The factory gear ratio for an A-code Toploader 4-speed car was 3:00 (normal level of performance).
The factory gear ratio to a HiPo K-code Toploader 4 -speed car was 3:50 (high performance option).
I've used the above two as examples of what happens when you change gear ratios. I make the assumption that the city/highway mileage was 18 and 20 mpg respectively for the A-code car. I was a young guy in my early 20's when the '65 was a new car. My dad owned a early '65, then '66 then '68. I owned a new '67 and my brother owned a '69 so I remember what kind of gas mileage these cars used to get when new. The mpg quoted may not be perfectly accurate but its close.
Knowing that, I predict what happens to the gas mileage when the gear ratio changes from 3:00 to 3:50. It will be worse for 3:89 and 4:11 by about the same percentages. Just plug in 4:11 into the equations and you'll see the mileage drops down.
I also provided the gear ratio in fifth gear for the T5 transmission which is 0.68 rather than 1.00 which is fourth gear for the Toploader 4-speed and also top gear (3rd) for the Toploader 3-speed.
EFI = electronically fuel injected.
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