I've done some light testing and figuring to get a rough idea the kind of gas mileage my car gets, but more as a "I just like to know everything I can about my car" kind of factoid rather than an actual measurement to judge my car by or to try to find methods to improve its number. I think I might have forgotten what its number was anyway at this point, but I want to say I put it in the 25-30 mpg range, I think 26 was my average. A lot of that though is due to the fact I live in Texas and with the jobs I've had and relatives living many miles away, there's a lot of highway driving. For most cars that's a fuel-saving plus, but for my car it turns out it isn't because it's only a 5-speed so to do the speed limits (and maybe a bit higher) my RPM tend to stay considerably high for long periods of time.
Fun fact before I move on: When I moved my car out to Hawaii, I thought it would get better gas mileage due to the slower highway and roadway speeds there. I also thought that with the elevation changes, even though I would have to climb hills, I figured I could coast down the other side and maybe that would grant me a net gain in mileage. Usually the highway speed limit is only 55mph there, with 60 being the highest I ever saw. And my commute to work was mostly on roads with a 35 speed limit. However, it got much worse mileage. I eventually attributed this to the ridiculous traffic, which in my opinion was worse in general than it is in Dallas (I'm convinced there are more considerate drivers in Honolulu however). With my car being a manual, stop and go traffic means revving many many times only to ease the clutch out enough to travel about seven feet before stopping again. As for the hills, I figure that the traffic played a role in that as well, as I would often times have to brake on downhills. I think that constant revving yet not getting anywhere, plus the fact that you traverse some mountains there, resulted in the worse fuel mileage.
That might have been a large buildup to a rather small observation, heh.
Every time I get gas, in addition to saving the receipt, I make a mental note of the cost and the number of gallons. I usually don't remember for very long, but as I pull away, I do a quick calculation of how my last tank of gas performed. I especially like when the number of gallons I pump is in the 12 gallon range, because my car's fuel tank holds 18 gallons. So I can quickly see that I still had a third of a tank left, having used two-thirds (yes, I'm one of those people who fill up when the gauge reads around a quarter). That's actually what lead me to realize my little epiphany. When I did, I noticed that my gauge was way closer to 1/4 than 1/3, yet I pumped 12 gallons. Meaning, my gauge was wrong. However, if your fuel gauge is going to be wrong, you want it to be wrong that way and not the other way around... this way makes you get gas before you need it instead of running out, wondering what happened.
The idea that struck me from this, was that one could pump gas at certain marks on their fuel gauge, like one-half or one-third, and note the number of gallons that equates too. If I did this, I would even take pictures of the fuel gauge and the pump readout or receipt. I would also do it at all points of the gauge, not just the more obvious marks. My theory is that fuel gauges tend to "fall off" below the halfway point, as it seems like my gauge always consumes more after that mark. If I were to note the gallons and gauge point regularly, especially with pictures, I could get an excellent idea of how many gallons I truly have left at any given gauge reference point. It would also allow me to see if the gauge is at least consistent in its inaccuracy. I'm positive it would be, but if it somehow isn't, that would probably be really good to know. It would also be cool to know when exactly the gas light comes on.
Things I have realized might make this a moot point:
- Most newer cars have a digital readout telling you the estimated mileage remaining. My car is a 2005 however :D.
- I think it's already widely accepted that fuel gauges "increase consumption" past halfway, at least nobody I've talked to about my findings sounded surprised. I bet my fuel gauge is spot-on in the upper half of the spectrum, but starts to alter after that. They might even be designed to do that to help prevent people from running out, especially those daredevils that know their car will still run below "E".
- This might already be a common practice when tracking fuel mileage. I suppose when I've calculated mine before, I simply did the math and that was that. Matching it up to the fuel gauge just seemed like a smart idea to me that I hadn't thought to do before!