I couldn't even fathom getting that kind of mileage.............
Do I need to check my valves?
Is my aftermarket slip-on causing a rich burn because of the reduced back pressure?
I am at a loss.
I've always thought that highway mileage should be better not worse
W/ my '08 in AZ (I'm going by memory...) I get low-to-mid 50mpg as a NORM, but I have had OVER 60mpg (US gallons, of course!) a few times. Very SELDOM does it ever dip into the 40s. As the US Gen 1 and 2s come w/out the oxygen sensor, I personally would NOT run an aftermarket 'can', as you are changing something on an "open-loop" FI system.
Have you EVER checked your valves, Jeff? Despite what the "book" gives as an interval, quite a few of us have checked them EARLY and found some TIGHT valves, and open up the clearances to the MAX end.
I 'did' them in my '09 at 10,888 kms (6,765 mi) and found ALL FOUR EXHAUST valves at .009", against the book limits of .0087 to .0122"; and the intakes JUST ABOVE .006", against the book limits of .0059 to .0083", so IF a dealer had done the check, they would PROBABLY NOT have done anything, as they ALL WERE W/IN LIMITS!
I, on the other hand, 'opened' them ALL up to very close to the MAXIMUM.
I re-checked them two years later (at 39,440 kms - 24,506 mi) and found that ALL valves had tightened SLIGHTLY, but not enough to require anything done.
On the GREEN HORNET TOO (my '15), I checked them at 26,243 kms (16,306 mi) and found 3 exhausts at .225 mm - just ABOVE the tight-limit of .22 mm; and the intakes had ONE tight, one just ABOVE min, and the other two a BIT looser, but I opened them ALL up.
I have no experience with the later gen versys' (versii? lol). I'm not sure if the 2nd and 3rd gen's have an O2 sensor or not....
I BELIEVE that all the Gen 3s have an oxygen sensor (MY '15 certainly does), thus operate "closed-loop", giving the better mileage.