Which has the greater impact on your perception of fuel economy?

[briantroutman]

I'd imagine that many roadgeeks are more concerned with fuel economy than the population as a whole. More MPG means more to see, more ground to cover, and more highways to clinch...with the same amount of fuel.

Obviously MPG is MPG is MPG, and the hard numbers tell the true story, but psychology behind perception of fuel economy is a little more nuanced. Your perception of economy can be affected by how quickly the fuel gauge needle moves downward–and by the "sticker shock"  of pumping 20+ gallons at a service station and seeing triple-digit price tag.

But which positive outcome do you think has the greater impact on your perception of fuel economy?

[Brandon]

1. You need to know how fast your gauge moves in relation to tank size.  My gauge is fairly large, and thus seems to move fast, but the car only has a 13.6 gallon tank, and each tick on the gas gauge (there are 19 between F and E) is less than a gallon.  It's better, IMHO, to measure mileage with the trip odometer and know the typical point at which the gauge should be with the mileage.  For me, a quarter tank gone at 120 miles is decent.

2. Filling up with small amounts merely masks the true cost of the fuel.  You might as well use up 10 gallons (over 300 miles for me) and fill it to full.

[hbelkins]

2. Filling up with small amounts merely masks the true cost of the fuel.  You might as well use up 10 gallons (over 300 miles for me) and fill it to full.

Unless gas prices are so volatile that if you drive to the point where you absolutely have to fill up, and then the next day gas goes down a dime a gallon, I think I'd prefer smaller fill-ups. I'd rather fill up with half a tank gone at $3.29 a gallon than fill up with my gauge nearly on "E" at $3.59 a gallon.

[sdmichael]

My motorcycle usually uses about 2-3 gallons after about 150-170 miles... So either way I have to fill up frequently. Seeing how little I have to put in makes it better.

[SteveG1988]

For me, i work out how much my car SHOULD use on 1/4 of the tank, for example my focus got 30-33mpg on the highway (4 speed auto) on a 14 gal tank, in theory i should get 400 mi per tank, and i felt proud when i did get 100 per quarter.

[wxfree]

2. Filling up with small amounts merely masks the true cost of the fuel.  You might as well use up 10 gallons (over 300 miles for me) and fill it to full.

Unless gas prices are so volatile that if you drive to the point where you absolutely have to fill up, and then the next day gas goes down a dime a gallon, I think I'd prefer smaller fill-ups. I'd rather fill up with half a tank gone at $3.29 a gallon than fill up with my gauge nearly on "E" at $3.59 a gallon.

It's generally pretty easy to predict large upcoming changes in gasoline prices.  I regularly check the futures market at NYMEX, owned by CME.  Once you learn the general relation between wholesale futures prices and retail in your area (for example, in my area retail prices are generally about 50 cents more than front-month futures), you can learn to predict big changes.  When the gap between front-month futures and the normal expected retail price becomes substantial, say 10 cents or more, I watch the futures more closely for the next few days.  If the futures prices don't close the gap, I expect a change in retail prices generally within a week.

http://www.cmegroup.com/trading/energy/refined-products/rbob-gasoline.html

Using this system generally produces no useful insight, as normal changes are slow, but at those times when prices are about to change a lot, you can get a warning.

Keep in mind that the front month rolls over the first business day of each month, and end-of-month trading can have weird effects, so I pay more attention to the second month out starting the last week of each month.

Also keep in mind that winter and summer formulations are different, with the more expensive summer blend having more energy and generally delivering more MPGs.  The increase in price approaching summer is usually less burdensome, and the decrease approaching winter usually less beneficial, than the price changes alone suggest.

[Laura]

Doesn't a car get better fuel economy at 1/2-full tank compared to running it down to E? I've always wanted to know if that was true or an urban legend.

I usually let it run to E and then fill it all at once.

[KEK Inc.]

For some reason, Full to 1/2 goes down in a heartbeat, but it takes forever to reach empty.  I never let the idiot light go off, but I've let it stay at 1/4, and it seems to last me a while.  Once it starts to fluctuate (when you go downhill, you may have less fuel than going uphill according to the gauge), I generally think it's a good time to fill up.

[jeffandnicole]

Doesn't a car get better fuel economy at 1/2-full tank compared to running it down to E? I've always wanted to know if that was true or an urban legend.

I usually let it run to E and then fill it all at once.

I would think the opposite: less fuel equals less weight. The difference would be minuscule though.

