How to compute mileage (mpg)?

I was amazed the other day when I had a chance to work with a brother (16) and sister (14) at a concession stand. They both would have worked circles around almost every adult that works there.

Oh, the reason - parents own a store.

Reply to
DTJ
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You need to mark the milage at each fillup. Then take the gallons at each fillup. Take the milage since the last fillup (subtract previous fillup milage from current milage. Take gas you put in this fillup. This is the gas you put in, even if there already was gas in tank, and the milage you drove since then, so it doesn't matter how much was previously in tank.

Then; miles PER gallon. PER means divide. So divide miles by gallons.

Now, there are two trains of thought on how to handle statistics and accuracy.

One thought is to take milage at beginning of several fillups. Now keep track of gas used in ten fillups, milage at last fillup. Divide cumulative milage by total of gas used. This MAY eliminate some small measurement errors. But, it yields no statistics by which you can measure accuray.

I prefer calculating milage each time, for ten times. Average the ten readings. Now, compute variance. This is done by taking each individual milage calculation and subtracting average. Square the difference. Some all the squares. Now take square root of that sum. Sounds like a lot of work, but with calculators it is a two or three minute job.

Now, the average you compute is only accurate to within this square root, with is the basic precision of your set of measurements. For instance, I tried ten tankfuls of regular, ten tankfulls of premium, to see which gave better accuracy. The value for regular was slightly bette than the average for premium, but the amount of the difference was only about half of the variance (that square root). Thus I could not conclude that regular actually gave me better milage- I have to conclude that they are the same within the accuracy I can measure my gas milage.

Reply to
Don Stauffer

You're so harsh... LOL

Reply to
Mark

While not an issue with my Toyota, I've had several vehicles where the odometer error is significantly less than the speedometer error.

Reply to
someone

Very likely. The odometer is a pretty simple beast. The speedo is quite a bit more complex, and friction can spoil accuracy of speedometer. Cables are better these days, but in former years the squeaking or howling cable not only was a source of irritation, but destroyed the usefulness of the speedo because of its effect on accuracy. Worn springs or magnets are other sources of error.

Reply to
Don Stauffer

The very best way is to ALWAYS fill the tank everytime you get gas, then divide the distance you went, 250 miles, by the new load of fuel, 12.5 gallons. The result, 20, is your miles per gallon. Fill the tank and reset the odometer (trip meter).

I consistantly get just a bit over 25 mpg, so in 300+ miles I put in 12 gallons of gas, for just over 325 miles, I put in 13 gallons, for 275 miles, I get 11 gallons.

It doesn't matter how much gas is left when you fill up. It does matter if you fill to the brim, or only fill to the first shut-off. I usually fill to the first shut-off, then top off to the next even dollar. I used to top off to the next even quarter, but gas is getting so expensive that topping to the next dollar is less than a half gallon, and this won't change your formula very much.

So, you go to the corner today and buy gas. If you filled the tank to the top AND reset the odometer the last time you got gas, then you can divide the distance by the new load of fuel and derive the mileage. If you did not fill to the top and/or reset the odometer, then this tank will be your baseline and you can make no calculations from it. Then, in a week or so when you visit the filling station again, then you fill the tank and divide the distance by the fuel load. The result will be your mileage. If you go

250 miles and put in 10 gallons, then you got 25 mpg. All you have to do is always fill the tank, never just get $5.00, or whatever. If you do just get $5.00, then you have to add the gallons to the next fill up and leave the odometer counting as if you had not gotten the few gallons.

After you do this a few times, you will be able to quickly calculate in your head to confirm that you are getting the same mileage that you always get. I get 25 mpg in my normal driving routine.

Reply to
Jeff Strickland

Correct.

Correct.

Sorta. Drive as long as you want and refill the tank anytime you want.

You make this much more difficult than it needs to be. All that is needed is to fill the tank where ever it is convenient to get gas.

Correct.

Correct. Repeat from Step 3.

Reply to
Jeff Strickland

Start with a full tank. record mileage. Drive. At next fill, record mileage and the gallons to fill back up.

With my truck I just go to the first click. Your mileage will vary depending on the type of driving, the weather (-40 idling uses a lot of gas) etc etc... so just keep tracking it.

I use a spreadsheet and track the average and the average for the last

10 tanks. I found the last 10 tankfuls to be a good indicator of my overall mileage. I moved to a more rural area so I spent less time in rush hour so the average mpg went up by a couple, but went back down in the winter... and went up a couple mpg when I finally changed the plugs and did a tune up.

