I would think the impact is minimal. Plus Fuelly doesn't track partial tank fill ups individually if I recall correctly. On a really small car I can see weight have some help, but on a vehicle weighing over say 1 ton (2000lbs imperial), 75-100lbs is not going to make much difference. Most vehicles weigh between 3500 to 5000lbs.
- Negligible weight advantage, relative to the weight of the car, as DTMAce pointed out, and bentheswift further qualified
- Non-negligible risk of fuel pump damage if you ever run completely dry. Modern fuel pumps are fuel-cooled, so running out is bad. Not to mention sucking in the last dregs with air/tank-bottom crud cannot be particularly good for the filter and engine (if anything makes it that far) either!
I like the myth of the "last of the tank crud". The fuel pump is pulling the fuel in from the BOTTOM of the tank all the time! There technically shouldn't be any crud in there anyway. When I pulled my 200k plus mileage tank off of my Z it was clean as it could be on the inside, with NO crud or dirt but it had bad outer rust and had a leak near the top, hence the replacement. I can see this being the case on some older vehicles, but most cars the past 20 years or more pull their fuel from the bottom of the pump/tank. But I will agree on the fuel being used to keep the pump cool, and it is still not advisable to run around on little to no fuel all the time. You shouldn't even do this during the winter especially, in case you get stuck in a ditch for awhile and need to run the car to keep warm. Been there, done that. Providing you don't plug the front full of snow during the episode.
I know this, hence my post. But as I said, I highly doubt most modern cars have "crud" in them anyway. If you are filling the car properly, there won't be crud in the tank! Unless you are a person that is constantly leaving the gas cap off and the lid open, and drive through dirty rain or something.... All I know is that when I changed my 200 thousand plus mile tank on my car it was clean, no "crud" or other debris inside. AND since the pump pulls from the bottom (as I said above) it WILL suck in whatever MAY be floating down there, and it gets trapped by the fuel FILTER.
So assuming the fuel system is in good shape and the components are working properly and the owner filled and maintained it properly, it will be clean as it was when it was new in there. Simple as that.