The background to this post is in this thread:

https://www.jaguarforums.com/forum/s...e-test-115462/

The results here were obtained under the following conditions:

- one vehicle for all observations
- no powertrain changes
- same tires, 38 psi
- the same route
- 300 miles per outbound and return leg
- 6000 miles total
- observation via onboard trip computer
- tank filled to first click-off at same station at the beginning of each leg
- reset to zero at top of highway ramp at start of each leg
- obd-ii logging used to confirm LTFT

- 80 mph cruising speed


The fuel economy observed:

regular grade gasoline with ethanol: 18 mpg (us gallon)

premium grade ethanol free: 22.5 mpg (us gallon)

therefore, the difference as a percentage:

(22.5/18)-1 x 100 = 25 percent

As the reader can see, the conditions are consistent through
all observation periods.

The onboard trip computer might have an error factor, but since
it is always the same vehicle, it is also consistent through all
observation periods. Therefore, an assertion that there was
a relative improvement is perfectly valid.

As a matter of fact, the distance is understated since the tires
are oversize and a higher tire pressure is habitually used. The
fuel economy numbers would be higher if a distance correction
factor were applied.

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