# What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

What is a realistic accuracy & precision of typical MPG measurements when measured by the consumer using the typical method of dividing their tripmeter miles by the gas-pump gallons during fillup?
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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 curiosity.
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Vic Smith wrote on 7/20/2017 8:29 AM:

+1
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.
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Rick C

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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 altitude.
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snipped-for-privacy@snyder.on.ca 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.
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Rick C

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On 07/21/2017 07:47 PM, snipped-for-privacy@snyder.on.ca wrote:

I'm a fairly economical driver but on longish trips I'm more concerned with getting there. 80 mph guarantees the fuel economy is going into the dumpster.
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rbowman wrote on 7/22/2017 1:29 AM:

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 mileage rocks.
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Rick C

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On 07/22/2017 07:22 AM, rickman wrote:

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.
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On 7/22/2017 1:38 PM, rbowman wrote:

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.
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On 07/22/2017 11:52 AM, Ed Pawlowski wrote:

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.
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wrote:

Where is their limit 55?
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On 07/22/2017 10:45 PM, Bill Vanek wrote:

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.
http://www.oregonlive.com/commuting/index.ssf/2016/06/oregon_rolls_back_speed_limit.html
but according to this the limit is now 70 on rural roads:
http://www.speed-limits.com/oregon.htm
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.
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On 07/23/2017 1:12 AM, rbowman wrote:

...

...
They're not the only seemingly bizarre place--between Clayton and Springer is 100 mi of open country with either 55 (or _maybe_ 60) that makes no common sense at all...
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dpb posted for all of us...

Where does one find common sense in da govt?
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Tekkie

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On 7/24/17 3:19 PM, Tekkie® wrote:

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
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The taxpayers are sending congressmen on expensive trips overseas. It
might be worth it, except they keep coming back.
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rbowman wrote on 7/22/2017 1:38 PM:

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.
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Rick C

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It is true that air resistance goes up a square of the speed, but the power requirement, and the corresponding rate of fuel consumption, goes up as the cube. Work=force*distance, Power=force*speed.
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root wrote on 7/24/2017 1:00 PM:

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?
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Rick C

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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 gearing
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snipped-for-privacy@snyder.on.ca wrote on 7/24/2017 9:46 PM:

What was the lowest speed you could use 5th gear in the 75 car?
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Rick C

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