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After I installed the TH700R4 in my 454" powered '71 I got 12 mpg at 50 mph but got 16 mpg at 70-75 mph. Logic would tell you the fuel mileage would be better at the lower speed so why did it get better fuel mileage at a higher speed?
It's all about matching components. My engine has a 220/220 duration marine cam so my maximum "lope" rpm is 1300. And at 50 mph my cruising rpm was also 1300. So my engine was running at a very inefficient rpm at the lower 50 mph speed.
By simply changing my rear ratio to 3.70 from the stock 3.08 I increased my 50 mph cruising rpm to 1600; putting my engine into a much better power band and substantially increasing the fuel mileage. And with the 3.70's I can cruise at 70 mph in 4th at 2385 rpm then kick it down into 3rd and be right at where my torque curve starts.
Yes, changing rear end ratios is expensive but absolutely necessary to match the cam's characteristics.
It's all about matching components. My engine has a 220/220 duration marine cam so my maximum "lope" rpm is 1300. And at 50 mph my cruising rpm was also 1300. So my engine was running at a very inefficient rpm at the lower 50 mph speed.
By simply changing my rear ratio to 3.70 from the stock 3.08 I increased my 50 mph cruising rpm to 1600; putting my engine into a much better power band and substantially increasing the fuel mileage. And with the 3.70's I can cruise at 70 mph in 4th at 2385 rpm then kick it down into 3rd and be right at where my torque curve starts.
Yes, changing rear end ratios is expensive but absolutely necessary to match the cam's characteristics.