Out of the box, consumer cruise control interfaces favor simplicity over efficiency. Even though it can be efficient to maintain constant speed, cruise control wastes a lot of energy downhill by braking to not go more than 1 MPH over the target speed. If cruise control systems allowed more variation around the target speed, softer and more spread out upper and lower bounds, they would build efficiency by letting cars build momentum and store energy downhill that they can use uphill.
I developed a brainless way of implementing this without having to overthink anything. This method is much simpler than driving without cruise control, and it only takes a little more attention than using cruise control normally. Using it on a 3 hour hilly drive, a round trip from Hanover, NH to Burlington, VT, I increased my MPG by almost 10, from high 38 to low 46. I got there in about the same amount of time, but with much more variation in speed. The control trip had cruise control at 72 in a 65. I didn’t deviate from that except for the occasional car. The temp both days was around 70°. Car is a 2008 Prius.
For the method, instead of deciding on a desired speed, you decide a desired MPG and minimum and maximum speeds. That’s three numbers to think up instead of one, but you can do it in a way that’s still brainless. Set your cruise control to the minimum, fix your foot on the throttle so that you’re usually above that speed driving at the target MPG, and only hit the breaks when you expect to hit your maximum. For this trip, my target MPG was 50, and my minimum and maximum speeds were 64 and 80 (so cruise control was at 64). For the most part, my foot is setting the pace and the cruise control is doing nothing. As I go uphill, the car decides that I’m not hitting the gas hard enough and it takes over. As we round the hill it eases off and I feel my foot get back in control (even though it hasn’t moved at all). Then, using momentum built downhill, I’m usually most of the way up the next hill before the engine kicks in. Momentum goes along way, especially in a hybrid. Hybrids are heavy because of their batteries. Over three hours, I was at 46 MPG and spent most of the trip around 70MPH.
This method probably doesn’t make a difference in flat areas, but it contributes a lot in hilly ones. I don’t expect to ever hit my target MPG, but by minimizing the time spent below that, I can count on approaching it asymptotically. A hypermiler would recommend driving a lot more slowly than 70, but they’d also recommend stripping out your spare tire and back seats, so take it and leave it.
Peak fuel efficiency on a Prius is crazy low, like in the 30s I think: a pretty unrealistic target for highway driving. But if there was no traffic, and if I was never in a hurry, I’d try it again with cruise control 45 MPH, a target at 60MPG, and a max speed of 90MPH, to see if I could hit 50MPG. I haven’t stayed above 50 on that drive before, but I still think I can do it and still keep my back seat.