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

Of course vehicle position and shut off point makes a difference. If you fill it up in a situation that winds up with more gas in it, then fill it up after your run in a situation that winds up with less gas in the tank, you think you used less gas than you actually did and you get slightly higher MPG.

Reply to
trader_4

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

This thread reminded me to run-the-numbers from my car's log book. I record each fill-up and a few times a year I'll plug the numbers into a home-made spreadsheet. I'm not sure if my car even has a built-in calculator - I'll have to check .. my brother's Buick will always show about 1 mpg better than reality.

in liters per 100 km my last 10 fill-ups are :

6.5 8.0 7.3 7.9 7.0 7.4 7.4 7.4 7.3 7.4 avg = 7.36 The adjacent high/low values indicate that the fill-up was filled-to-the-brim or not-quite-full due to the pump sensivity.

Interesting to compare winter driving with winter tires :

8.3 8.7 8.2 7.9 7.7 8.0 8.5 8.3 8.0 8.0 avg = 8.16 2015 Kia Rondo 6 speed auto ; 2 liter gas ; rated 10.6 / 7.6 city/highway at Natural Resources Canada
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John T.

Reply to
hubops

Agree. I was thinking of filling it full once.

Here's related question. Many new cars measure and show MPG. How do they measure how much fuel is being used? It's not via the tank gauge, because it can change almost instantly. Do they have something that measure fuel flow directly? Or do they do it by knowing how long the injectors are open and using that? Anyone know?

Reply to
trader_4

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

Based on my checking over a tankful and what the meter says, they just take an optimistic guess. It is always higher, but there seems to be an explanation

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Roger Clark, senior manager of GM's energy center, explains that the fuel economy gauge makes a calculation by counting the number and duration of pulses made by the fuel injectors as they squirt gasoline into the combustion chambers of the engine. The onboard computer system divides the distance the car travels by this estimated fuel consumption.

Clark says the gauge is "dead nuts accurate" ? if you consider all the variables at work during driving, including temperature, driving conditions and driving style. The biggest fluctuation occurs because ethanol, which is blended with gasoline in varying amounts, contains less energy.

"When you fill up, you are paying for a gallon of gas, but the energy in that gas varies significantly," Clark says. This means that while the car's computer assumes the gasoline is providing energy to drive a certain distance, the fuel might have less energy and not propel the car as far.

Reply to
Ed Pawlowski

On 07/20/2017 11:59 AM, Ed Pawlowski wrote: ...

I think that's hogwash, too. The mpg is volume/distance; the computation is independent of the energy content; simply a higher consumption rate/lower mpg value will be computed.

_Unless_ they don't use actual odometer reading and distance is imputed from other measurements at least one of which is based on engine efficiency under given conditions.

I don't have inside info on the precise algorithm, I'd always figured it was, in fact, simply the instantaneous volume*t integrated and the accumulated is simply the ratio of the two sums (integrals) of fuel volume over total distance since totals were last reset.

The Buick longer-term values have always worked out to be pretty-much identical to logbook records on long trips for several different models from the old LeSabres to the (relatively) new Enclave and Lucerne even though they're now getting long in the tooth by car sales standards...

Reply to
dpb

Why is it hogwash? They seem to be saying the same thing you are, that the energy content means that what you see as MPG will vary because of it. If you have gas with higher energy content, then you'll see 25 mpg. If it's lower energy content, you'll see 23 MPG. How much the energy content varies, when it's labeled as the same product, eg reg unleaded, IDK. It would obviously vary depending on the amount of ethanol added.

Reply to
trader_4

Seems like a computation based on a given fuel energy value rather than just volume. An accurate measure of volume consumed versus miles driven would be a consistent and accurate readout. Evidently they all use the same method as I always got high readings.

My Buicks were always 2 or 3 mpg high. I've not driven a Buick since that last one fell apart. I used to be a fan of GM until they screwed me. I should have wised up sooner.

Reply to
Ed Pawlowski

Seems to be some confusion. Sure, the pure gas will give you more distance per gallon. The computer readout though will say the same for pure gas, gas with ethanol, or a bottle of bourbon.

The readout on the computer still says 25 even though you only get 23. Sort of like saying if you had pure gas this is what you;d have gotten but you got something else.

Reply to
Ed Pawlowski

I don't see how the computer readout will still say 25 even though you get 23. Like DPB said, the car computer is simply computing miles traveled by volume of fuel injected. With the lower energy content gas it will have to use more gas to go the same distance and the number calculated will show it. Instead of 25 mpg, you'll get 23. Take it to the limits. Suppose you had gas that had very little energy content and a gallon could only propel the car 1 mile. What would it show? 1 MPG or 25 mpg?

Reply to
trader_4

No it won't. Put very low energy fuel in the car, one that can only propel the car a mile with a gallon of it. What will the car MPG computer show? In my world, it's computing how much volume of fuel it injected and how many miles it went. 1 gal, 1 mile, it's 1 MPG.

Not if it's doing MPG the rational way, which, like DPB said is simply distance travelled and gallons injected. It's not calculating BTUs injected, at least not from the description given, nor is there any reason to factor in energy content, since the car doesn't know what it is.

Reply to
trader_4

This is interesting:

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It seems to me that I am getting slightly lower mileage this summer but maybe because it has been very hot and AC is on all the time.

I prefer to store summer gas for my generator because winter gas puffs up the sealed plastic cans in summer heat.

Reply to
Frank

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