I have an 87 780, into which the 230FT/M46 engine/transmission from an 89 740 wagon has been transplanted. Apart from a rough idle, it seems to be running pretty well.
It does seem to me to be quite thirsty however. I am getting about 300 km on a 50 litre tank of gas - mixed cityl/country driving. This works out to about 18 mpg (US gallons). When I went to the EPA website and checked their ratings for 780s with the 230FT and 4-speed automatic, this doesn't look far off what they say is normal.
I would appreciate anyone who has this engine/gearbox combo in heir car/wagon or anyone who has the 780 turbo to let me know what their mpg looks like. If this is normal fuel economy, that's OK by me - I'll learn to live with it and stop trying to figure out what's wrong with the ECU/ injection/ignition control systems.
Thanks in advance for any enlightenment you may be able to send my way.
Bryan Colleran

Bryan,
you situation is a particular with the motor/trans from an 89 and the rear end gears from your 87. If I remember correctly the gear ratios in the 780's were around 3.73:1? This is just a guess. My 89 780 was a pretty thirsty lad as well. I managed about 22 mgp. On the freeway! That was the best I did-on the other hand it would whip the pants off the manual 740 turbo that I have now - no contest.

jes

Sorry to bum you out even more, but 50 l for 300 km works out to 14 miles (22.7 km) per US gallon (3.78 l). And I thought I had it bad on my 760 at 16.7 (average) miles per US gallon, and that car has a 2.8 l V6!

What I have checked on mine so far:

Ignition: new part from last year, should still provide good performance.
Fuel: pumps new, air filter almost new. AMM replaced a few years ago, O2 sensor fairly recent, catalytic converter less than 1 year old.
Tiere properly inflated, but noticeable oversteer yet tires show no marked "out-of alignment" wear marks.
Driving 20 to 100 km each time, AC off, cruising at about 100 to 120 km/h

Sorry I can't help as to statistic, hope my checklist can help. In any case fuel consumtion appears to be high IMO. Then again, I'm new to Volvos and have been spoiled driving a Diesel Toyota Land Cruiser for 15 years which at over 4000 pounds can cruise on the highway doing 30+ MPGs at 90-100 km/h...