Pleasant surprise
I pulled my Jayco about 150 miles or so on Sunday and back the 150 miles this afternoon. I don't know how accurate the onboard mileage computers are but mine said that I was getting a little over 12 ...
Forum Discussion
ShinerBock wrote:4x4ord wrote:
The computer accurately measures the fuel used. When hand calculating there is room for error if the truck is filled differently from one fill to the next. (either different method of filling or on account of the truck sitting a little different, different nozzle). I drove the truck from Texas to Alberta Canada pulling a 20,000 lb trailer when the truck was new. I hand calculated at every fuel stop and found it to be remarkably accurate. Then out of the blue my hand calculation disagreed with the display. The display showed worse than the calculator. I thought it was odd that all of a sudden the truck was getting a little better fuel economy......then the next fill the hand calculation worked out to be worse than the display and I realized that I must have not quite got the truck full the one time.
Huh?!? That makes even worse sense.
Your fuel tank can only hold so much and there isn't going to be a major difference(i.e. more than 1 gallon) from one fill up to the unless you were filling up on a huge incline difference from the last fill up or the pump is off. Either one is highly unlikely statistically. Even if you did put a half a gallon difference (which is unlikely) then it will only alter the result by .3 mpg and even less if you were towing since the less fuel mileage you get the less difference it makes.
And no, the computer does not physically measure fuel used. It uses an algorithm with readings from multiple sensors to determine the estimated fuel used with miles traveled.
I can't say what I did wrong when fueling the one time but the point is that I made an error and when my calculation was different than the computer it was the hand calculation that was off.
As far as the computer measuring fuel used goes, I imagime it would work something like this: Depending on the power demand, the computer tells the injectors to open for a very specific length of time "knowing" at the specified rail pressure exactly what amount of fuel will be injected on each power stroke. It might only be a fraction of a cc each time but it can easily keep a running total. This is likely why the after market programmers that tamper with the rail pressure can screw up the accuracy of the mileage meter.
