Its 17.125 gallons, but dont ever try to squeeze that out of the s14 due to the design of the tank.
Also my gauge has read about 1/8" over full, at full, since I got it in 99 with 28k on it. I now have 115k and it still does it. Also my car has always gone about 70 miles before dropping, and then after 30 miles (100 total) it will rest exactly on 3/4 tank (keep in mind its off by about 1/8").
I consumed 10.9 gallons to 303 miles last time. I was kinda down mentally and drove like the average commuter with a mix of highway and city, but also raced to a buck 2 times (which is very very rare anymore on the highway, but having taken my neighbors mustang in the on/off ramps the opportunity arised for a clean 75-100 blast where my auto took him

although minimally) and sat in a dead stop traffic jam for 40 mins as well... So yeah my modded auto s14 got 27.78 mpg last time, and averages between 22 and 28.
Having dismantled a few gauge sets for other reasons pertaining to other reasons, I can honestly say the problem lies in the placement of the needle.
The needle sits on a shaft and is turned from its laden state to its on state when the car is on and getting a signal from the float / ecu. If the needle happened to be positioned 1/8" off at the factory it will still read exactly the same as a properly aligned one, but will read 1/8" above full when full, and 1/8" above empty when empty. Or, you could say, it will still have the same range of motion as a properly set one, in degrees, as long as it isnt being restricted from the trim.
If you want you can care fully take the cluster out, and take the gauge out. Mark where it rests in its off position, with out any interferance (there should be none if you remove the fuel gauge from the cluster, as the needle goes lower then the trim). You could lay a small peice of tape on the face under where the needle sits and mark this position. You can carefully remove the needle and press it back on in the corrected location, or IIRC (dont quote me as its been a cpl years since I cracked open an s14 cluster) you can simply rotate it CW till it stops, and rotate it just slightly more the amount needed, so that it will spin on the shaft. Let if fall and it should be in the correct location. Put it in the cluster and try it out. It should be ok now.
Thats exactly how I would do it, but its not a big deal to me. When I stuck the float down cornering agressively and had the neddle plummet the rise 1/4 notch then plummet to rise and fall rise and fall, I was a bit concerned
