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?

Reply to
Mad Roger
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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.

Reply to
Vic Smith

Repeatabilty is terrible. Accuracy can be pretty good over multiple tanks. Can be pretty good even on single tanks IF there is a way to ensure the tank is always filled to EXACTLY the same point (like a level in the fill - tube, with the vehicle parked at EXACTLY the same place for each fill-up). Relying on the auto-shutoff of the pump can cause variance of several liters per fillup.

Reply to
clare

My year old car gives constant mileage and appears very accurate when I calculate based on fill-up. It even has a moving mileage meter going up to 80 mpg when just coasting down hill or maybe 10 mpg going up hill. I recall many times on long trips mileage varying all over the map probably because I was filling up at different stations.

Reply to
Frank

IF the odometer is accurate and you do the math out to the 10ths of a gallon the pump shows it can be VERY accurate.

Reply to
Steve W.

Vehicle position or the auto shut off point won't make any difference. You read the amount of fuel you pumped off the pump itself. The only real issue is odometer accuracy. That can vary with tire size variations and factory calibration.

Reply to
Steve W.

If you just do it one time, you can not be sure you put in the same ammount of fuel that was taken out.

If you keep a running total of the ammount of miles and fuel over several tank fulls , the ammount of fuel will sort of average out.

Say you park so the back of the car is up hill and you fill the tank. Go a number of miles and fill up again. This time the back of the car is down hill. You may burn out 15 gallons, but only put back in 14 gallons. Ot it could be the other way around and you burn 14 gallons, but only put back in 13 gallons. From tank to tank full there could be a large variation. Over many tanks, the variation will average out to a lessor error. After say 10 tanks used you only have to contend with one or two errors caused by the exact ammount of fuel put in the tank. Probably just the last tank full would be where the error would come in. So instead of 1 gallon of error like the example above, you would have about .1 gallon of error if the pumps are correct, which they should be.

Reply to
Ralph Mowery

If you're getting MPG on a long trip, you only fill the tank all the way when you start and when you finish the trip. All the other gas stops you go by the meter reading. Very little room for error if you write down the meter reading.

Reply to
Vic Smith

Trip meter miles depends on circumference of driving wheels. I know my speedo closely matches readings of roadside radar displays or my GPS, so I guess trip meter miles will be accurate too.

Reply to
Mike Coon

10%
Reply to
dsi1

No, steve. You are wrong. The amount of fuel you put in is the amount you can squeeze into the empty portion of the tank. The amount you used is the amount that used to be in the tank. You need to fill it to the exact same point each time to get an accurate reading. You may have filled your 72 liter tank to only 71 liters the last time you put in 50 liters to fill the tank. Now, at a different station, with different levels, you may squeeze in 73, or only 68. COSISTANCY is the key - and where most will fall down, because, like you, they just don't REALLY understand.

Reply to
clare

If the first and last are identical, none of the others matter. The difference of 1, or 10 liters spread across many tanks becomes , more or less, just noise. On the short term, like 1 tank, it can be a pretty large percentage of error.

Reply to
clare

You buy gas by the gallon and mileage is miles per gallon. Summer gas has higher density so you get more gas by weight for your money but it costs more per gallon in the summer. Sounds like you can't win.

Reply to
Frank

It is not pointless.

A large drop in mpg can indicate an engine problem.

Andy

Reply to
Andy

Every reading a mom and pop does has inaccuracies that, I posit, are tremendously higher than most people seem to think they are (at least most people who quote mpg figures with decimal places in them).

Most people have a tripmeter reading and a gas pumpmeter reading. Where they fill the tank and reset the tripmeter before driving away.

I can't find any reliable source that says what the accuracy or repeatability of that mom-and-pop tripmeter/pumpmeter calculation, but basic logic dictates that the errors compound such that there is likely (IMHO) no way to get anywhere near decimal-point accuracy, and worse, probably plus or minus 1 mpg is the closest anyone can get in terms of repeatability and precision.

Even the EPA's $360,000 machine only claims plus or minus 2% of the indicared reading. I can't find where I got the notion that a mom and pop can't possibly get closer than about 4% with a tripmeter/pumpmeter mpg calculation - but I'm still seeking those numbers as we speak.

Reply to
Mad Roger

On 07/21/2017 11:51 AM, Mad Roger wrote: ...

...

Why do errors compound in your view?

And, it depends on what you mean in terms of accuracy -- in terms of absolute one needs to know the calibration error of the odometer; most folks are satisfied to just assume it's close enough for the purpose.

If you look at simply a single fillup, it's not unreasonable to expect a few tenths of a gallon difference between the first fillup level and the subsequent; if you try it on shorter distances than a full tank then the fractional error goes up.

OTOH, if one keeps track over longer periods of multiple fillups and take some care to use the same filling pattern and only fills up after using near the full tank capacity, then over time plus/minus targets _will_ tend to cancel out and I have no qualms in believing a relative performance number in the 0.1 mpg can be determined.

As noted, I've done this on long trips a number of times (generally on first trip or so with a new vehicle, either actually new (rare) or (most often) new to me) just to see how it compared with previous and have had quite good comparisons on recent ones with the computer-computed results. These would be over total distances of 1500 to 2000 miles, not just 20 miles test runs.

Reply to
dpb

+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.

Reply to
rickman

The EPA doesn't have the time to do accurate MPG numbers Plus/Minus 2% is good enough for the EPA. I'm sure my "mom and pop" MPG number are more accurate than that. But so what? The MPG I get depend on the driving circumstances. For instance, I've measured my MPG on a number of cars on 3000 mile round trips to Florida. I can tell you the EXACT total MPG I got on those 3000 mile trips because I carefully noted the exact metered amount of gas I used, and I verified the odometer accuracy using mile markers. The only real useful thing that gives me is my MPG for the entire trip. That includes local traffic when getting off the highway, and my travels at my destination. But I know my approximate MPG at steady highway speed because I sometimes do tank to tank calculations by filling to the filler tube. That too is an EXACT calculation, but is still only approximate MPG because maybe the terrain and weather may vary. So before you ask about "accurate MPG" you have to define what that is.

Reply to
Vic Smith

I get 17 MPG most times. On longer trips, I'll get around 22 MPG round trip. One way, it can be as high as 27 MPG. All of that doesn't matter because a tank of gas will give me 17 MPG consistently.

Reply to
dsi1

s good enough for

trips to Florida.

I carefully noted

acy using mile

at my destination.

do tank to tank

tion, but is still

My guess is that the EPA gets very accurate mileage numbers. They use the s ame startup/warming procedures and routes for the cars they test. They prob ably compensate for temperature and weather and other factors. They know ex act distance traveled and the exact amount of gas used. These guys are whit e coat scientists and technicians. Why would you think your mom would do be tter? I'm sure she's a nice lady though.

Reply to
dsi1

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