Better for MPG: rolling hills or flat terrain?

Gang,
Traveling to and from work got me wondering while in transit. What topographic condition do you think is more efficient use of gasoline?

Here's my thought: rolling hills.
Why:
You seex when traveling on flat terrain, you cannot use gravity to your advantage and inertia induced energy is basically wasted.

When you go uphill, you can ease off thrust and use vehicle inertia to climb. As a side effect, losing speed gives you less aerodynamic drag. Once you reach peak and start heading downhill, you can use as little thrust as possible and let gravity help accelerate the vehicle downhill.

Of course, this takes careful choreography and concentration to get just right, but in my belief, one may actually be more efficient on rolling hills vs driving constantly on flat terrain.

Thank you! Please share opinions!

 

If you want to hypermile your truck, this is the truck for you. It's even a dually.

 

That thing is SWEET! I would buy and make my kids drive it around

To answer the original question… Flat ground.

Flat ground let’s the engine operate in a steady state where fuel/air can be close to stoichiometric. Pulls hills - even when backing off the throttle a bit - is asking for an increase in power which usually runs rich in order to not go lean. Plus, it adds more heat which has to be cooled with either electric fans or engaging the fan clutch. Flat landing typically slows for less heat and the airflow through the radiator supplying enough cooling.

 

Very interesting experimental automobile @GODZILLA ! I was trying to search for those narrow tiny WV beetle type little pickups that I see sometimes driving around(mitsubishi?). They're literally the size of that tunDARRR Chinese pickups, but gasoline powered. If they had one on Amazon for like 2k, I'd probably pick it up!

@blenton , that's a valid point, but I do believe gas is cut off when you let go of throttle on newer vehicles. It was super pronounced on my Suzuki xl7(2000) from a while back. Even engine tone was noticeably changing when letting go of throttle.

 

I thought about the fuel cutoff as well. I don’t think it really kicks in except for high braking load and higher rpm after 30 seconds or so. I don’t recall exactly where that little tidbit came from, so I may be totally wrong. But I’m pretty sure the fuel cutoff doesn’t happen just because you are going down hill. And being rolling hills, I interpret that as lighter grades instead of real climbs that trucks pull at 35 mph.

 

I get my best mileage in mountainous conditions. When it's steep enough (>6% grade or so) you can be totally off throttle on the DH and the system is in Fuel Cutoff.

 

If you have an OBDII reader you can see that Fuel Cutoff happens quite often. Usually above 1500rpms and it's "instant". To be fair, it probably doesn't happen quite as often on rolling hills.

 

Interesting. I’ll have to see if I can find that in one of monitors.