Sigh... actual vs. computer mileage
 

Am I the only one whose computer mileage readout is all over the friggin map compared to actual mileage? I just filled up this afternoon, and despite the computer saying 50.3 for this tank, I actually calculated 42.7 (a *HUGE* difference to my way of thinking). The tank *immediately* prior, the computer showed 48.2, and I calculated 50.1! What gives? I know there is usually a discrepancy (mine has averaged 8%), but the computer almost *always* reads higher than the actual calculated. How can it be so variable when I'm doing the *exact* same driving in the *exact* same conditions? Nothing changed from one tank to the next. Same gas station, same pump, same gas (presumably), same routes to and from school & clinicals, same everything. Wildly different results. The scientist in me is perplexed.

Butch

Re: Sigh... actual vs. computer mileage
 

BUtch,


I have been very consistent on 6 tanks of my hand calculated mileage (which is what I report in the database) being 2% over the display.

Mike

Re: Sigh... actual vs. computer mileage
 

Mine is always off a little, but the computer is always low. Sometimes by as little as .4 MPG, but the most off was 2.5 MPG...again, calculated has been higher by usually 1MPG on every tank. I'd prefer it be low than high. Never had it be very far off, though.

Re: Sigh... actual vs. computer mileage
 

By actual mileage, I assume you mean mileage calculated by topping off the tank manually? Any single measurement by that method may not be particularly accurate either. Over time, it probably becomes a more accurate measure. The computer measurement may be off, but it's probably off in a more consistent way, making it useful for determining mileage change as per drving behavior change.

Re: Sigh... actual vs. computer mileage
 

My displayed milage is quite consistently 4% worse than actual. I think this is too much, too.

Re: Sigh... actual vs. computer mileage
 

IMO the computer is best for comparing one tank to another. The difference you speak of is most likely due to fill inconsistancies. These errors average out over time so the "at the pump" method is best for computing a long term average.

My display is usually around 3.5% too high so I use fill data when reporting mpg to the database.

Re: Sigh... actual vs. computer mileage
 

Lakedude is right on. Each fill-up is different, so as mentioned earlier in the thread, unless you fill up manually where to the top of the filler tube (not recommended), you will get discrepancies from tank to tank.

How much discrepancy obviously is a factor of how consistent one is with filling to the exact same spot in the tank.

I don't know how consistent the computer's mileage is, but I bet it's no worse than the inconsistencies introduced by different tank fill levels, so maybe we should not be complaining about inconsistencies in the computer, but inconsistencies in the fill level from tank to tank.

Case in point, usually when I fill up it's at the same station, even the same pump. I fill it on "medium fill speed", then two more clicks at very low speed and I'm done. My mileage difference between computer and computed is usually about 2 mpg. This last fill I went to a different station and did the same routine. But the second click at low speed didn't click off like I expected. It kept filling, and I let it. By the time it quit, it had added another gallon and a half. The result? The differnce between calculated and computed was 8 mpg! Following a fill, I usually get between 50-60 miles before I lose the first bar on my fuel display. This time I'm at over 80 miles since the fill up, and the display is still showing all bars. I don't blame the computer for this, I blame the fact that I filled the tank higher than I usually do.....

Re: Sigh... actual vs. computer mileage
 

I fill at the same station and often the same pump at the same time of day and just to the mechanical fill click. No topping off. Tires are checked and maintained at 36.

MPG meter is always below calculated. This is always between o.4 and 1.2 with the hand calculation always being higher.

Reasonably consistent.

Re: Sigh... actual vs. computer mileage
 

I finally filled my tank for the first time over the weekend after having my car for a week. I was able to get 518 miles (still had 2 bars left) but the computer was showing I had 41.3mpg but when I calculated by hand I got 50.1 mpg. Are my calculations more accurate?

I did notice a huge difference in how soon I lost the first bar of gas. When I picked it up from the dealer I got close to a 100 miles (or I may have even reached 100) before losing the first bar. However when I filled it up (put in 10.3 gallons) I noticed that I lost the first bar much much quicker, so I know that I will not be able to get as many miles on this tank probably.

I want to start logging my milage on this site but haven't yet, should I use my calculations or the computer?

Re: Sigh... actual vs. computer mileage
 

If you had 518 miles with two bars left, then yes, your calculations are far more accurate. There will be small variances in the accuracy of the calculations at the pump (per tank), but over several tanks, it will be FAR more accurate than the computer, because it's your actual fuel cost into your actual driving. The only variance is actually the FIRST tank fill when it was filled before being driven. Over the course of the car's life, any 1 gallon discrepancy from the first fill becomes so minute that it will not affect the average at all. I have noticed that the computer is .5 to 2.5 MPG low...never had it more than that.

