Best cruising speed for highest MPG.
 

I was curious to see if anyone has figured out the best HIGHWAY speed to set cruise control on for the highest MPG?

I think for cruising (in 6th) driving at the lowest speed possible without stalling. IIRC there was a study some years ago showing that, on average, cars were most efficient at around 45mph. It would be nice to know what the specific range is for the twins.

Would you believe in 1100 miles my car has yet to hit 65? It's been about 62 or 63 indicated. I never need to use a highway.

Anyway, I believe I've heard using Cruise Control can actually negatively impact MPG. Depends on a few variables. As far as the best speed overall on a highway, I'm going to guess 65 mph, but be willing to lose a little speed on inclines and pick up a little naturally on declines.

This is not a matter of opinion - it's basic physics.
 

It's physics, not opinion. You need to determine the engine speed that is most efficient for gas consumption. Then, find the speed at which the highest gear the engine is turning at that number of revolutions. Set the cruise control at that interval.

Of course this assumes a level road. Under changing elevation, you may have to downshift to maintain the correct engine speed without lugging.

My off-the-cuff guess is that optimum fuel consumption, without lugging, is probably around 1800 rpm, or roughly 45mph in 6th or thereabouts. This is obviously sub-optimal for normal driving.

If you drive above 45mph wind resistance increases at a much faster rate, so driving faster will increase fuel consumption regardless of rpms.

Maybe we should limit the scope of this to highway driving then?

Highway, the slowest speed you can maintain safely and smoothly. You don't want to be going too slow that hills drastically slow you down nor faster than necessary. Best MPG is going to be like Havoc and Frisson said, typically 35mph-45mph in OD but varies depending on conditions.

BTW, topics like this should go in Tech area.

Pick the lowest highway speed that you are comfortable and safe driving at, and adjust upwards if conditions warrant it to maintain the general highway flow. Use cruise control, but keep in mind that it will cause the car to consume extra gas to maintain speed on hills. If you have the opportunity, slow down on hill climbs and allow the car to speed up on descents - that can save fuel as well (note: do not coast or put the car in neutral). Using cruise control on relatively level and uncongested highways will usually boost efficiency.

You might want to dig up a test that Popular Mechanics performed measuring the most efficient way to accelerate a car to highway speed. They found that accelerating slowly is not the most efficient method. Instead, they recommend getting to your terminal speed at twice the maximum speed of the car. In other words, if your car goes from 0-60 in 7 seconds under maximum acceleration, you will get the most efficient performance by going 0-60 in 14 seconds. They found that the gains in getting to the highest gear quickly is more efficient than getting to it as slowly as possible.

Should be fairly easy to figure out. Just get on a flat stretch of freeway and set the cruise control at different speeds for a mile or so each, in increments of 5 mph, and see what the instantaneous mpg readout is. My guess out of the above choices is 60 mph.

Cruising has a time factor involved... does no good getting better MPG at a much slower speed if you are going to be on the road for possibly DOUBLE the time compared to a faster speed that isn't as efficient - but not 1/2 as inefficient. Stick to as fast as you can on interstate/long distance travel without excessively consuming fuel: it's a compromise/balance of time and fuel consumption.

On my Tacoma, cruise speed is ~3k rpm which works out to about 80mph...that gives 27mpg on 1,000+ mile interstate trips, and the Tacoma is an aerodynamic brick. I'm sure slower would be better for the actual fuel consumption rate. But the fuel consumption rate is only 1/2 the equation for cruising....you also have a time factor for the time spent on the road while consuming fuel; by going slower you are increasing the amount of time fuel is being consumed.

45mph average for 1000 miles is ~22.2 hours
75mph average for 1000 miles is ~13.3 hours

Is burning fuel for ~8.9 more hours really saving much more fuel? Only if the fuel consumption rate is at LEAST 1/2 of what it is at 75mph... and I don't see how the Gallon/hour rate would be less than half if you got say 28mpg cruising at 75-80mph and 35mpg at ~45mph. I'd much rather save 9 hours than a few gallons.

45mph on the interstate is not cruising, it's more akin to hyper-miling. Cruising is being somewhat efficient with the fuel by keeping the motor in it's most efficient torque rpm range while also being efficient with the trip time - in other words you are trying for as short a trip time as possible without burning through fuel at too high a rate.

Generally cruise speeds should be ~75-85mph but the speed is really determined by the torque rpm the motor likes the best for sustained higher speeds. All my vehicles have gotten EPA or better rated highway MPG at these speeds. I also don't use cruise control...cruise control doesn't work correctly for the ups and downs of overpasses and hilly sections. You should carry excessive speed into the base of the incline with gentle acceleration before hand and use the incline to slow the vehicle back down to crusing speed via engine braking and either maintain speed going down (if going to be flat for a while) or use the declining slope to build up the potential energy (if another incline is coming up). Only if on flat terrain I will use Cruise Control...but there's still traffic that needs to be negotiated which requires kicking off the Cruise control to keep from being shoehorned behind 18 wheelers and other "turtle racers" and such [now those are really annoying things that will really eat at your MPG's! Keep right people and don't take MINUTES to pass somebody while both of yall are going 10-20mph below the flow of traffic!]

