MPG drop from 65mpg to 35mpg in the last 4 weeks

have you cleaned the egr circuit? how many miles on her?

a lot of things can reduce prius mpg, but dragging brakes is an obvious one.

running a little rough can be plugs/coils/injectors or even a minor head gasket leak

 

What’s the miles? London Ontario or England?

 

Are those MPG numbers from a built-in display........or from actually dividing miles driven by gallons used ?

It is HIGHLY unlikely that you EVER really got 65 MPG for anything but a short "perfect" drive.

When you reset the MPG calculation, it is common to come up with an unrealistically high number for the first 5 miles or so, which then "averages out" the more you drive.

I think your 35-40 is "normal" and does NOT indicate a problem.

 

Disconnect the 12v battery for 10 minutes.

 

IF......you were not doing the math......miles divided by gallons.......then you are just chasing a ghost here.
Something might have changed causing you to not be able to fill the tank completely.

It is a shame that you actually believe that you EVER were getting a true 65 MPG.......or anywhere close to that.......because it just is not possible with normal driving.

AND.....are you really quoting MILES per US GALLON.......or something metric ??

 

It would be helpful of OP could clarify: London, Ontario, Canada, or London, England. The latter has a bigger gallon.

 

It's amazing how many people have trouble grasping this sort of thing just like flipping on the consumption screen and believing you're getting 94 miles to the gallon yeah for a 1/10 of a second etc but these conversations will go on until the next big thing easy or whatever and then we'll be talking about front ends falling off cars and charging 10 times a day in the winter. Big fun

 

I’ll
Come back later and post my summer 67 MPG for 9,999 miles photo

 

Sure. You do that.
And I, for one, won't believe it for a split second.

 

I think @Grit's on the level, but IIRC he takes some extreme measures, weight reduction for starters?

 

IF you drive slow enough, you can get great fuel mileage...
It's a persons normal everyday driving that counts...

 

9,999 miles would be amazing at slow speed

 

especially if you have to pay someone

 

I'm surprised no-one has mentioned fuel - moving from 100% gasoline to 10%-15% ethanol doped gasoline can cause a drastic change in mpg. Though a consistent 65 mpg average on a 3nd gen seems to be a bit of a stretch.

I just did the math and it doesn't work out. 485miles/65mpg=7.5 gallons; 7.5gal*40mpg=300miles. 50 miles shy of the second tank full stated. 47mpg would yield the stated 350 miles a tank; which seems to be a more normalized number for a gen 3.

 

This was the halfway mark photo on my phone. , the 9999 miles at 67 mpg photo is archived on a computer which I’ll have to find the photo this weekend. I’m sobering up from from the purple wine staying up a bit later than usual to find the photos and searching the forum here because I posted the photo once

 

Add downhill only driving also!

 

Get a chevron fuel system cleaner and do this reset.

 

I guess that depends on your definition of "drastic".

The commonly accepted figure is 3%.

So if you were getting 45, the fuel alone could account for a drop to 43.6.
Not quite drastic in my book.