Highway speed to optimize gas mileage


Practically, especially on a highway, that is true, but I wonder, theoretically, what your fuel economy is at 10 MPH in second gear, letting the engine idle without touching the gas, or letting it idle in third gear without touching the gas.

Our Prius at work has a dashboard display setting that will show your current fuel economy. Idling around campus, I’ve seen it go up to 100 MPG on battery-only power. I’m sure your Yaris isn’t a hybrid, and I know this is all academic, but are you sure you can’t go slow enough to make your Yaris get 80 MPG? I’m not.


our 2012 prius got 52mpg at 60mph. i went out to wyoming few yrs ago and it was 80mph there. i got 32mpg approx at 80mph. our 15’ civic gets about 45mpg at 60mph and it drops to 35mpg at 75-80mph.


I had a boxy first generation scion xb that was estimated to get 35 on the highway and it would get about 38mpg while averaging 60 to 65 on the highway. At 80mph, it would drop below 30mpg

I once drove it from Anaheim to San Francisco on the right lane at 50mph. On long hill climbs, I let the speed dropped to semi like 35mph. Everyone passed on the left without giving me the finger or a honk. It got about 42mpg on that trip. Not bad for a box shaped car that was meant for the Japanese domestic market


Everybody has their own motivation- here’s how I see it:

I’m going on a 500 mile trip.
Let’s say I can get 45mpg @ 50mph but only 30mpg @ 80mph.
That means at 50mph I will use 11.1g but at 80mpg I will use 16.7g for a delta of ~5.6g.
5.6g at today’s prices around here ($2.55) is $14.28.

Now look at elapsed time.
At 50mph, it will take me 10h but at 80, it will only take 6.25h.

So to save myself 3 hours and 45 minutes, it cost me $14.28.

I kept up with prevailing traffic, arrived almost 4 hours earlier and only cost me $14.28.
I’ll take that trade off any day…and do.


+1 to @TwinTurbo

Very clearly explained economic consequences to reduced highway speeds.


The xb wasn’t as boxy as it looked though. All the front surfaces had big curves on them. It was sort of an optical illusion


Maybe the difference is in a four cylinder versus a V6. A four is working pretty hard at 80, but a V6 is just in a happy spot.


I don’t have an instantaneous mpg readout, but from what I have learned from drivers who use fuel consumption gauges, the Yaris consumes 0.167 gallons per hour while idling. Using that amount of fuel while traveling 10 mph comes out to around 59 mpg.
You would likely have better results by using a technique that the hypermilers call “pulse and glide”, accelerating to around 20 mph at a power level that has the engine running at peak thermodynamic efficiency, then switching the engine off and letting it coast until you are down to around 10 mph, then letting the clutch out to restart the engine and repeat.
A lot of gas mileage records have been set using this method.
You are using the engine intermittently at near peak efficiency instead of continuously at an extremely low power level where engines are inefficient.
Hybrids simply switch to electric propulsion at speeds too low for good engine efficiency. Letting the electric motors pull the car up one car length at a time at the drive through bank teller lane.


Never understood that mentality. Much faster to park the car and walk into the bank. Last time I used a bank, there was a line at the drive-in window, no one inside.


The only time that I have ever used a drive-up window at a bank was about 10 years ago when my left foot was in a cast.

If somebody has a physical handicap then it certainly makes sense to use those drive-up windows, but for those of us who are blessed with good health and the use of our legs, it just doesn’t make much sense to sit on one’s butt in a car, wasting gas and inevitably taking much longer for a banking transaction.


When it’s -20 out and a drivethru is available, my butt is staying right on the seat heaters. :wink:


When it’s -20 my window will go down but won’t come back up!

I walk in and experience the majesty of nature. For 30 seconds.


Fix the window. That’s an emergency situation. :wink:

Ford, by chance? Our work Escapes do the same thing, which gets awesome when we need to roll them down to pay for parking.


2007 Town and Country. So far I’ve cleaned and lubed the channels, but that made little diff. It really needs a new motor/regulator assembly. If I need one more summer project. that would be not be my first choice, but it would be a gratifying improvement.

If it’s above about 5F, it works, especially if I’ve driven it and warmed up the interior. And parked with the door facing the sun, if present!


The shape of the vehicle is part of the formula.

Since wind direction can’t be determined…it’ll probably be a wash in the end. Sometimes wind will be with your…other times it’ll be against you. The formula is sound.


While I realize that going fast exponentially increases the effort required to push the car through the air, the biggest factor I find in my area (where there are no level surfaces) is an uphill or downhill grade. Even a very slight grade will make a difference. My '14 Civic, which is just a 1.8L four-cylinder, albeit a spunky one, has a CVT and runs 80 MPH at about 2,200 RPM. If there is just a very slight (as in nearly imperceptible) downhill it needs VERY little throttle to maintain 80 MPH, and the instant MPG readout will stay around 50 MPG if I keep a steady foot. Other than that, it really depends on the gearing and how much drag the body design and tires have.


The problem with grades is that every down grade is followed by an up grade and while you get spectacular gas mileage while going downhill, you pay for it on the next uphill where the instantaneous gas mileage can easily go into the single digits.
You also gotta remember that getting 20 mpg on a mile long uphill followed by getting 60 mpg on a mile long downhill does not result in a 40 mpg average fuel economy. It only averages 30 mpg. To correctly average gas mileage, you have to use the reciprocal of your instantaneous gas mileage, convert miles per gallons to gallons per mile and then take the gallons per mile average and convert back to miles per gallon.
This becomes obvious when you consider an extreme example. Let’s say you get 10 mpg climbing a hill, and then you get infinite miles per gallon going back down the hill using zero fuel. This does not result in a round trip gas mileage that splits the difference between 10 mpg and infinity. It actually results in a round trip gas mileage of 20 mpg.


This is one reason why the metric approach of liters per 100 kilometers is a bit more intuitive.


B.L.E…You are correct! There are some trips I make around here where my destination is at a lower elevation than my point of origin…I haven’t really studied it much, but watching the instantaneous MPG gauge and the avg. MPG gauge, I suspect that the uphills do more damage than the downhills save…


Not if you ask most people over 50 about their walk to school compared to kids today :wink: