MPG reading appears way off??

It is difficult to judge by one result.
You need to accumulate statistics.
Any measuring instrument can have an error, including a trip meter, a meter on the dashboard or a meter on a gas station pump.

 

If I were you, I would not "fill with gas as much as possible" You risk triggering the Check Engine Light for fuel vapor capture issue. Let the pump click off and stop there.

 

Well, first thing you need to do is stop pumping gas after it clicks off. You risk damage to the EVAP system by continuing to force gas into the tank. Secondly, it takes effort to get good mileage out of the Prius. You can't just drive with a lead foot all the time and expect great fuel economy.

If you want to keep track of your fuel economy, use Fuelly.com.

 

What fuelmiser and coocoocachoo said about stop pumping gas after it clicks
It’s going to cost you big

 

I've topped off both Prii and the RAV4 HV since day one with no problems. In fact...I have done this with every vehicle I've owned since HS and I'm 66 now...and a career mechanic.

 

It is unlikely that the tank is filled to precisely the same level each time, so filling after consuming only a few gallons will amplify error in the gallons part of your measurement. You can probably get almost a gallon into the tank and gas evaporation system after the pump first clicks. That will ultimately damage that system and may cause it to fail environmental checks in regions that have that.

If the engine isn't broken in and you are driving in NH in February that also won't be great for efficiency.

My car typically estimates about a mile per gallon, or about 2%, more than refill calculation indicates.

 

I've also put the max amount of gas in the tank for the last 18 months with no problems.

 

My understanding is that there is an evaporative emissions system in all modern cars with gasoline engines that routes vapor from the filler neck to a charcoal cannister.

When impoverished student Winston sees gas for $1.10 when it's $1.45 everywhere else and clicks the nozzle until he can see gasoline in the filler neck, he ends up dumping liquid gasoline into the charcoal cannister. When that happens enough, the evaporative emissions system stops working and you can have problems with passing emissions testing.

That understanding of mine may go back to Click&Clack episodes.

 

You're doing ok, especially for those speeds.

Toyota Prius MPG - Actual MPG from 9,177 Toyota Prius owners

 

Correct.

Also, liquid gasoline in the evap canister ruins it. And there are two other valves that are part of the system that can be ruined if the canister gets filled with liquid. There's a vent valve that allows fresh air into the system that is supposed to only allow air in one direction, but can get stuck open if charcoal particles get stuck to it when they are carried by liquid fuel. This will result in gas vapor escaping the vehicle into your garage, house, whatever. There's also the evap purge valve that releases the collected fuel vapor into the engine to burn it off. If that valve fails, you'll get a check engine light and the car might stall out.

The charcoal canisters are often very expensive, and some are placed in terrible locations that means you have to tear apart a good chunk of the car to get at them. I've heard of cases where the rear subframe, exhaust, differential, etc. had to be dropped to get at the canister. In the gen5, the repair manual says the exhaust has to come down. And the MSRP for the charcoal canister is $680. So to have it replaced at a shop would probably run you $1000-$1500 depending on whether the vent and purge valves are involved. You could do the job yourself for as low as $375(max discount on canister and no labor), but that's still $375 down the drain for what benefit? Is that extra half gallon in the tank really that important?


Here's a video from TheCarCareNut on the topic. It's one of his earliest videos, so the production quality and his delivery are way below his current standard. Still, it explains everything(including basic diagrams) and it's from someone with real experience and not a Joe Blow like me.

 

Probably you should believe neither. However, people claim that the Gen 5 mpg gauge is accurate unlike Gen 4’s, the latter of which overestimates by 5 mpg.

4 gallons is not enough for an accurate mpg estimate. You should aim for 9 gallons or more next time.

Also, the car needs to be on a slight downward slope when you fill up; otherwise, it will take 1–2 gallons less than the rated capacity. This is probably what happened, and you were shy about 1 gallon from a full tank, which works out to exactly 39 mpg as on the gauge.

 

Presumably for a more accurate fill?

Considering the possible detrimental effects to your evap system, and just to expedite fill-ups (especially when there's someone behind you, drumming their fingers), it's simpler to just pump, back off a little when it approaches full, and then stop, when the auto-shut-off first activates. This'll be approximately the same from fill-to-fill, and while there might be slight differences, it all balances out.

 

Was your previous car a limited AWD model equivalent ?