No, that wouldn't effect MPG
The filler hose would be the issue
1988 should have this type of filler hose:
https://m.media-amazon.com/images/I/61FcG6ngdoL._AC_SX679_.jpg
If you fill a coke/beer bottle with water and flip it upside down to drain the water out it comes out very slow because it has to let air in to let the water out, they have to share the small opening in the bottle
If you do the same with a glass of water, the water comes out fast because air can come in fast
The filler has a separate IN(gasoline) and OUT(air) hose
In the above pictured filler the gas goes in the smaller hose, and the air comes out the larger hose
The Air comes out the smaller holes in the metal part of filler
So if gas can't go in fast then either the smaller tube is kinked or blocked, or the air can't get out, so both have to share the smaller hose
There is another Vent on the gas tank, the EVAP system
In the engine bay is a charcoal canister usually mounted on rad support drivers side
This has a hose that runs back to the top of the gas tank
As gasoline flows in the air from the tank flows out and thru the charcoal filter in the canister, limiting gasoline fumes(pollution)
If that hose/filter gets clogged up it can limit how fast gasoline can be pumped in
Pull the hose off and blow thru it