Airplane "cruise" speeds are not at the stall end of their operational airspeed envelope but rather a whole lot closer to their Vmax speeds.

How does time figure into a miles per gallon/fuel efficiency discussion? You have miles driven and you have gallons of fuel burned. There is no time factor in that equation.

The issue of what your time is worth, especially on long trips, is another discussion entirely.

It depends on the drag coefficient of the car but it is probably somewhere around the 50 mph range. Once the car starts pushing air rather than sliding through it the amount of energy required to achieve each additional MPH grows exponentially.

An interesting automotive factoid that demonstrates this is that the Bugatti Veyron only requires 270 hp to reach 155 MPH but needs the other 730 HP to reach it's 253 MPH top speed.

1mph in 6th gear manual car :)

I dont know how accurate but when i was going 75 on the interstate my read out was showing 37 mpg as the average.

The minimum posted speed limit will get the best mpg, if you can do 45 in 6th it will be fabulous.

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.

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.

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.

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

Survey Says!
http://images.thetruthaboutcars.com/...urvey-says.jpg


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.

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

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!

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.

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.

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.

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.

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.

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.

Not terribly scientific, but while I was out today, I spent some time with the cruise set to 50, 60, and 70 mph in 6th gear on a flat stretch of road. Thanks to traffic and a 55 mph speed limit I wasn't able to hold the lower and higher speeds as long as I'd have liked, but was able to do so for at least a mile. I used the average mpg instead of the instant mpg readout, since the instant readout bounced around too much, and reset the mpg display each time I changed speeds. In general, there was a 3-5 mpg drop in mpg each time I increased speed by 10 mph. 50 mph was the best of the speeds I tried.

I don't think that's right. The DI never stops firing, you can hear it at idle. You might have it backwards... at high RPM, the port injectors stop firing.

Low RPMs: DI and Port simultaneously fire
Mid RPMs: DI and Port both fire but staggered
High RPMs: DI only

Fuel mileage is figured regarding gear, speed, powerband and duty cycle (throttle/Injectors).

I would imagine the best mileage is going to be in top gear (6th our case) so the most efficient speed would probably be at the slowest speed we can travel in 6th gear without bogging the engine or requiring an unnecessary depression of the throttle (bogging). I'm guessing this to be round 40-45 mph as this was about the lowest speed the car was comfortable with in 6th gear on a flat road.

okay I didn't buy the FRS or hypermile it......but I did buy it because I figured it would get good real world mileage...and it does...

My typical 300 mile trip on the freeway averages 32.5mpg and I drive FAST...typically 75-85mph.....as traffic allows....

AT I assume? I'm currently at 27.9 avg. I drive mostly highway miles but I admit that my accel is not gradual and my shifts not at lower rpms.

i dont know why i only got 10.1 avg Is that the MPG reading? How come i see people with like 2XMPgs... .... =[ is it because i mostly drive in town and get a load of stop signs and intersections? How can i improve on the mpg.. i don floor my car either and i slowly build up speeds too... i always see the people infront of me get ahead at intersecions before i finally reach their speeds... i usually go on road from 40km/h - 80km/h... please help

The question is gallons per MILE not gallons per HOUR.

Popular mechanics article on tips for fuel economy
 

I found a relevant article http://www.popularmechanics.com/cars...-this-weekend/.

Two questions grew from my reading of the article.

1) Your example of taking 7 seconds instead of 14 seconds makes some sense, but I did not see how to get that exact result from the link. Perhaps, you were just trying to give a rule of thumb you invented? The graphics in the article were a little hard to understand, imo, and they do not tell whether the tests were made with an automatic or a manual transmission, e.g., , but they do seem to suggest the general idea of your comment is correct.

2) The link article says that leaving the car in gear and descending a hill is more fuel efficient than putting the car in neutral and indicated that newer cars have a special fuel shutoff that works only when in gear and the accelerator is not engaged. First of all, do the twins have that feature? Also, one of the comments says that this turned out not to be true for the commenter who apparently experimented on a hill using the car's mpg sensor. Any comments?

Brian in Atlanta

You guys getting 30MPG need to start a newsletter or something. I'm thrilled when I get 25.

"Whenever possible, anticipate that a light will turn red and ease off the gas. Generally, the less you have to brake, the better your fuel economy."
The typical driver around here is either too stupid or too distracted to do that.

"But it turns out that nursing your speed up to the limit too slowly also lowers mpg."
Flooring it is always the answer. Not does it give you some space between you and the idiots, it saves gas!

I get 32 on a regular basis. Dropped all the way down to 31 with the snows on and cold weather.
Mind you that is at 98% highway driving with cruise set at 75!
When I do drive city I do exactly like you said above with the addition that I actual row through all the gears down shifting with my foot off the gas. This keeps my revs up if I need to move quick but I am not using any gas just compression.

Anybody that coasts down a hill in neutral is a tool anyway! You just turn your nice shiny sports car into a soapbox racer with no hope in hell of reacting fast enough if something goes wrong and you need to power out of it!

31.5mpg is my best record with PSS. I imagine the result would be closer to 35mpg if I had the primacy hp.

If you granny drive, you can easily get 35. I got 36-38 during my break-in period, and that was about 50% city / 50% 75-85mph freeway. Helps to have an automatic.