It seems that every time I fill up, I'm at about 55 MPG 50-100 miles in. Then, I start to gradually decrease until I end up about 48 or so. Is this common for you all, too, or is it just the fact that I tend to fill up on weekends and then let my economy plummet during the week?

it seems to be true for me, i was at 200 miles at half tank (aprox 33 mpg) and after going 80 miles im at 1/4 tank. (31mpg) i also seem to have less power than normal (its been shifting to third on my local hill). i think it may have something to do w/ the fuel pump having to work harder, either because it has less pressure from gas, or the filter is more cloged when the tank is lower.

I think its just that you fill up on weekends.

I've seen this too and don't understand it.

Another puzzler for me is how many gallons in the tank. When I refill with the flashing bar, 7 gallons does the trick. My odometer was showing 343 miles but my avg mpg was only 40 mpg.

343/7 = 49 mpg and I only hit that once, on the highway, briefly. My mpg declined to 40 mpg.

I'm wondering if I have more gas than the gas tank shows.

Re: Best mileage at beginning of tank?
 

Sorry to post on the Prius forum ;)

I have the same thing on my HCH... matter of fact my first 100 miles on this tank were well over 50mpg... now after 600 total miles it's down to 47. The last 250 miles were at 43 (I reset the 2nd trip meter 200 miles ago).

This is long-term, it happens almost every tank, and I fill up at different times. My driving habits don't really change and the ambient temps have been relatively stable over the last period of time during this tank.

Re: Best mileage at beginning of tank?
 

Mine does edge down, but nor very much really. I think it's the law of averages. It's easy to increase the average early into a tank because of the law of averages. Halfway through a tank it takes a quarter of mile on all electric to move the average a tenth or two but early on it can jump as much as half a gallon over the same distance. It seems the same distance accelerating or on all ICE propulsion will drop it about twice as much. I do see it slipping as the tank goes by but I am fanatical about my mileage and focus on improvement almost every driving instance. The reward - I just did 63.0 mpg actual my last tank (400 plus miles).
:D

Re: Best mileage at beginning of tank?
 

As a suggestion, try keeping the weekend mileage separate from the weekday mileage:

1. top-off late Sunday
2. top-off Friday

There are techniques that work on the weekend that don't work on the work week. You'll find that you're driving patterns will change and adapt to maxize performance of each style of driving.

Bob Wilson

Re: Best mileage at beginning of tank?
 

We notice much better MPG's now that winter is over and the temps have been > 50 or > 60 when we drive the prius. Today, we got 59 MPG for the first 17 miles after filling up. Now the Read Out is saying 51.3 after another 40 miles of 50/50 highway and city driving.

Re: Best mileage at beginning of tank?
 

I filled up last night just before my 43 mile commute home and when I stopped to pick up a take-out order at the 99, the MFD indicated 37 miles w/ 58.9 mpg. This by far the best I've ever experienced at the start of a new tank. I will admit I was driving at 60 mph for about 25 miles hwy and really worked to optimize the battery usage for the non-hwy miles. The temp was about 63 degrees and dry road surface.

Re: Best mileage at beginning of tank?
 

Other things being equal, one might expect slightly poorer efficiency at the beginning of a tank because of the greater weight of the gasoline itself.