my fuel mileage increased to 10 mpg. Later I installed 10:1 domed pistons and a 236/236 duration camshaft which brought my fuel mileage up to 11 mpg and with the later addition of ram air my fuel mileage jumped to 13.5 mpg; a 69% increase over the original 8 mpg.

Overdrive transmissions will extend the engine's life but often at the expense of reduced fuel mileage; especially if the engine has a high performance intake manifold and longer duration camshaft because at the lower rpm fuel droplets can come out of suspension inside the runners and ports which results in incomplete ignition (wasted fuel).

After many years of being without overdrive transmissions were reintroduced in 1982 when better fuel management and intake manifolds became available. The Tuned Port Injection intakes had smaller cross section runners that were about 17" long which gave much high runner and port velocities that provided more bottom end torque and increased fuel mileage.

So overdrive transmissions can be great but only if all of the engine's components are designed for them.
 
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