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The TCH gives us average mpg since our last fill-up. The car is very new to the market and many of us are on our first, second or third tanks of gas. As we get more experience with it I'm just wondering how the mpg reading compares with real life.
My first tank was several mpg higher in real life than on the display. However, the dealership may have topped off the tank whereas I stopped when the pump shut off. So it may really be off by several mpg, or it could be that there was less gas in the tank after my first fill-up than when I got the car.
My measured gals/miles varies with the console readout which I reset with each fillup. The readout has been both high and low of the actual by almost 2 MPG. After 4 fill-ups, 3 have read high on the car's computer and one low. I suspect the computer is more of a weighted average with the more recent milage having more weight. On the other hand, it may just be inaccurate. The error has been about +/- 5%.
Mine's all over the board. I've been within a tenth and off as much as 2 mpg. I was really disapointed yesterday as I babyed the car on a trip to get it to 39 mpg before I filled up only to discover my real mileage was 37.
The disturbing part is that it is not consistant. I can drive like it's not a hybrid and get 36.5 on that trip. I drive in the limit, coast downhills and back off going up hills to get the 39 indicated and yet only got 37. It makes me wonder if it's reading real time volumes and milages or simply averaging the 1 minute "bars" on the consumption graph (which could lead to rounding errors.
Any one have ideas as to why it's not consistantly off? I typically fill up at the same station and stop at the same lever (click off and two squeezes just to make sure it stopped becuase it was full)
Are you sure it is so far off? Isn't there variation in when the auto-shutoff mechanism kicks in on gas pumps? Let's assume the gas pump you most recently used allowed .6 gallons more than another pump would. I calculate you got 38.9 MPG. Or perhaps the next-to-last pump you used cut off .3 gallons sooner. Anyway, I think that small differences can really screw up your single tank average. I pay more attention to a 3-6 tank moving average, or at least an average of 2 tanks to try to smooth out this type of variation.
I too had the same experience with the digital mpg display and manual calculations of mpg. The display read 36.8mpg, but when I filled up with gas the pump shut off at 14.5gallons. With 570 miles on the odometer that comes to 39.3mpg. So I am at a loss too as to why the digital readouts are less than actual miles. I am sure there are a few variables that one has to consider, but almost 3mpg difference is a lot.
I pay more attention to a 3-6 tank moving average, or at least an average of 2 tanks to try to smooth out this type of variation.
I do as well. It does seem to have no consistancancy as some are very close and some off as much as 2 mpg (high and low).
I've filled up 17 times and think I have a pretty good feel for the way it kicks off and how much more the tank will hold. I have not found the problem with the TCH like many of my past cars where the temperature, emptiness of tank or other environmental or physical circumstances would affect the nozzel kicking off early. One time I tried to top off my tank and it did not really take that much more gas. Now I let the nozzel kick off and give it two more squeezes to verify that it did indeed kick off because it was full and not because of a vapor pressure issue or something else.
HOWEVER, Over my current 6471 miles I have kept the indicated mpg per tank. I just went back and calculated the "theoritical" lifetime MPG based on actual tan milage and indicated mpg and calculate that I should have 39.18 mpg but I actually have a calculated value of 38.38. Granted the inaccuracy is only 3.5 gallons over 6471 miles but it does amount to almost 0.8 mpg error with the indicated versus actual (actual being lower). It does indeed show the sensitivity of the calculation with a high FE vehicle, but it still is an inaccuracy I believe.
Just for fun, let's assume the FE calculations done by the computer are based on the amount of fuel being used by the fuel injectors divided by the miles measured on the odometer. More specifically that 1) the odometer is correct (well within 1%), and 2) the fuel measurements are based on the commanded volume of fuel of each "squirt" of each fuel injector (4 cylinders @ 0.5 intake stroke per revolution is 2 squirts per engine revolution). For the sake of simplified calculation, let's calculate the squirt volume given for 40 MPG for 40 miles (1 gal @ 40 MPG) at 60 MPH (1 Mile/Minute - which is 40 minutes), with the ICE turning 2500 RPM.
2500 Rev/Min * 40 Min/Gal * 2 Stroke/Rev =
200,000 intake squirts of gas for 1 gallon of gas.
That equates to about
5 millionth of a gallon per squirt, or
6 ten-thousandths of an ounce per squirt!
A bit more for lower MPG, and less for a higher RPM. This is probably a fairly typical volume of each squirt for my TCH.
The bottom line is that a very large number of extremely tiny amounts of fuel are being added by the computer to get the mileage calculation. I am quite impressed that it stays within 5% accuracy on my TCH!