Has anyone here that has converted to these or another fuel injection from a carb noticed any DROP in fuel mileage? I have a friend who recently install a new FI Tech and lost from 11 down to 9 when he installed it on his 383 street cruiser. Now I know him well enough that he is very meticulous with his calculations. He says it starts, idles, runs better and has more power. He is taking it to a friend who is going to help with the tune up. I am advising a customer on a purchase and will install the system. Any real world experience here?

calculations are one thing.
How has his driving changed?
If he stomps on it more now with the efi, 2 mpg isn't a huge drop.
it is also possible he was under carbed or lean on his original tune with a carb.

There are a lot of variables here.
did he ever us an o2 sensor for tuning with the carb?

If the carb was well tuned switching to EFI will not improve mileage much if any, but once the EFI is tuned properly it will give the same mileage. The EFI is very easy to tune and will constantly try to adjust to the proper AFR. One situation could be his carb was way too lean and now he is operating with a proper AFR.

In my case on my 528 Hemi although I have not driver it much, from a few short drives I know the mileage is better. I never have checked the mileage, no point it was so bad, but to warm it up and take a 20 mile drive with the carb would use 1/4 of a tank. No exaggeration. I've made that same drive a couple of times now including some WOT blasts and the fuel consumption is noticeably less. But I never could get the Demon carb on it adjusted good. It was always just trying to get a balance between idle, transfer and main circuits.

My reading says the #1 cause of symptoms you describe, are exhaust or header gasket leak.

Or he could have a malfunctioning water temp sensor or O2 sensor.

Or his car really likes vacuum advance and it has been disconnected.

Good Luck!

His problem can only be fuel or spark.

Does he have a wideband o2 gauge so he can see his a/f while driving? The base calibration in his efi controller may be looking to cruise at 14.7:1 where he can go more lean. Running good and feeling good by the old seat-o-pants meter does not necessarily equal optimal.

How about ignition timing? Does his efi system do timing? If yes, perhaps he has not tuned his spark table to account for the equivalent of vacuum advance. If his EFI system does not do timing, perhaps he did not properly hook up the vacuum advance when he changed from a carb to throttle body.