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Slightly off topic but relevant.
I used to have a Ford Festiva that I did all sorts of mods to. One of the things I was most interested in was gas mileage. I found that with the stock 1.3L motor and 5-speed, and 155/80-13 tires, it would actually get these mileages, when driven at a constant speed for about 90 miles:
45 MPH = 62.5 MPG
55 MPH = 58.0 MPG
60 MPH = 52.5 MPG
65 MPH = 45ish MPG (I forget exactly)
When I replaced the stock motor with a 1.8L Mazda SOHC, bolted to the same transmission, and had some head work done on it, the HP increased 50% over the stock 1.3L, and the gas mileage only suffered by 10%, when driven as in the previous tests.
55 MPH was a good sweet spot. Not much MPG improvement at slower speeds, and you got someplace quicker than at 45 or 50 MPH. Above that speed, wind resistance and higher revs kill mileage.
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