In general, this has been a controversy among drivers and expert analysts.
Analysts have repeatedly reported that modern cars don't get any measurable “mileage” benefit by having the higher Ron (or higher Octane, as the yanks say). At least nothing above Ron 91.
But actual owners say different. I’ve been driving cars since the ’80 years. Even my little Japanese economy cars used to knock when you push down on the throttle in a hurry, with only 89 Octane in the tank. So I used to experiment with 91 and settled for 97 religiously in my cars. Lots of us say, go for the highest Ron, and poop on any analysts!
I think no matter what Ron you use, you won't have any chance to see any sort of benefit, if you're the typical driver who doesn't have much smooth self-control with his foot.
I too have the 2017 XE, but it’s the 25T with the last of the Ford engine cores. I have only experimented with 91 versus 98 (the other numbers are hard to find around the Northeast USA cities).
But to be honest I was trying to compare the petrol BRANDs (BP versus Canadian Shell), rather than the Ron.
I found the Ron 91 made the shiftpoints feel less “urgent”, so that even going slightly uphill the XE didn’t “leap” into sudden acceleration as much (I’m sure you’ve experienced what I mean by the “leap” that the XE does when it’s brand new---scares the bees out of your passenger sometimes).
The Ron 91 also made the exhaust growl “less obvious” going from 2nd from 4th gear (the gears you tend to hear the most when going from traffic light to traffic light in city driving).
Mileage with Ron 91 was actually the best I ever had. And though I did not measure the numbers, I was grinning at the fact I could get an extra day of driving before the instrument panel showed only 10 miles worth of petrol left.
Now, on the 98...
The Ron 98 feels more “impulsive”. Not aggressive or more powerful (the XE is always powerful to me, and I spend most of my time subduing the temptation to just take the highway from everyone), but quicker to obey even the lightest touch on the throttle. I find myself overdoing the throttle more frequently using the Ron 98 on the highway, than when I was using the 91 petrol. And… the car growls more audibly, even when I’m just pushing the throttle just a tad quicker at a random traffic light. Not the stealthier more mellow growl of the Ron 91, but the more audible familiar growl that Jaguars and Mazeratis are famous for.
But my mileage was LESS with the Ron 98. I would have to refill my tank in 8 days (where the Ron 91 would give me 9 days before refill).
Why? Could it be the different Brand? (Ron 91 was Shell, and Ron 98 was BP).
Could it be that the Ron 98 made the ride feel more “enthusiastic”, and the enthusiasm actually caused me to put a tad more pressure on the pedal, to accidentally pee away more petrol per hour?
Was the “less urgent” response of the Ron 91 actually training my foot to be more reserved and gentler with the pedal over time?
I don’t know.
I decided to run with Ron 91 for this whole winter, because it helped me treat the vehicle more relaxed during the winter driving, and put away all my sporty fantasies (which is always a good habit to follow during winters in the city).
But my preference is the Ron 98. With some focus and practice, I bet I can gain the same higher mileage that the 91 seemed to be giving me.
And dealing with the more impulsive car that sings a bit louder with from the exhaust pipe...is just all-around more fun.