[SteveG1988]

I rarely let the needle drop below 1/4 of a tank, you never know what gunk is sitting in the bottom of an old gas tank.

how often do you put fuel injector cleaner in?

I try to dump it in every oil change, and whenever it is on sale i try to grab a bottle. It may be snake oil, but it cannot really hurt the injectors.

[english si]

Do you guys not get mpg on inboard computers? Seems a common thing here on most mid-range cars to be able to display one of journey mpg/tank mpg/momentary mpg/journey average speed/fuel range (user choice, can rotate through) somewhere on the dash.

[Brandon]

Do you guys not get mpg on inboard computers? Seems a common thing here on most mid-range cars to be able to display one of journey mpg/tank mpg/momentary mpg/journey average speed/fuel range (user choice, can rotate through) somewhere on the dash.

No.  My car does not have an inboard computer for that.  I calculate it after filling up.

[J N Winkler]

Do you guys not get mpg on inboard computers? Seems a common thing here on most mid-range cars to be able to display one of journey mpg/tank mpg/momentary mpg/journey average speed/fuel range (user choice, can rotate through) somewhere on the dash.

I think it is only very recently that trip computers have come to be considered "midrange" in the US.  My 1994 Saturn (top-of-the-line when new, with ABS, four-wheel disc brakes, variable-assist power steering, and traction control) doesn't have one, though it was available as an extra-cost option on the 1986 Nissan Maxima I had previously.  (I am not sure it was possible to get the trip computer without a digital dash, which at the time was associated with Lincolns and Cadillacs.)  In this family we also have a 2005 Camry which probably has MPG readouts only because it is at the highest trim level.  However, the 2009 Honda Fit has a trip computer, even though essentially only aluminum wheels and possibly a convenience package elevate it above base trim.

In regard to gauge behavior, we all but "pack" the tank when we refuel:  that is, we back off on the nozzle flow rate as the tank fills, pump in a little more after the shutoff engages, and rinse and repeat until the shutoff has engaged a total of three times.  The result is that the gauge moves much more slowly from the "F" mark to the first tick toward "E" than it moves from one tick mark to another.  We tend to expect good mileage from a given tank when it takes about 100 miles of highway driving for the gauge needle to start to move.

Unless gas prices are so volatile that if you drive to the point where you absolutely have to fill up, and then the next day gas goes down a dime a gallon, I think I'd prefer smaller fill-ups. I'd rather fill up with half a tank gone at $3.29 a gallon than fill up with my gauge nearly on "E" at $3.59 a gallon.

I absolutely don't like frequent fillups.  Gasoline contains benzene, which is a known carcinogen, and I prefer to limit my exposure.  It annoys me greatly that the driving range of my Saturn is limited to about 300 miles when I could usually count on 420 with both the Nissan Maxima and the Honda Fit.  (These cars have registered one-tank mileage maxima of 550 and 570 respectively.)

Also, for preference we refuel at the supermarket so we can redeem fuel points, which are already paid for through higher prices on food.  At the pump, fuel points are redeemed as a set discount per gallon, you are given no assurances that unredeemed fuel points will be rolled over as a smaller discount at the next refueling (indeed, you are told explicitly that fuel points cannot be carried over from month to month), and the gallonage limits for fuel point redemption are all significantly in excess of tank size on any of our cars.  It makes sense for us to minimize losses from unredeemed fuel points by refueling when the tank is as close to bone-dry as possible.

In fact, until the grocery store cracked down a few months ago, it was not uncommon for shoppers to try to fill up multiple cars on the same pump cycle, which requires split-second timing to move cars around at the pump and thereby encourages unsafe behavior.

[formulanone]

Doesn't a car get better fuel economy at 1/2-full tank compared to running it down to E? I've always wanted to know if that was true or an urban legend.

I usually let it run to E and then fill it all at once.

Since increased mass decreases fuel economy, then yes, you would get slightly better fuel economy with a lighter tank. However, the difference between a full and empty tank in the average vehicle only represents about 3-5% of the total curb weight - possibly not enough to be noticeable.

In racing cars, it is a much larger weight ratio: between 20-30% of the total vehicle mass decreases between full and empty. Sounds like a big deal, except the engines are not designed for what we'd call "fuel economy"; even though the car becomes lighter and should theoretically handle better, it prompts the driver to drive faster.

