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I'm not so sure on speed killing MPG but I have a very different experience with it. My Monte used to only get about 15-18 mpg with the original engine/trans and 3.73 gears. One year I needed to do a lot of driving for a summer job/internship so I found a 2.29 rear from another car and threw it in. Doing the math to see what RPM was 65-70 I was cruising at like 1600RPM and the engine was lugging and got terrible mileage. So I sped up to whatever speed was 2000RPM and jumped up to 25MPG consistently. So faster was more efficient but more so because of BSFC of the engine than the speed itself.
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-Mike
2007 Ford Focus ZX5 - 91k - SGII, pending upper and lower grill bocks - auto trans 
1987 Monte Carlo SS - 5.3/4L80E swap - 13.67 @ 106
2007 Ford Focus Estate - 230k - 33mpg - Retired 4/2018
1995 Saturn SL2 - 256K miles - 44mpg - Retired 9/2014
Cost to Operate Spreadsheet for "The New Focus"
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