I have a 1998 3406E (550 hp) in Volvo 770 with a Super 13 speed tranny, 3.36 gears and 455/55R22.5 X-One Energy Drive Tires.
I averaged 9.2 mpg in January and 9.05 mpg in February driving 57 mph.
I highly recommend replacing the stock exhaust manifold and turbo with the Pittsburgh Power ported/polished exhaust manifold with larger turbo including blanket and exhaust wrap that picked my fuel mileage up 6/10ths with that change alone.
I also have the PP Box, PP Balancer/dampner, FASS, Walker Megaflow Muffler, Fleet Air Filter, VistaPro CAC, Airtabs, Real Wheel Covers, TST tire pressure sensors and use Mobil Synthetic lubricants in the motor (5W40), tranny and rears.
I highly recommend MicroBlue treatment on wheel bearings, tranny and rear ends to pick up between 1.7 to 2.0 mpg. Well worth the investment.
The Cat 3406E motor is a better fuel mileage motor than it is given credit for once the crappy stock exhaust design and small turbo is replaced with the PP ones.