Best cruising speed for highest MPG.

[Synack]

Also depends on if you're going uphill/downhill. Driving north up to Dallas from Houston I averaged 32 and down back to Houston it was 35.

[civicdrivr]

I averaged 31.5mpg round trip (not based off the dash read out, but miles driver / fuel volume) from VA to NY last week. That was at 70-80mph with the a/c on. Its a flat run for the most part. For comparison, my trip last month from VA to Kansas City averaged ~28mpg round trip. That was at the same speeds, but many more elevation changes.

So as FRiSson said, its not as cut and dry as a set speed in 6th.

[eriktherod]

Call me skeptical but I don't think it's as easy as saying "least rpm in the highest gear." Obviously, we'll assume sunny weather and fairly low winds and not sub-optimal conditions outside our control.

However, I do believe there is a sweet spot at some point that isn't the minimum speed. Reminds me of S2000s that got better mileage when they'd stay in the 3-4k rpm range compared to at the lower ones (please correct me if I am imagining this all incorrectly).

Think of it this way, do you think lugging around at 30mph in 6th gear is more optimal than say in 5th? I don't think so personally. So given that a lower gear may be better at some speeds than a higher one, it would only make sense that the highest gear would have a sweet spot at some point too where it is more efficiently holding a certain load at a certain rpm. I voted 70mph because IIRC, that's right around 3K rpm which FEELS like a good efficient spot in the car to hold speed at.

I suppose when I think about the physics of it, there's a certain amount of energy that's being created by combustion and used to hold a velocity at load. You have resistance of the engine, wind resistance, friction, and other factors that negatively impact your efficiency. However, at higher velocities, you will travel more distance in shorter time increasing efficiency (more miles at the same amount of gallons = more mpg). For example, maybe you use 10% more gas but travel 20% more miles, so you gain efficiency still. I believe the biggest variable here is how the engine was designed.

Then again, I could be entirely wrong and 55-60mph could be more efficient.

[#87]

With every car I have driven, the mileage decreased as speed increased in the highest gear. Mind you I have never driven any true sports cars before and the closest to this car would be civic si.

And of course the responses are assuming you are not lugging the engine...

For me the lowest speed I can tolerate without wanting to gouge my eyes out on journeys covering 100s of miles is 60-65 mph

[rice_classic]

Survey Says!



45mph theoretically but 55 within reason on a freeway. I chose 60 because of the options on the poll, that's the closest one to being right.

[regal]

This car is different, below 3k the direct Injection drops off like a rock and the port injection takes over. It may be interesting to find that the typical 55mph highway efficient mpg is burning more mpg than 65 mph in this car.

[#87]

Originally Posted by regal

This car is different, below 3k the direct Injection drops off like a rock and the port injection takes over. It may be interesting to find that the typical 55mph highway efficient mpg is burning more mpg than 65 mph in this car.

The car has an instant MPG readout which is accurate enough to test these ideas out. We should all stop being so lazy lol

[eriktherod]

Ooo, right. I forgot it does have that "instantaneous MPG" readout. I wonder how accurate it is. I guess it's time to set cruise control for various speeds and see what I get over the course of a couple (relatively flat) miles!

WE HAVE THE TECHNOLOGY!

[GNS]

1) 6th gear at the speed limit, or 10 over the speed limit is perhaps not the most ideal way to max fuel efficiency, but it's the most practical

2) Speed up when coming up to hills, in order to build momentum while you're still on relatively level ground. Do it gradually and ahead of actually reaching the hill, then let off the gas (car still in gear) and let momentum do the work for you.

3) Don't get stuck behind people slower than you, that forces you to use the brakes, which wastes your momentum.

[Dadhawk]

Originally Posted by eriktherod

Ooo, right. I forgot it does have that "instantaneous MPG" readout. I wonder how accurate it is. I guess it's time to set cruise control for various speeds and see what I get over the course of a couple (relatively flat) miles!

WE HAVE THE TECHNOLOGY!

I made an (admittedly unscientific) attempt at exactly that, right after I got Olivia. The results are are here. I had planned to try and duplicate the results later under more controlled circumstances but never got around to it.

What we need is someone with access to about two miles of relatively flat, straight road where you can have a half mile run up to stabilize and then run the test in both directions to account for wind and road variances.

[Xenith]

I recall reading in a Thermodynamics textbook this past Spring that the optimum speed for fuel efficiency is 55mph (typically) and then fuel efficiency decreases 10% every 5mph higher you go. I'd have to double check that but I am pretty sure that's what it said. (this was a newer textbook from 2010)

As for my own experience, I drive a lot on the highway at 65mph and my average mpg has been between 33-34mpg for all of my 1900miles driven.

[EsoBOFH]

It's been stated elsewhere that Toybaru deliberately put the torque/hp dip at 3000 rpm to hit fuel economy targets at cruise. So for this car, I would say being in that range in 6th is going to give you good economy - probably not the best available for these cars when simply deciding on the facts/science of it all, but in terms of usable economy (going the speed you want to go on a highway) - i'd stick around this area, which is essentially 100km/h (60mph) in 6th.

[eriktherod]

Right! I don't think any general statement like "all cars are most efficient around 55mph" can be correct, especially with today's transmissions with more gears and better aero. While that may be true for "most" cars, I don't think it can be apply to any 6-speed car in the last 5-6 years.

[Miniata]

Originally Posted by Dadhawk

What we need is someone with access to about two miles of relatively flat, straight road where you can have a half mile run up to stabilize and then run the test in both directions to account for wind and road variances.

That's just about all we have around here, flat, smooth, straight roads. I'm getting ready to load the kids in the BRZ and run into town to get my license plates for the car, so once it gets up to operating temperature, I'll see if I can do some testing with the cruise set at various speeds to see what the onboard mpg readout says.