Best fuel economy depends on the car engine (the tachometer specifically). Any "posted" or "advertised" best speed is bunk unless its someone talking about a Mazda3. I think keeping the tach at around 2500 for our car in a coast scenario (not pulling) is best. Of course for BEST fuel economy, just drive down hill!
IMHO the sweet spot would be at the highest gear with the lowest rmp, at 70km'h at least... At high gear, the least amt of power is needed to maintain the speed with the engine working less harder at low rmp.
Maybe the cruise control engineers could better answer this question since the ECU would have all the parameters to determine the best operating environment...
You dont want to underdrive the engine either, gas mileage is usually related best to the RPM you are cruising @. If I am going 70km/h I usually keep it in 3rd which is about 3250 RPMS, that is reasonable, 4th is about 2500 (I think) @ 70 km/h.
I just drove from Calgary to Red Deer going 130+ the whole way and I thought the car still got decent mileage
Tricky question. It depends on a few variables. Think about humans running. We get the best results, time to distance ratio, when we jog as opposed to sprinting or walking. Find that speed in your car for the given terain and maitain it as close as possible. On the highway stay within 5miles per hour of your target speed for best mileage. In the city, don't sprint off then line and then jog once optimum speed is reached. Instead start out relaxed but confident, steadily accelerating to jogging speed then maintain. Same for slowing down. Don't come to a sudden stop, slow down gradualy. The engine is more efficient making gradual changes than sudden ones.
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