When I first got my HHR, I was really concerned about the gas mileage that it read on the dash. According to the car, I was getting 17-18 MPG driving on the highway (given it is rather hilly). But gas just didnt seem to leave the tank fast enough for those numbers to be true. So for the next 8 tanks or so I did the math on miles driven VS gas burned and according to that I had been getting 26-28 MPG. I belive that much more as my commute is 25 miles of a 55mph hwy. What I am asking is what gives? Is this a common thing on HHRs? Is there a reason for this or can I do anything to change it? I have checked my speedo against other cars and it reads the same so I know its not wheel size.

 

Which MPG reading are you looking at? Inst or AVG.

The inst gives you the mpg at that second AVG gives you mpg since the last time it was reset. Inst will vary infinitely; like from 0 to 99 based on the conditions.

It is in the Owner Manual. See the link in my sig.

 

Note. You need to reset the AVG mileage back to 0 when you fill up. That way it will display the average that you are getting with the current tank of gas.
I'm going to guess that it's never been reset.
Info on how to reset it is in the Owners Manual as Donbrew stated.
If you don't have a manual, there's the link in Donbrew's sig (as he pointed out)

 

Welcome to the site, it’s normal for low numbers, after a run to the sore part way thru a tankful with stops and starts hills and valleys.
set it to zero at your next fill up and don’t look at until your next fill up. See how it is then.

 

Welcome...

 

I just set the trip odometer at each tank full in all my vehicles.... then do the math at the pump.

 

I was looking at the AVG one and while I hadn't reset the AVG after every fill-up for those 8 tanks, I did reset at the beginning of my investigation. At the end the AVG read around 17 but on every tank I had gotten at least 26 according to math. This is definitely something I can live with as it doesn't affect anything, I was just wondering if anyone else has had a problem/quirk like this. I love my little HHR, the only things I have done to it are the an SS spoiler (not wing, I'm not a ricer) and a dukes Dixie horn from boom blasters. Since ol chevy lee is a salvage shes got her quirks but its my favorite car in the world.

 

My 2 HHRs the AVG was very close to pencil calculation, as was the miles to empty (range). I once literally ran out at the pump.

 

Welcome KL8T.....

 

I too checked my Actual vs Avg on the dash....at most, its .7mpg off....but many times its been Spot on!
...so id hv to think yours seems to be Way off.

Its possible that hving the/some module/s Re-flashed would render this huge miscalculation