Close to 100% accuracy if done right. I've done it on long trips. But MPG will vary
depending on terrain, weather, wind direction, stop-and-go traffic, etc.
So if you want "true" MPG for your car, you have to do it for the life of the car.
Once you do it initially, it's kind of pointless to do again except to satisfy your
I measure my gas mileage on every fillup. I get 19 to 20 MPG every fill
unless I do a lot of around town driving. Very consistent. I watch it to
see if it drops off which would mean something is wrong.
Occaisionally on a longish trip I'll see how well I can drive for
economy - to see if I can better the last time I did that trip. This
is generally over pretty close to a full tank - and small differences
in driving technique can make a HUGE difference. So can a small change
in route.Or a difference in the wind. I've registered a good 25%
difference in mileage between 2 trips, both trying to squeeze the last
foot out of a liter of fuel. Round trip averages out the difference in
firstname.lastname@example.org wrote on 7/21/2017 9:47 PM:
Nearly all my driving is on secondary highways so I pretty much am driving
at pretty optimal speeds for mileage although there are some traffic lights,
they tend to be miles between stops. I have developed fuel efficient habits
so I nearly always squeeze every last MPG on my trips. I have a manual, so
I slip it out of gear and coast to lights and nearly always accelerate
gently. I leave a lot of room to the car in front so I can ease up on the
gas rather than hit the brakes. I think I am doing about as well as can be
expected all the time, so my mileage seldom varies unless I do more city
driving. High 19 or low 20 MPG, very consistent.
I forgot, I can tell the difference in fuel economy by driving 65 MPH rather
than 60. Driving at 65 very much (only about 1/3 of my trip allows that)
will assure that I only get 19 mpg rather than pushing 20.
There is a 10 mile stretch with only one traffic light and a posted speed
limit of 45 MPH. If I can get up to 50 so I'm solid in fifth gear my
I should look at the instantaneous readouts versus mph to see if the mpg
falls off gradually or if there is an efficiency sweet spot around
65-70. Except for around the cities the interstate speed limit in this
and some of the adjoining states is 80. Drive 65 at your own risk.
I tried that one day on a flat stretch so there would be little
variance. This was on my regular trip to work. Speed limit is 65. One
day I did 70, the next 65, then at 60 is was dicey, the next day I tried
55 for about 30 seconds and decided not to risk my life.
I forget the details, but 60 was better than 70 by a couple of mpg.
Problem is, I prefer driving 75. If I could get away with it I'd go 85+
but don't want to pay the fines.
At under 70 my car usually is in the 35 mpg + range; at 80, it is more
like 32. I get even better mileage in Oregon with its 55 mph speed
limit. I also get bored out of my mind. There isn't a whole lot of
anything between Ontario and Bend but I figure as soon as I get up to a
decent speed a OSP cruiser will materialize from the sagebrush.
That stupid speed limit is the least of Oregon's problems.
The last time I was there US20, US395, and other 2 lane roads in eastern
Oregon. Apparently the raised it to 65 in March of 2016 but are rolling
it back in some places.
but according to this the limit is now 70 on rural roads:
70 on Rt. 20 would make a lot more sense if that is indeed what it is
now. I'm not planning to check it out personally though.
Yeah, as @patsajak noted (and especially in uber-liberal states like
Oregon), politicians have learned that it's more fun to over-control
people and tell them how to live than it is to fix the potholes
The taxpayers are sending congressmen on expensive trips overseas. It
might be worth it, except they keep coming back.
Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.
You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?
No, because the speed doubling takes only half the time, but 4 time
the power. Not necessarilly 4 times the fuel, because the engine may
be "on the cam" at the higher speed, running more efficiently.
An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.
Didn't work on the 1976 model - same body (and engine) but different
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