Hey Landy, can you put up full specs, too much guessing as you haven't supplied much.
What is the L/100 really? kays to 3/4 tank is way too raw and not accurate. Fill tank, reset odometer to 0, drive till low fuel light comes on then fill again and note litres used. Now report back.
Auto's will chew more than manuals. if you lost economy with swap, you got something wrong somewhere, time to visit and diesel mechanic who has a pump-bench.