Re: Sigh... actual vs. computer mileage
 

To have that much of a difference, I have a feeling you got some "bonus" gas from the dealer. In other words, it was probably a bit over-filled when you picked it up, whereas when you filled it yourself, I imagine you just filled to the first click. So you actually burned more gas during your first tank than what you filled back up with. However, I think you will find (with this next tank), that the computer is about 2 mpg lower than calculated. I always use calculated for adding to the database.

Re: Sigh... actual vs. computer mileage
 

So the 50.1 mpg probably isn't acurate, right? I would have to know how full the dealer filled it and divide my miles by that. Well I put 10.3 gallons and had 2 bars of gas left, I didn't want to fill it too much.

Should I just start entering my mpg from the 2nd tank forward and not count my first tank?

Re: Sigh... actual vs. computer mileage
 

Maybe you can wait until you calculate the next tank and see how big a difference you see. If it is a large difference, leave out the first tank if you want. Over the long haul, the little difference of your first tank won't have much impact on your overall mpg (as JordanCS pointed out). If I had to guess, I bet your next tank will come in around 44mpg (calculated), assuming your commute is similar to your first tank. Just a guess.... Keep us posted!

Re: Sigh... actual vs. computer mileage
 

The display on my '05 is usually 2-3 mpg over actual; occasionally slightly under. The wife's '06, OTOH, has been a bit below actual so far.

Re: Sigh... actual vs. computer mileage
 

My computer is (despite the occasional oddball reading, either high or low) is generally staying about 8% higher than my actual calculated mileage. It typically shows in the neighborhood of 48-49, while my calculated average (this is over many tanks of gas) has been running in the neighborhood of 45-47.

Note that I live in a very mountainous area, so I'm not doing as well as a lot of folks who live in the flat lands do. On a recent trip to NC, I averaged 52.4 for the tank (calculated), while my computer said 50.1 for the same tank. Always take the computer's ideas with a grain of salt.

Butch

Re: Sigh... actual vs. computer mileage
 

I see yours is a 2003, but mine had the same issue, a 2006. It was directly related to the service advisory which delt with the odometer resetting every 63 miles...however mine never reset...it just never read accuratly, going from the mid 30's to over 60 mpg in a given hour!! Now I look back and should of snapped a picture of 60-70 mpg tank at 400 miles...

They replaced the odometer and now its within 1/2 mpg of actual. Very happy. Too bad the experience of having it changed out wasn't so great. Now it goes up to maybe the low 50's at best...but thats ok, its reading correctly now. With more miles on the tank...it settles into around 48-50 mpg.

Bob

Re: Sigh... actual vs. computer mileage
 

A change in the levelness and/or sensitivity of the pump cut-off could make a difference too

Re: Sigh... actual vs. computer mileage
 

It's been under by 1 or 2 miles the past 4 or 5 fillups. I tend to stop
after the first click of the pump. So I figure about 1/4 to 1/2 gallon or less could fill it to the top. It a good enough approximation with me.

It's cool with me. Considering I went from 24 MPG with my older commute vehicle to the 50+ MPG I'm getting now and the sheer fun of driving the hybrid. I'm pretty content and not worried about the gas calculation. As long as the mileage odometer is correct and the NASCAR wannabes are able to pass me safely (burning up the gas and then grumbling about the price of gas when they stop at the pump, tsk tsk) I'm good to go. I have to say the the Civic Hybrid has given me a better outlook on driving.

Re: Sigh... actual vs. computer mileage
 

My computer data is sometimes high, sometimes low, but always off compared to the calculation, by up to 2 mpg or so. If you track overall mileage, after 5 or 10 tanks these little inconsistencies have a negligible impact on your lifetime mileage, so I really don't worry about it much. Computer inaccuracies, pumping inconsistencies, it's all part of the game. No individual measurement is going to give an accurate overall picture, so keep some big picture data and you'll find that these minor differences are insignificant over time.

Re: Sigh... actual vs. computer mileage
 

It's actually almost 2 gallons to fill up from the first click to the filler neck.

Re: Sigh... actual vs. computer mileage
 

The first gen HCH's computer usually overreports, the 2nd gen usually under reports a little. This is, of course, just a trend, but a very common one I believe. Individual results may vary! ;)