[aerohead]

I found a CAR and DRIVER road test for the Volvo 760 Turbo-Intercooled Wagon.
It's 'larger' and heavier than the 245, but you may be able to adjust the data using delta-frontal area, and delta-curb weight.
* the larger 760 has 2.16-meters'squared area, vs 1.95 meters-squared for your car.
* the 760 is 1467.82-kg curb weight. I don't know the weight of your car.
* both 760 and 245 are Cd 0.41.
* the 760 has the BOSCH LH-Jetronic fuel injection.
* the 760 has AiRsearch TB03 turbo, with 14-psi boost.
* the 760 has Pirelli Cinturato P7, 205/55 VR-16 tires, Cfrr- 0.0154413.
* the 760 has 4-speed trans with electrically-locked overdrive, @ 41 km/h per 1,000-rpm.
* 137.954-kW power.
* 205-km/h top speed, @ 4984 rpm.
* EPA HWY 9.0467- L/ 100 km
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Guestimating from known aero-mods, depending on what you'd be allowed to do, without adding any length to the 245, there's a high probability that you could reduce drag down to Cd 0.266, delta Cd 0.144, delta drag 35.12%, with an mpg increase of 17.5% according to 1978 SAE automotive metrics.
If you added length, you could do better.
Mods would include:
A-pillars
wheel covers
rear skirts
belly pan
rear diffuser
rear roof wing
aero mirrors
rear tire deflectors
wrap-around airdam extensions
rocker panel extensions
partial grille block
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* synthetic engine oil would make a minor improvement
* synthetic differential lube would make a more significant improvement
* LRR tires would make a significant improvement ( Cfrr 0.0055 Bridgestone Ecopia for example ).