[hbelkins]

Until recently, I had always been one to wait until my vehicle got to around a quarter-tank or less before filling up. Exceptions were when I was traveling and had to stop at a gas station anyway for a restroom break, or to get something to eat (this was usually a Sheetz phenomenon), or at the end of the day when I wanted to start out the next morning's drive with a full tank. I always filled up; I didn't just put in a few gallons.

Now, with gas prices ranging from $3.699 to $3.799 in my area and me watching my budget much more closely than I have in prior years, I have taken to not filling up when I get gas. Instead, I will only get about $20 or $25 worth of gas at the current inflated prices. For a long time, it seemed like my luck ran such that the day I needed to fill up, prices would go up between the time I got to work that morning and when I left for home in the afternoon.

If I only get a few gallons at a time, I can hope that prices will go down in a few days, making it more economical to buy a full fuel load at that time.

I remember the days when people would only buy a few dollars worth of gas ($3-$5-$7, etc.). I presume purchases that small are a thing of the past, since that will only get you a couple of gallons at most.

[Duke87]

Do you guys not get mpg on inboard computers?

My car has this feature but I ignore it because by my own data logging and math I have found it to be quite inaccurate. It thinks I am averaging about 35-36 MPG. I am not, I'm only averaging about 30-31.
It is by no means a feature every car has, though: my father's car, which is newer than mine, does not have it.


In terms of fuel economy, I generally accept that the car performs how it's going to perform and anything I can do to make a difference is likely to be lesser in magnitude than natural variation based on driving conditions and thus not measurable or noticable. I do, however, hate letting my car idle. People who on hot summer days ask me to turn the car on and run the AC for a few minutes before they get in get glared at by me. I will open all four doors and let the car sit like that for a few minutes to let it air out a bit before I get in on a hot day, but I will NOT turn it on until I'm driving away.
On a similar note, I am also conscious about not running the AC if I can be comfortable without it.


With regards to buying gas, I will plan so by location along my trip if I know in advance that one area will be cheaper (e.g. buy gas in Massachusetts rather than Connecticut), but I will not expend any effort researching this prior to departure. I generally will just fill up wherever I am when I get down to 1/4 tank or so, and the gas costs whatever it costs. I honestly care a lot more about avoiding full service stations (you've read my rants about this before) than I do about finding cheaper gas. To the point where I will happily pay 30-40 cents per gallon more to buy gas in NY instead of NJ.
I always fill the tank when I get gas both because I have to in order for my perpetual fuel economy calculations to work, and because I fail to see the point in not doing so - it doesn't actually save you money, it just makes you need to stop for gas more frequently, which costs you convenience.

As for perception of fuel economy... what is this perception of which you speak? I keep a notebook full of odometer readings and volumes of gas purchased and do math. Screw perception, I measure it objectively.

[MikeTheActuary]

My car has this feature but I ignore it because by my own data logging and math I have found it to be quite inaccurate. It thinks I am averaging about 35-36 MPG. I am not, I'm only averaging about 30-31.

About a month ago, I bought a new Subaru Forester.  It has the ability to display realtime MPG stats.  The numbers are inaccurate on a nominal basis (it reads 1-2 MPG high between fillups), but I do find the proportionate information plausible.    I get great gas mileage when coasting downhill; I do OK cruising at 40mph; and the efficiency appears to plummet above 70mph, or when going up a steep slope.

So whiile the data may be inaccurate, I have been letting it inform me, and my driving style has changed already.   It's become a bit of a game, seeing just how high I can get the displayed MPG to read.

(And man, am I enjoying the improved efficiency over the now-retired 98 Jeep Cherokee.)

[PHLBOS]

My car has this feature but I ignore it because by my own data logging and math I have found it to be quite inaccurate. It thinks I am averaging about 35-36 MPG. I am not, I'm only averaging about 30-31.
It is by no means a feature every car has, though: my father's car, which is newer than mine, does not have it.

Actually, Ford has had that feature as an option for many of its cars since the 1980s when it was originally called the Tripminder Computer.  One of the first cars to offer similar (don't laugh) was the land-yacht-sized Lincoln Continental Mark V coupes of the late-70s (displaying MILES TO EMPTY).  Ironically, the one in the link's a Canadian model showing km/h as the primary speedometer units; I couldn't find a better photo of a US-spec'd model showing the feature.

My '07 Mustang has a similar feature that your Focus has and I find that it usually yields a 1 to 2 mpg higher average rating than what I manually calculate.

