vipereatr Posted June 19, 2008 Report Share Posted June 19, 2008 There have been several posts on other threads where very good (read: unbelievable) gas mileage has been shared. An example is driving above 65 mph and getting better mileage than at 65 mph. I don't doubt the posters - I doubt the computer. There is an easy way to check your computer against actual mileage (I think) and that is to compare the computer gallons used to the gallons shown on the pump. If pump gallons are always higher then our computers are overestimating mileage. There is a more (sinister) possibility - we are not getting the gallons we pay for. But every pump is supposed to be certified, so that possibility should be very low. Another variable that can effect this test is fuel shut off point on the pump. If we use the same pump and let it shut itself off and don't try to top off then the tank should be filling to the same level each time. Does your pump gallons equal your computer gallons used? Link to comment Share on other sites More sharing options...
Sherri Leicht Posted June 19, 2008 Report Share Posted June 19, 2008 Mine seems to be off by a half gallon. It takes a bit more than the computer says. Link to comment Share on other sites More sharing options...
mustang crazy Posted June 19, 2008 Report Share Posted June 19, 2008 Mine seems to be off by a half gallon. It takes a bit more than the computer says. mine is also about half gallon in fuel and 1 mile in mpg calculatated by miles into gallon 68 mph I made 23.7 on cruise control only no boost on a 100 mile interstate trip. Link to comment Share on other sites More sharing options...
07SHELBY GT Posted June 19, 2008 Report Share Posted June 19, 2008 Same here.all of the S197 Mustangs I've owned have needed more gas than seems to be required.Evaporation? Mileage is always 2-3 less than computer shows. Link to comment Share on other sites More sharing options...
Vogon Posted June 19, 2008 Report Share Posted June 19, 2008 Same here.all of the S197 Mustangs I've owned have needed more gas than seems to be required.Evaporation? Mileage is always 2-3 less than computer shows. Here too, it's off by about 10%. Both my Canadian vert and US coupe. Both with Canadian pumps and US pumps, all the time. It's the computer, trying to make you feel better. Strangely, it works... Link to comment Share on other sites More sharing options...
08-VB/W Posted June 19, 2008 Report Share Posted June 19, 2008 Hmmm . . .. Mine is usually dead on, allowing for normal filling level variables. And I share the 23.7 MPG figure for a laid-back road trip. Link to comment Share on other sites More sharing options...
mciarochi Posted June 19, 2008 Report Share Posted June 19, 2008 5006.2 miles, 262.8 gallons, average 65 mph. 19.049 mpg. 474 miles, 20.7 gallons, average 75 mph. 22.89 mpg. 869 miles, 49.89 gallons, average 76 mph. 19.64 mpg. Those were out of my notebook (I keep one in the car) and not from the computer. I write all gas, oil, trips and stuff in a little notebook in the console. I just now added up the gas and mileage the trips. The speed average in the note book is from the computer, but the gas volume is from the pump, and the mileage is based between first and last fillups on the trip. The first line is the Birthday Bash trip, and includes a day of racing, the Bullrun, and a couple weeks of around town. The second line is pretty much flat highway, a Georgia/Alabama trip. The third line is highway through hills and bad weather, Georgia/Kentucky, and included two rush hours (I started in early Atlanta rush hour, and thanks to a time change, caught the same rush hour in Nashville). There are other trips, but that's pretty much the deal. As you know, average speeds are lower than what you are doing on the highway, normally I'm set at 85+, weather and traffic permitting. During a few of those trips, I reset the computer and set the cruise control at various speeds. Even if the computer is off, it should be consistent with itself. 65 mph gas mileage was awful. 75 was much better. 85 was a little better than 75. 90 was about the same as 75, but it's hard to maintain 90 because you keep running into traffic. Based on my observations, 80 to 85 mph yields the best gas mileage. Also, based on many k's of highway miles, I found it doesn't make sense to try to drive much faster than 85. Faster than that, and you just run into a pack of traffic, and by the time you get through that, all the arseholes that we dogging you 20 minutes ago have caught up again. Then again, every once in a while, you go over a hill, and there's this loooong, empty stretch of road in front of you. Adios, Ameeegooooooooossss......... Link to comment Share on other sites More sharing options...
VaporDude Posted June 19, 2008 Report Share Posted June 19, 2008 I think the problem is that the computer know that the car has a 16 gallon gas tank and it doesn't take into account that the neck of the tank can hold any gas. Plus there is a little slosh room at the top of the tank that is not counted in that 16 gallons. Now when the float in the gas tank goes down a little at a time, the computer is calculating how many miles it has went compared to the how much is in the tank at the position of the float. Now since the float didn't go down much because of the extra gas in the neck of the tank divided by the miles driven to that point, it give you a favorable MPG reading like .4 or .5 MPG difference. If you are seeing this much difference, slow down the speed of the gas going into the tank and when it clicks off the first time, pull the nozzel out and place back into the pump. Now calculate your MPG the old fasion way and see if the computer comes closer to your calculation. You may have to do this a couple of times but on the 2nd tank, it should be very close to the computer estimate. Link to comment Share on other sites More sharing options...
