A car going up and down hill acts a bit like a pendulum swinging or a kid on a swing..
You push the kid or the pendulum on the downward part of the swing and it slows on the upward part. Let your speed gradually slow a bit going uphill and coast or speed up slightly going downhill and accelerate some on the flat at the bottom just before you start going uphill.
The heavier and more aerodynamic your car is, the better this works. If you are driving one of those ultra boxy Scion micro vans it won't make that much difference.. My wife had an Infiniti J30 which is quite heavy and pretty slippery and with that you could definitely tell the difference in gas mileage by driving this way.
I have towed a dual axle flatbed trailer with a race car on it with my daughter's 97 Expedition that has the MPG readout from the factory.. I could get better mileage driving at 8800 lbs total vehicle weight than my daughter can without the trailer just because I pay attention to the MPG display and adjust my driving accordingly. Seventeen MPG at 60 mph and 8800 lbs is not too bad.
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94 Altima 5 spd.. Stock.. 29 mpg combined with basic hypermiling techniques ..
89 Yamaha FZR400 Crotch rocket, semi naked with only the bikini fairing, no lowers, 60 plus mpg
87 Ranger 2.3 5spd.. Does not currently run..
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