I just bought my 2005 s60 with 68k miles about 3 months ago.
I'm impressed, i filled up my tank, set the trip to 0, then drove until the gas light came on. Then divided the amount of gallons filled up by the miles driven.
Using 91octane my first tank did 28.7
Using 89 octane my 2nd tank did 28.9
Using 89 octane my 3rd tank did 28.9

All 3 were identical work routes, mostly highway.
Not bad. The computer avg mpg was showing less at 27.6 so I got better and more accurate mpg than the computer.

 

Very Nice i have s60 AWD and get 26mpg .. My suggestion sick with the higher Octane.. It will benefit the car over time.. 91 is OK 93 is better .. For the pennies per Gal its worth it..

 

26mpg for awd is awesome! Is your s60 2.5t or T5?

 

+1 what MYS6T said. Get the highest octane you can get. You may think you are saving $$ but it will cost you in the long run.

 

2007 s60 2.5t with AT
Averaging 24 mpg calculated and 22 mpg on computer on 2 tanks in a row of 91 octane. Roughly 100 miles every 1/4 of the tank.

Noticed on one stretch that has cruise control and reading instantaneous mpg.
60 mph - 27-29 mpg
65 mph - 24-25 mpg
70 mph - 26-28 mpg

 

Just did my 4th and 5th tanks, and they both were calculated 28.8mpg, on 89 octane.
I will try 91 to see, but honestly I cant tell the difference on mpg nor horsepower.
I guess we can agree that the computer shows less than what it actually is once you calculate it.