A c20let with converted to fwd f28 6speed box will be good on fuel, as long as its running properly, ive had many c20let's in nova's- Corsa's and all have been good on fuel due to the light weight cars, ive also had a couple of z20lets ie mk4 gsi and 888 there not so good fuel wise and only 5 speed, i now have a nurburgring vxr and i would say its not bad on fuel av 30mpg and its around 300bhp

It also depends on how you drive.