Hi Tony:
The value being read accurately depends upon a couple of things. First, there is a bend the builder must make in the arm of the float sender. I would surmise that bending this arm a little off would change the readings on the guages.
You could easily start the test by taking a multimeter and checking the ohm reading on the sender. If the tank is full, you should be seeing about 33 ohms if I remember correctly (this is in the assembly manual if he gave it to you). The low end is around 2 or 3 hundred ohms. If at full you're reading many more than 33 ohms, you know that there's a good chance the bend in the float arm is a little off.
You can do the same at the low end (i.e. by draining the tank). Again, check the manual for the correct ohms you should be showing. Someone that reads this can probably post the exact values (I'm at the office and don't have it available right now).
Your fuel gauges are basically a couple of ohmmeters marked in gallons instead of ohms.
If you're bothered enough by this (I know I would be) you could purchase a fuel gauge that can be calibrated, such as the Electronics International unit ($$$, but very good and a great company) or similar. Since they can be calibrated, it doesn't really matter if the arm swings through the full range, as long as the resistance changes throughout the range of travel of the arm.
Hope that helps,
