Not to play devils advocate, but...
I've had a C43, C63 and a C63S, and fuel economy isn't too dissimilar. In fact, long runs, I actually averaged better in the C63 than I do the 43. I think the best I ever saw in my C43 was 35mpg on a trip up to Old Trafford, however, last time I drove to Old Trafford in the C63, I averaged 36.2MPG
even have pics to prove it I was so shocked haha
My C63S I think is almost identical to the C63, give or take. Around town, again, not massively different between the V6 and V8, maybe 10% in it which isn't really that bad when you're talking the low to mid 20's