This turned out to be a very useful discussion!
Answering a couple of posts here.
1) Ratio doesn't change with engagement of AWD. It's the same at front and rear, and not variable in any way on our cars.
2) The gear ratios, to the degree we can trust the details of Volvo's published specs, are available by drilling down into the year->model->specs pages at http://volvocars-pr.com. The 6MT 4th gear ratio is 0.95:1.
3) I don't know what I did in my math, but I agree that the published 2005 Volvo specs are wrong (not the 2006), and the final drive must be 4.00:1 in the AWD's.
Here's an analysis for my tires and a 3.77:1 final drive:
Michelin Pilot Sport A/S 205/55-16: 823 revs/mile (specs)
3.77 final drive
0.95 4th gear
2210 engine rpm.
OTOH for a 4.00:1:
Michelin Pilot Sport A/S: 823 revs/mile
4.00 final drive
0.95 4th gear
2345 engine rpm.
This is 95 rpm less than I observed, but clearly is the better match to the data. In fact, 95rpm is just about exactly 2mph, which is basically roundoff error.
Now we know why the AWD suffers worse fuel economy from the FWD!
In addition to the extra 130 lbs, and the increased drivetrain drag (though these are small)...
It's the rpms. 6% higher rpm's means 6% more air going through the engine per mile and therefore 6% more gas. 6% of 25mpg is 1.5mpg right there.
On the other hand, and putting my loony hat on, I can see why this is done.
The main reason is likely the Haldex unit. If the final drive ratio is numerically lower, the taller gearing means the difference between wheel slip and traction is going to be small. This means the Haldex won't be able to detect it as quickly, or as certainly. It will be less effective. And, high input rpm's will give its oil pump more action.
Second reason is probably takeoff. The AWD pulls with 4 wheels and therefore pulls down more torque. Getting the engine into the powerband earlier is a god match to this. And, an AWD driver is going to be looking for this.
OTOH, the FWD is going to actually want less torque at first, in order to avoid chirp. But the longer gearing will make up for it and give it a better run.
Jameson, I envy your ability to be able to evaluate the car both ways. Wish I could feel the difference first-hand!