If the prior miles were done at high mpg and the next 47 were done at low mpg, the reducing average mpg will mean the "how much can I get out of a tank ?" will drop more than you expect.
My cruising mpg will be dropping rapidly (I don't look at it too much, just at the petrol gauge) because I started this tank with a higher mpg run up the motorway and now I'm back to stop/start city traffic.
About same mpg. I have read that these meters are not that accurate but the apparent discrepancy between actual distance travelled and decrease in cruising range driven shocked me.
It was triggered by the cruising range dropping from 7 to 0 miles in a few hundred yards - I was about 1 mile away from a motorway turn off where I was going to get some petrol, 7 miles left according to the car (I've read the threads about a large reserve when you hit empty) than pow, the cruising range dropped to 0.
Think it depends on a lot of stuff :
How it measures the fuel remaining : is it Full - measured fuel used or a gauge in the tank
If it's a tank gauge, then that gets affected by the fuel sloshing around (slowdown, exit ramp gradient)
How long the sample time is on the averaging, longer = more consistent average mpg but it'll lead to bigger variations in estimated range as more or less fuel is used.
I don't think you'll ever get a totally accurate cruising range figure, best to not take chances with the estimate and fill early (and according to the fuel gauge) in case the station you were stretching the fuel to get to is behind a big traffic queue (or shut !).
PS That's also me indulging in my healthy distrust of technology ...
PS2 It could also be the colder weather having an effect on things ...
Not at fault. The cruising range is based on several estimates and Sleepypete alluded too. If you think the fuel gauge defaults on the conservative side the cruise range is even more conservative.
Here is my SWAG of how I think it works:
(1) It is based on the remaining fuel gallons (2) It is based on you recent average speed (3) It is based on you recent average mpg
and more important factored in with the previous 3,
(4) Your instantaneous speed (5) Your instantaneous mpg
If you have been cruising along at a relatively high speed and a relatively high mpg and you then slow down and then accelerate fairly aggressively in the power zone and I were your cruise computer, not knowing any better i would assume your new driving behavior was going to continue, I as your dumb computer processing inputs would significantly drop your cruise range estimate.
Bottom line, it is just an estimate trying to predict what you as a driver are going to do in the future and is therefore very, very conservative on the low side. However, once you reach 0 miles in the cruise range you will be able to drive an additional 100 miles or more.
The range indicator is there to tell you when to fill up; It does NOT tell you when the car is going to run out of gas. There is a difference. If it told you when the car would run out of gas, you'd be playing the guessing game of how many miles to go before you should fill up. Like midcow said, it always errs on the conservative side to keep you from running the car out of gas. Just drive the car and stop stressing over the range. When the gas light comes on, fill the car and go on with life.
I really only use the range to plan where I'd like to stop on trips. I know it is 220 miles from my house to my parents. When I visit them on Friday, I'll check the range. If it is ~300, I'll not plan on stopping. If it is ~200, I'll plan on stopping at Weston. If it is ~100 I'll plan on stopping at Flatwoods. It gives me a basic outline of where I'll be stopping on my trip.