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Old 12-20-2019, 02:53 PM   #2 (permalink)
mpg_numbers_guy
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Location: Michigan/Virginia
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Insight - '06 Honda Insight MT
Team Honda
Gen-1 Insights
90 day: 81.29 mpg (US)

Mom's Prius (my driving) - '08 Toyota Prius
Team Toyota
Last 3: 60.96 mpg (US)

Insight Delivery Driving Log - '06 Honda Insight MT
90 day: 70.7 mpg (US)
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It depends on the car. In my Insight, I suspect that cruise control would cost me 10-20 mpg on trips (normally gets 80-100+ mpg, so 10-20 MPG loss isn't that much). However, on my mom's Prius I find that the cruise control actually improves fuel economy by a decent amount over driving without it.

I suspect that most modern CVTs fall somewhere in between. Part of the reason cruise control reduces fuel economy on most cars is because it over-revs up hills, only to ride the brakes down the hill. You don't lose economy when you accelerate, rather you lose it when you brake and turn your car's kinetic energy into brake dust.

When I had my Civic, I used cruise control on flat terrain, but turned it off on hills. If you find that there is still a fuel economy loss on level ground, try using cruise control "conservatively" - turn it off for a while, and then turn it back on for a while when your foot gets tired.

I'm surprised that the CVT runs at higher RPMs on cruise control. Is this on level ground? Cruise control on a CVT will constantly adjust the RPMs, supposedly for maximum efficiency. On hills this causes the RPMs to rise, when in a standard car you would keep it in overdrive and slightly lose speed, or do a complete downshift to maintain it.
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