I think that the biggest factor determining you mileage after this change will be in how you actually drive the car. The natural tendency for most people after making a change like this is to get on the car more often because it feels so much stronger which is why people change gears and being more aggresive with the go pedal obviously drops the gas mileage. If you drove the car in a conservetive fashion after making the change, your mileage would likely not change much at all from what you are now getting. In normal driving, the better gear multiplication will require less throttle opening to get the car up to normal operating speed, which in day in day out driving helps to balance out the increased cruising rpm. This assumes that you do not do a lot of higher speed highway driving - if you do your mileage will likely drop by 2 or 3 mpg. As mentioned, IMO the use of your right foot will be the biggest factor
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1965 K code coupe - numbers matching - restoration ongoing. 1987 LX notch, 5 spd, stock heads (minor porting) and intake, 65mmTB, 3.55's, ram air, pulleys, shorty's, 2.5" exhaust with cats 13.89@100.25
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