Two things:
1. This one is probably pretty obvious but all other things equal, bigger fuel injectors would probably under most/all comparable circumstances decrease fuel economy. (Right?) What my thoughts are though are not necessarily fuel economy differences, but rather how the ECU sees the differences (or not...). Will the ECU know the difference?
When I use my OBDII scanner, my Cooper S reports impressively high fuel economy. Today for instance, it reported out 54 mpg on a ~5 mi stretch of mostly highway at a steady 55 mph. (EPA rating is probably 30 mpg highway, max)
My car’s got 380cc injectors instead of the stock 330cc ones. That’s 13% larger. Pretending that that’s the only non-stock mod (it’s not), would it be safe to assume that my car was actually getting (.87*54=) 47 mpg at that time??

2. Also today, I launched (well, rolling launch) from 4th gear. Obviously it’s slow but the question is the fuel economy comparability of this. Again, let’s pretend all other things equal: On one hand you have a fourth gear launch which can get you up to speed (~40 mph) with say 75% throttle but only 1000-1500 rpm range. On the other, you have a normal first through third gear launch, let’s say 25% throttle but a higher 2500-3000 rpm.
What is technically more (fuel) efficient? Are they equal?


Post-replies edit: (hopefully some of you read this) Thanks so much everybody! You all made really good points and I’m so glad to learn everyday from Oppo!