OTHO, my '97 Crown Vic does not have that feature... although models made before 1990 and after 2005 actually offer it; optional for the former, standard for the latter.

Generally, I always make it a point to let the car get down to roughly a quarter tank and then I fill it as a means of better tracking my gas milage.  I've done such almost religiously with all my cars I've owned since the late 80s when I had my '85 Mercury Grand Marquis.

Since my Crown Vic has a 20-gallon tank & a trip odometer (that I reset after filling it up every time); I can glance over the odometer reading when the needle is at the 1/2-way mark to roughly determine my average fuel economy without using a calculator; I simiply move the decimal point over one place to the left (odometer reading 200.0 = 20.0 mpg).

[roadman]

I remember the days when people would only buy a few dollars worth of gas ($3-$5-$7, etc.). I presume purchases that small are a thing of the past, since that will only get you a couple of gallons at most.

I told you to put more than five bucks worth in! -  Spaceballs

[PHLBOS]

I remember the days when people would only buy a few dollars worth of gas ($3-$5-$7, etc.). I presume purchases that small are a thing of the past, since that will only get you a couple of gallons at most.

One topping off a tank of a rental car prior to returning it to an the agency is likely the only place one will see gas purchases that small.

I remember when my mother would only get $2 to $3 worth of gas for her car back in the mid-70s.  My brother & I joked about such even then.  Heck, I think she still did similar during the 80s (she might've bumped her max. to $5 and her min. to $3 then).

[getemngo]

My car has this feature but I ignore it because by my own data logging and math I have found it to be quite inaccurate. It thinks I am averaging about 35-36 MPG. I am not, I'm only averaging about 30-31.

About a month ago, I bought a new Subaru Forester.  It has the ability to display realtime MPG stats.  The numbers are inaccurate on a nominal basis (it reads 1-2 MPG high between fillups), but I do find the proportionate information plausible.

My car has a broken fuel gauge - it will only stay on E or F, and it might switch to E at any point between 5/8 and when it actually is almost empty. I have no choice but to fill up the whole tank each time and use the trip odometer as a rough guide.

How do most cars' computers calculate fuel economy? You'd think it would just divide fuel consumed by distance driven... but my Pontiac, with its broken gauge, can't tell me how much gas it's used (it always says 99+ gallons) or how miles I have left on the current tank. Yet it will display current and average fuel economy, giving fairly reasonable sounding numbers. Would it go by engine speed, perhaps?

[J N Winkler]

Yet it will display current and average fuel economy, giving fairly reasonable sounding numbers. Would it go by engine speed, perhaps?

These days I think most trip computers get efficiency information by back-calculating from injector pulses to get fuel consumption, and using the odometer (which generally is electronic even in cars with analog gauges, which have been driven by stepper motors in most models since the early 1990's) for mileage.  I don't know if there are any models that actually have flow meters in the fuel system.

[froggie]

"None of the above".

In part because my Tiguan has a whole display menu related to fuel economy and "range".

I also reset my tripometer after every fill-up.

[roadman]

For some reason, Full to 1/2 goes down in a heartbeat, but it takes forever to reach empty.  I never let the idiot light go off, but I've let it stay at 1/4, and it seems to last me a while.  Once it starts to fluctuate (when you go downhill, you may have less fuel than going uphill according to the gauge), I generally think it's a good time to fill up.

My parents had a '75 Buick Century wagon with a fuel gauge that behaved like that about every third or fourth fill up.  However, that was rarely an issue for them except on long trips thanks to the large (22 gallon) tank.  And, once I got used to calculating the expected consumption (usually 5 to 8 mpg  ) based on miles driven, I never had a problem with it.

[hbelkins]

Do you guys not get mpg on inboard computers?

My car has this feature but I ignore it because by my own data logging and math I have found it to be quite inaccurate.

I checked my Saturn Vue's reading vs. doing my own calculation and found them to be reasonably close.

I passed through Lexington Tuesday and got gas at Sam's Club for $3.539. Gas is ranging from $3.799 to $3.859 in Lexington and Sam's is about the cheapest place. I filled up at that price and reset my trip odometer.

I was back in Lexington today and filled up, again at Sam's and again for $3.539. I let the pump shut off and did not top off the tank. When I checked my mileage the old-fashioned way, the reading was 0.4 MPG less than what my vehicle calculated. That's close enough to me.