The gauges and senders are the same for 15 and 22 gallon tanks, as far as I know. Just as a general rule, you should NEVER trust a gas gauge to be completely accurate. Those senders only read a comparative level in the tank - but you can't count on that to be accurate. This is doubly true if the vehicle in question has an oddly-shaped tank, as many today do.
My daily-driver Chevy S10 has an 18-1/2 gallon tank and I know that when the gauge needle reaches 'E' there's still another 3 gallons left. However, I don't like to test my luck, so once the gauge reads a 1/4 tank I start kooking for a place to gas up.
My '88 Dakota needed a fuel pump two or three years ago, and I had to completely drain the tank. When I put gas back in it, it took five or six gallons before the gauge needle even twitched. Later that day, when I got it to the gas station, it took another 17 before it clicked off, and the needle then was firmly against the right peg.
I think the logic of this is to give the driver a reserve so he (or she) doesn't end up walking - which, trust me, is a good thing provided you don't try to outsmart it by routinely driving the truck with the needle on 'empty'.
The only way to be sure about your tank's size is to empty it completely and see how much fuel it takes to refill it. The easiest way I know to do this is throw a (full) five-gallon gas can into the bed, run the truck till it sputters and quits, dump the gas can's contents into it and hit the very next gas station.
Not very dignified, to be sure, but it works. Be sure to add the five gallons to the pump reading you put in when you do get to the gas station.
Oh, and yes, I *DO* suggest a five-gallon can. For some reason, Murphy's Law demands that a vehicle when run out of gas will come to rest in such a way that the pickup tube or fuel pump is on the 'high' side of the tank - and a single gallon is seldom enough. Go ahead, ask me how I know that....grin.
Last edited by xaenon; 05-17-2011 at 06:53 PM.