My truck usually averages around 12-16mpg with the odd highway drive above 20.

Why the average? Because unless you drive the same road under the same conditions at the same speed with the same brand of gas filled to the same spot in the tank you're introducing all kinds of "error" into the equation. Averaging that over 10 tanks will give you a more "real world" mileage.

If you have excel I can email you a copy of the spreadsheet... Ray

Reply to
ray

One normally assumes that the tank becomes equally full right after each fill-up. Therefore, all you need to do is reset your trip-odometer after each fill-up, but first, divide the number on it by the number on the pump. (In other words, the amount of gas you just put in equals the amount you used between the previous fill-up and this fill-up.)

Reply to
John David Galt

Ask your little sister, she'll help you.

Reply to
The Real Bev

Well, it /is/ slightly overkill - but if you want a little more accuracy on your fill (let's say you are doing an economy run test for a magazine or newspaper article, or just want to get more accurate numbers on each fill) it's a simple and logical way to get it.

Using the same gasoline pump means you will use the same fill nozzle, and are parked at the same level (or not level) attitude on the pavement. Oh, and you want to use the same fill speed setting on the nozzle, usually the slowest speed.

Gasoline nozzles will vary a bit between different ones exactly when they trip the automatic shutoff, but the same one will probably kick within a quart from fill to fill. This will get you a slightly more accurate fuel mileage reading than using any old pump at any old station.

The only way to get better numbers would be to install a temperature compensated calibrated fuel tank with a fill view port on the car, like the ones your local County Weights and Measures guy uses to check the calibration of the gas pumps. (Megabucks, and a waste of effort.)

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Reply to
Bruce L. Bergman

Moves lips when reading stop signs?

Reply to
Norm De Plume

It's a basic math exercise usually found in the form of word problems. For the results to have any value you have to fill up every time you do the calculation.

Number of miles driven divided by number of gallons equals the miles per gallon.

A cumulative average can be computed by using the same numbers as above but just keep adding them in and redividing. After a while the number becomes glacially slow in responding to change and a performance problem may not be noticed for several tanks.

A moving average covering some moderate time period like five fillups is probably the most useful because it retains some responsiveness to short term changes while also reflecting performance over more than one fillup. Unfortunately it also requires the most work.

Reply to
John S.

A variation of a half gallon or so on 300 miles isn't going to have a significant effect on the mileage.

Reply to
Jeff Strickland

FSVO "significant effect".

With a car that returns 30 mpg, your "a half gallon or so on 300 miles" represents a difference of 5% or thereabouts. Heck, with some econoboxes (of the European variety, at least) it could be as much as

10%.
Reply to
Robert Briggs

If a car gets 25 mpg, and goes 300 miles, it will take a new fuel load of 12 gallons. If you cram in 12.5 gallons, the mileage will be 24 mpg, if 11.5 gallons then it will go to 26 mpg. This might be significant on a single tank of gas, but over time it won't matter much.

The way to measure mpg is to fill the tank and reset the trip meter. Then, on the next trip to the gas station, fill again and divide the distance traveled by the quantity of the fuel load. Reset the trip meter, and repeat.

Unless one is doing a science project where the exact figures are critical, then getting gas at Pump 1 on one trip, and Pump 6 on another won't make any difference. Getting gas at one station (Brand X) and then getting gas at another station (Brand Y) will have a greater impact than jumping from pump to pump within the same station, or different stations of the same brand.

Reply to
Jeff Strickland

Has anybody determined what the original poster is going to do with this information? Does he really NEED 3 decimal places accuracy?

Reply to
The Real Bev

The Real Bev wrote: : Has anybody determined what the original poster is going to do with this : information? Does he really NEED 3 decimal places accuracy?

No, if the uncertainty was less than1mpg (that is, 28 means definitely somewhere between 27.5 and 28.5), that would be fine.

Reply to
Newbie

Seems to me the way to eliminate the pump cutoff point problem is to fill till you can see fuel in neck. Variation then should be very small.

Reply to
Don Stauffer

A few years ago I did tests to see whether my Neon with spark knock sensing would get better milage on premium vs regular. With ten fillups on each, the variance on each set of measurements was 0.2 mpg. Regular showed 0.1 better than premium, but since variance was 0.2 I had to conclude no difference.

The fact that variance was so close on both sets of measurements (I think it was 0.17 on one, 0.18 on other) indicates to me that at least the way I was doing the testing was accurate (precise, actually) to about 0.2 mpg.

Reply to
Don Stauffer

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