I know its hard to an accurate read on the results with so many variables and stuff. I did my wife's car while on a road trip so it was a little easier and more straight forward test all highway miles. But if you can at least take the same route to and from work on two separate tanks gas you can at least get an average. Like I said I did experience a gain in mileage which meant I had to stop less for fill ups and I did get more then enough mileage to pay for the extra cost. Also I think the XK is meant to run on 89 octane so I'm curious to see what you get out of it. Looking forward to your results.