Running the engine at low RPMs will cause formation of deposits on valves and, as one of the consequences, rough idle and poor starting. The now commonplace term "Itelian Tune up" is aimed at undoing just that, and the cure is simple - driving for ~5 min. at high revs (5,000-6,000 rpm) on a lower gear. Volvo engines love to be revved. If you let you engine cross 5k line every now and then, it will ensure cleaner chambers, spark plugs and valves, and, as a result, you'll get better fuel economy.

I consistently get 19-21 mpg in a mixed 60%hwy/40%city driving, no matter what gas brand or tranny mode (S/E) is used, which is fine considering my driving style: I like to floor it off the line and can't stop enjoying the strong pull of the LPT engine. The RPMs jump over 5k quite often, and I have never had any problems with idle or poor starting. I check the plugs every 3-4 months, and they have been just fine since I put them in new abt. 8 months ago.

Cheers,

Vadim.