First 50 miles or so sets the average MPG for the rest of the tank?

That's how averages work. The more data is available, the more average the average, and the harder it is for the average to move around.

For example, if a baseball player has 2 hits in 3 at-bats, his average is .667. Next at bat, has 2 hits in 4 at-bats, average plummets to .500. Each piece of data is major compared to the total data pool. When you have driven 200 miles, the next mile is pretty minor in its ability to change the average. It would take a nice 20-mile downhill to run this average up significantly.

 

As barcelona said, the average is just what it indicates. The ODO you are saying is probably reading the INSTANTANEOUS MPG not the average. Trip computers calculates the MPG through the fuel injection pressure (or rate). The culmination of the thousands of readings (instantaenous) makes your average MPG while the the random ones you see on your dashboard is only what it is at that specific time.

So back to the original question. Does the first 50 miles set the MPG for the remaining tank? No. If you have a lead foot the first 50 miles on the tank but the remaining 550 you feather the gas, you can still end up with a high MPG of >50mpg. On the other hand you can feather the gas for the first 550 and lead foot the last 50, you will end up with the same MPG as the other situation.

The main thing you should remember is that its a TRIP computer and should be taken as a close estimate. The best way to calculate your MPG is to gas up at the same pump as before at the same time as you did last time and take the miles you traveled since you last filled up and divide by the gallons gassed up. It is better to use the same pump as you did before because every gas station and even every pump can vary when to cut off when it thinks your gas is full. That variation can be pretty significant when youre calculating MPG. It is also good to gas up at the same time as before because of the varying density of gasoline due to temperature during the day (which quite honestly isnt that significant but some people are pretty anal).

 

Quit staring at the tank MPG. There are two trip meters. Use one for the tank, the other for individual trips. Drive with the individual trip meter showing and reset it either upon shutdown or startup. That way, you get a better idea of what your MPG is for a particular outing instead of wondering what the +- on the tank MPG really meant.

 

Or just go get yourself a scangauge and it will tell you the tank the day and prior day mpg avg at a much more accurate level if your mpg's are that important for you to see.

 

For me, PER TANK MPG is the only number I care about. I reset my trip each fillup. I use Fuelly for the math, always a bit below the onboard computer's calculations.

Your first 50 miles may very well dictate the per tank average if the rest of the tank is about the same mpg. We generally drive the same commute, the same way, in the same weather, week after week so the tank mpg in the first 50 miles is pretty much what it is in the last 50 miles because the driving loop is identical.

It matters very little to me if I can eek out a 30 mile stretch at 80mpg if the overall tank averages out to mid-40's. When someone asks me "how many mpg does it get?" I'm not going to give them an answer with 3 asterisks after it (*over a short distance, *downhill, *no climate control, *etc.) What folks should hear is the per tank average imo otherwise your just picking nits.

That's just how I do it though, obviously others do it differently.

 

There's an assumption in the above that's not true, though I know it can seem like it. Per other responders:

It gets increasingly difficult to budge the MPG down (or up) as you accumulate distance since the last trip reset. Just the way it is with averaging.

One tip when doing calculations: use your odometer readings (the total distance travelled by the car, since factory) to calculate the distance travelled between fill-ups, and just use the trip meter as a check, and for feedback about MPG. It is too easy to accidentally reset the trip meter.

 

Agreed wholeheartedly...short jaunts, etc. don't paint the big picture and can be most deceiving.

 

Mathematics. It all comes down to math. At the beginning of a tank, any variation is a big component of the total average. At the end of a tank, it's not very substantial.

As an example, let's look at money. Say you have a five dollars in your pocket and you lose a buck. You have lost 20% of your total money. Now let's say you have $100 in your pocket and lose the same buck. In this case you have only lost 1%. It's the same loss, but the percentages are different. Percentages depend on the size of the base: the amount of money in your pocket, or the number of miles already driven.

Tom

 

That's why, when I fill up and go to reset the "tank MPG" trip, I note what it is. Driving with the "short jaunts" trip showing gives me an idea of what certain trips and paths yield for mileage and gives me better feedback on my driving. If you're into increasing the "tank MPG", then you can just strive to make your "trip MPG" greater than your "tank MPG". There's no need to stare at it for all of the 500 miles. To each his own, of course, but this works for me and has given me better mileage.

 

I find that by resetting another trip gauge daily, the far more frequent and detailed feedback of what works and what hurts helps to push up the overall tank average. I hate having tanks fall to the 40s.

My commute pattern is more varied than most, providing a choice of four basic routes, with different typical mpgs. The choice is usually settled by traffic conditions, which vary by time of day (my schedule is flexible) and numerous other variables, many of them random. So my first day of a fresh tank is a poor predictor of the whole tank's average.

 

This is good advice and actually the perfect use for the "Trip B" odometer (which I haven't reset in over 6000 miles ).

 

Some people have tried to use one of their trip meters as a "lifetime" stat, but there are problems with doing that, like losing the data when it is serviced, as people have said in this thread. In that thread, you can see that others also suggest using one trip meter for individual trips. stefano5777's suggestion to get a ScanGauge is also good if you want to see more details, but ScanGauges aren't cheap.

 

Yup,
And of course the first inches, yards, 10ths of a mile are very volatile. The longer you run on a tank the greater the "pool" of data.
Thus, less volatility.

 

I think also, what are you doing after that first 50 miles that you expect your MPG to skyrocket? As I said above in my earlier post, if you are running the same loop all tank (as I think many do) then there's no reason to expect your average mpg to change in any significant way from one 50 mile segment to the next. Same roads, same weather, same traffic, same mpg.

However, if I spend half a tank up here on my regular commute then head south to flat land for the second half then even 300 miles into a tank I can "move the meter" quite a lot because the conditions I'm driving in have changed (improved) dramatically (flatter, more stop-and-go city, etc)

 

Pencils are...

I usually tabuguess my mileages by the tank (usually half a tank)....at the pump for all 4 of my vehicles. Five if you count the G3.
That's just how I roll.
I wouldn't want to deprive anyone the unmitigated joys of monitoring their hydrocarbon throughput minute by minute. I've simply found that I can monitor my vehicle's performance with a Mk-1, Mod-0 pencil...and the sales slip from the gas pump.

Besides...the MDF reading in my G3...and others, from what I've read, is optimistic by about 5%.
I'm sure that the Scangauge is more accurate, and it's useful for more than just monitoring---but I always seem to find other things to spend the 150 bucks for.