I can go 250 on the first half of my tank, and then only 180 on the second half. The needle seems to move linearly to the 3/4 mark and then just nosedives....

Is this normal? Does everyone's fuel gauge behave in this manner?

Also, my pathfinder will hold 20 gallons of fuel, with the tank size listed as 18.5; same as the max. I sputtered into the gas station today and she took 18.5 gallons. I assumed the tank would actually hold 19.5 to 20, the same as my pathfinder. Is 18.5 gallons the limit?

 

yea, mine does that too. should be normal

 

I think I know what you're saying, and take it from somebody that had 3 fuel sending units replaced under warranty. What was happening was the needle would drift from above F to 7/8 full.

When you say 3/4, I believe you mean full. If that's what you mean, I would say NO with a capital N. What happens is, when the tank is topped-off (illegal in NJ), the needle is above F. It takes about 70 highway miles to bring it under the F line. That's not linear at all, it's slow to react. By the time it reaches 1/2, it is linear, all the way to empty.