300 hp: an anecdotal reading, from any car 'mpg' data, is just that...an anecdotal slice.

I often show 34-34 mpg on my VetteVert, after fillup/reset of mpg, from sliding away from the pump, back onto the highway at sane speed, for a qtr - half the tank.
(It 'cruises' in 6th at <1500 rpms).

That readout showed almost 32 mpg after 9,000 miles of our 13,000 mile Road Trip last summer...nice to see and take a pic of, but I was doing the arith on every tank, keeping records. That Vette averaged ~28 mpg for the entire 13,000 miles, so I am very confident in that mpg 'average'.

Our '01 X shows 20.6 on our OBC, but that reading has only been reset a few times in almost 11 yrs and 80k miles...but, it is a good indicator of what mostly highway trips can return, over a long time. In the real world, our X's per tank mpg varies from 17 low to 23 high...I have a small notebook full of tank fills/arith on it.

So, I am not skeptical, per se. It simply is not a real world mpg 'reading' based on a trip, or even a full tank consumption result. Could a gas E70 get '29' on a tank? Doubtful...though maybe a road trip at 50ish, in cooler weather, no A/C, tires pumped, and no uphill or loads, might get a tank in the mid-high 20s.

Not disparaging, just not realistic to expect '29' anytime, other than a short term 'readout' from any car brand mpg 'reading.'
GL, mD