Anyone get BETTER mileage than the trip computer estimate?
I've always heard the MPG readout on a trip computer is wildly optimistic, with the real figures coming in around 1-2 mpg lower than what's indicated. Well, I only have a few fill-ups logged so far, but going by Fuelly's calculations, I'm averaging about 3-4 MPG higher than the trip computer says. The trip computer has been at 17.2 to 17.5 MPG for the last two weeks. I've only seen better than that on long trips, but never in my daily commute.
The figures I'm entering appear to be correct, but something just seems off to me. I wouldn't be too shocked to get 1-2 MPG better. I live in the DFW, TX area, and my miles are at least 80% highway, but it's stop and go a good bit of the time in the afternoon.
If it's true, I'll take it! But like I said, something seems off here.
17 MPG, what are you driving a tank? Like you say, 90% of the time, the computer reads 2 to 4 MPG over the actual figures. The only thing I can think of is that you didn't fill the tank right to the top, or it could have shut off early, I know you guys in the US with your huge tanks have automatic fuel pumps that shut off when the tanks almost full.
As far as I can tell, I'm getting a complete tank. I stop after the first click every time. I've been filling up at the same station, but not necessarily the same pump every time.
I know I'm not intentionally hypermiling, but I'm pretty easy on the gas. No hotrodding or anything like that. Heck, that thing doesn't have enough power for hotrodding, anyways!
I'm trying to think of ways that you or the car's computer got bad data. Otherwise, it would seem the dash mpg is simply reported low. Possible, but not likely.
Previous work done, or tires slightly different sized than OEM, would effect both calculations. A short fill is the usual reason for off results, and if that is the case it will become apparent with a couple more a fills logged.
The car is working out the MPG from fuel flow, throttle position, engine speed and load, it puts all those values into a pot and comes up with a number. Now these values are fairly accurate but not 100%. There are a lot of variables that can change the calculation, I'm actually surprised that they can be as accurate as they are.
You are working out the MPG from the mileage travelled and the amount of Fuel added. Unless you can gaurentee that the amount added is exactly the same as the amount used then there is going to be a discrepency.
Using mileage travelled and amount of fuel only requires two parameters to be accurate, your milometer in the car and the guage on the pump. The guage on the pump has to be within a certain tolerance by law, and I believe your milometer is also governed by certain laws to be within a certain tolerance.
Over time your calculations, using the mileage travelled and the amount of Fuel used will be as accurate as you can get. I would use these figures in preference to the ones the car calculates.
You can check your milometer against a satnav over say 10-15 miles, that will give you an idea on how accurate it is.
Can't help with checking the pump though
In summary, stick with your own calculations, using mileage and fuel used has been shown time and again to be more accurate than the MPG guages in the car.
Don't panic, you don't have an issue and you are not doing anything wrong.
The car guage works the same as a Scanguage, it takes various readings from the ecu and tries to calculate MPG. The car manufacturers define the algorithm to get the closest number to actual MPG, the key word being closest.
If you get a Scanguage, there are various settings that you can adjust to get the Scanguage to read as close as possible to your manual calculations.
The most accurate way to measure MPG is to use the distance travelled and the fuel used. The cars guage, like a Scanguage is only ever going to be a best effort, albeit very close to the real value.
The value to be gained from either the cars guage or a Scanguage is not the absolute accuracy of average MPG over a tank of fuel, it is the instantaneous readouts and the ability to see how changing your driving changes the values, hopefully for the better.
Don't expect the car calculations to match your own calculations, they will always be different.
Compare like with like, so for example if you set your trip MPG each day and today you got X mpg going to work and then tomorrow you get X-1 going to work, then you know you have improved.
Similarly for you own tank calculations, if on the last tank you calculated Y mpg and now on this tank you got y-1 then again you have improved.