Vogon Posted June 20, 2008 Report Share Posted June 20, 2008 So here's the REALLY important part: can you trust the computer's "Miles to Empty" value? I calculate that there's some grace above the readout, maybe half a gallon?, but I don't know if the pump will get all the registered fuel out of the tank. Fortunately, I haven't found out the hard way, but I've come close. I think I was running on fumes when I pulled into the station at Shelby, Montana, of all places... Link to comment Share on other sites More sharing options...
ilmor Posted June 20, 2008 Report Share Posted June 20, 2008 The way I see it, the mileage is what it is. The computer provides a good indicator that the mpg is generally in the range it should be, then I would look for significant differences or variations to appear, which would indicate a problem of some sort. Otherwise, keep on driving and have a great time! Still, I would like to check the actual mpg only out of curiousity. Link to comment Share on other sites More sharing options...
08-VB/W Posted June 29, 2008 Report Share Posted June 29, 2008 I'm not sure, but I don't think the float figures into anything but the "miles to empty" calculation. I believe the economy is calclulated with a flow meter. At least that's the way it is on all my other cars. Link to comment Share on other sites More sharing options...
vipereatr Posted June 29, 2008 Author Report Share Posted June 29, 2008 Well, here's my modified question. Lots of GT 500 owners are measuring better mileage at higher speeds, especially above 60 mph. My original assumption was that the reported mileage was an artifact of the computer, but too many people are reporting actually measured results. So how does that work? Is there something about our super charger waste gates that results in better fuel economy at higher speeds? It just seem contrary to my engineering sensibilities and my experience with non-super charged cars that a combination of higher rpm and speed results in better mileage. Anybody know for sure what's going on? Link to comment Share on other sites More sharing options...
08-VB/W Posted June 29, 2008 Report Share Posted June 29, 2008 Purely an educated guess - as was my last post: The tuning map runs a higher AFR at light throttle and 2000RPM?? I haven't had a chance to measure mileage since I installed the CAI. Dunn'o if it affected it or not. Link to comment Share on other sites More sharing options...
mciarochi Posted June 30, 2008 Report Share Posted June 30, 2008 Well, here's my modified question. Lots of GT 500 owners are measuring better mileage at higher speeds, especially above 60 mph. My original assumption was that the reported mileage was an artifact of the computer, but too many people are reporting actually measured results. So how does that work? Is there something about our super charger waste gates that results in better fuel economy at higher speeds? It just seem contrary to my engineering sensibilities and my experience with non-super charged cars that a combination of higher rpm and speed results in better mileage. Anybody know for sure what's going on? Gears first, then road and wind resistance, then engine efficiency. Imagine driving at a fixed RPM, say 2000. Sitting in neutral. Drive in first gear. Drive in second gear. Third, fourth, fifth, and sixth. All things being equal, you're going to get the worst gas mileage in neutral, better gas mileage in first, better in second, and so forth. The engine simply has to turn over more times in lower gears to go the same distance. Once you have accelerated to 2000 RPM, you're going to go further in sixth than you are in fifth in the same amount of time. That's where rolling and aerodynamic resistance (parasite drag) come in to play. As you go faster, parasite drag increases as a hyperbolic function (I think). Below 50 or 60 mph, parasitic drag is not much of a factor compared to the inertia of the car. However, once it becomes a factor, it gets worse faster. There is a speed where the rate of increase in parasite drag meets the efficiency of the gearing, and that would be your maximum fuel efficiency (mpg). That appears to be in the 80-85 mph range for my car. Engine efficiency is going to nudge this number around a bit, but the parasite drag is the main factor that keeps you from getting good gas mileage at higher speeds. You might be surprised how a little aerodynamic efficiency makes a big difference in your gas mileage at highway speeds. Bicycle riders are acutely aware of this, because they can literally feel the difference. -Mike Link to comment Share on other sites More sharing options...
sicilianjg Posted June 30, 2008 Report Share Posted June 30, 2008 So here's the REALLY important part: can you trust the computer's "Miles to Empty" value? I calculate that there's some grace above the readout, maybe half a gallon?, but I don't know if the pump will get all the registered fuel out of the tank. Fortunately, I haven't found out the hard way, but I've come close. I think I was running on fumes when I pulled into the station at Shelby, Montana, of all places... I can tell you this with my experance it got down to TWO miles on the computer. When I got gas it took 16 gallons!!! Link to comment Share on other sites More sharing options...
Rarecat Posted June 30, 2008 Report Share Posted June 30, 2008 The highest my onboard MPG indicator has ever registered is 17.2 MPG at the end of a 2 hour interstate run at about 77 MPH.... with the top down and stereo up! Maybe my Shaker is ruining my MPG..... Link to comment Share on other sites More sharing options...
gradona Posted July 1, 2008 Report Share Posted July 1, 2008 I have not and probably will not ever check my fuel mileage. I do know, however, that I am getting at least 50 "Smiles" per gallon :happy feet: Link to comment Share on other sites More sharing options...
lambbo Posted July 1, 2008 Report Share Posted July 1, 2008 We don't want to go there, 450+ miles since the computer was reset and it says 12.8 mpg!! Yep, I like BOOST!! Link to comment Share on other sites More sharing options...
mciarochi Posted July 1, 2008 Report Share Posted July 1, 2008 The highest my onboard MPG indicator has ever registered is 17.2 MPG at the end of a 2 hour interstate run at about 77 MPH.... with the top down and stereo up! Maybe my Shaker is ruining my MPG..... Top down part is what did you in. You sacrifice a lot of aerodynamics when you go for the tan. I'm thinking if you re-did the shakers, and pointed them backwards, maybe that would help? Link to comment Share on other sites More sharing options...
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