Gas tank issue? Or something else?
 

I've got a problem I can't figure out on my Blazer, it's a 97 LS 2WD. Everything I've read says the LS has a 19 gal. fuel tank and the other models have an 18 gal. I've run it out of gas twice (unfortunately, due to my next issue) and when I've filled it from empty (not starting) it's only taken 17 gal... What gives? Is this a pump/pickup issue or something else? It just turned over 105k miles, should I check into a tank cleaning?

Also, my gas mileage lately has become very inconsistant after some routine maintenance, plugs, wires, cap/rotor, fuel filter, air filter, 0² sensors and new catalytic... My dad has a co-worker that had a similar problem and switch from Shell gas to a different brand (Swifty I believe) anyone else had this problem? I went from 19mpg down to 16mpg, then bounce between 16.5 and 18mpg right now, after being very consistent for a long time...

I checked on a pump/pickup/float unit and it's over $300, so trying to find any other possible solutions first...

Thanks!

Also, but possibly unrelated? My fuel gage is off, I'll go from full to 3/4 in 25 miles, and ride the last 100 or more miles in the orange... So I always just go by gas mileage/Trip odometer...

The 4dr model has an 18 Gal tank, the 2dr model has a 19 gal tank.

The fuel gauge issue might be caused by sulfer in the gasoline. Switching to a different brand might help as well as a fuel system cleaner that states it will remove sulfer deposits. It also could be the sending unit in the tank but if it were mine I would not bother changing the sending unit just for that.
Gas mileage can be affected by many things to cause 2-3 mpg difference. If the vehicle is not level each time it is filled and if it is not filled to the same level (not all nozzles click off at the same level) will throw your mileage calculations off. Where & how you drive. Cold weather makes the mileage go down particularly on short trips. Cheap or defective 02 sensors ahead of the converter can cause poor mileage.
Unless you keep pluging up fuel filters I see no reason to clean the tank.

When I purchased my Blazer in Feb it had some fuel gauge issues. Mainly if you filled it up it read full for about 200 miles then all of a sudden it showed empty. It also was VERY noisy. I had to replace it recently due to someone not supporting the tank when it was dropped to check for leaks. Not going to mention names on that one! I replaced it with a AC fuel pump. Now my fuel gauge works perfectly. It has the normal GM swing but it's now very accurate. Also it's super quiet. The old pump was an Airtex one. It might be just a bad sending unit.

Well I've switched brands for now to see how that does, all of the 0² sensors ahead of the cat are new (less than 3 months) as well as the cat itself. The fuel filter was replaced due to some poor starting issues/rough running and that seems to have cleared up (the fuel that dribbled from the filter was brown).
I do have the 4 door, so it would seem that 18gal is it. Most of my driving is semi-highway (20 miles to work each way on mostly highway/backroads).
I was hoping the cold weather would help due to no A/C drag and the -10° air temp = 1% HP roughly... (what I've always read in Car Craft).
The gas gauge has been this way since I bought the truck a year ago, and from what I could tell from the photo on the Autozone websie, the pump/sending unit/float appeared to be all together...

my truck is a little fubar'd in the gas determining dept too...i can fill it up drive 300 miles it drops to a little above 3/4 but if i hit a pot hole suddenly i'm full again...hell the one day my friend drove it until the tank was dry and it still read full...(i kinda thought it was weird i drove it 200 miles and the needle didn't move) unfortunately had the truck for bout 4/5 months..no warranty on it at all and thinking i should have taken it to my mechanic before buying it..could have gotten it for 2400 instead of 3g$ if i had a report of all the things that were wrong...but for the shape its in i couldn't have gotten a better deal..but it does bugs me about the fuel issue...and my mechanic wants 200$ just to drop the tank to find out WHAT it is...and if need be..charge me another 200 on top of labor costs to fix w/e is up with it...sh't life ain't cheap..not like it used to be..

You are right in that colder weather will produce more hp and no Ac drag which help mpg. But the richer mixture during longer warm up will decrease the mpg. Of coures how much depends on the length of the trip(s). Then if in snow the extra rolling resistance burns more gas.

Well a month ago the fuel pump finally went out. Got it replaced, mileage is a little more consistant, and my fuel guage is a little more accurate now. Maybe part of my problem was the pump?
Also found out it appears my TPS may be starting to go bad...