Heavier car = More MPG on the highway?

[Palm]

If you were to drive to a location and back the same way, wouldn't a heavier car = better fuel economy? Of course this is provided you don't brake or use engine braking. My thinking behind this is that you use more engine power on the uphills which boosts engine efficiency and reduces pumping losses etc. Because of the higher weight you also have a correspondingly higher kinetic energy at the top of the hill enabling you to coast in neutral further than with lower weight.

Is there some merit behind this way of thinking?

Also, instead of the technique used by some to drop speed on the uphills and gain it on the downslope, wouldn't it make sense to use the throttle generously on the uphill and then also coast downhill of course? Yet again for the increased efficiency.

[P-hack]

if you can still stay off the brakes, some extra weight isn't much of a penalty. Though you will see a bit more rolling resistance. It depends very much on circumstance too.

[Frank Lee]

When you need more power to get up the hill you need more fuel. Recapture is never as good as not expending in the first place. And rolling resistance will be somewhat higher.

[t vago]

Engine size would really play a large part in this, too. An engine that is properly sized to the weight/mass of the vehicle it is propelling, rather than an engine which capable of much more maximum power than is warranted, will also help fuel efficiency.

[Palm]

Originally Posted by Frank Lee

When you need more power to get up the hill you need more fuel. Recapture is never as good as not expending in the first place. And rolling resistance will be somewhat higher.

Yes I can see now the problem with my way of thinking. Even at load, a gasoline engine is inefficient and extra kilograms will not give back as much kinetic energy as it takes to get that weight up the hill in the first place. These things can be confusing at times!

Originally Posted by t vago

Engine size would really play a large part in this, too. An engine that is properly sized to the weight/mass of the vehicle it is propelling, rather than an engine which capable of much more maximum power than is warranted, will also help fuel efficiency.

Yes! Most engines today are very overpowered if the user isn't going to pull caravans or drive on the Autobahn etc.

Theoretically I guess it would be best with an engine at full throttle all the time? In practice though the problem would probably be that most cars have fuel enrichment at very high throttle levels.

[Frank Lee]

Maximum engine efficiency is found at as full a throttle as possible without triggering the fuel enrichment schemes. I generally figure the sweet spot load range to be 60-75% of maximum rated power.

[ECONORAM]

Originally Posted by Frank Lee

When you need more power to get up the hill you need more fuel. Recapture is never as good as not expending in the first place. And rolling resistance will be somewhat higher.

I am seeing this first hand with my RAM. The aerodynamics are improved over OEM, but when I hit a hill the fuel flow climbs. If all my roads were flat, that'd be great!

One thing on keeping the load applied with the throttle; since I put a warm air intake on my truck I've noticed it won't hold as much load. It forces me to pay more attention when passing or climbing.