[Tommy]

Lightningrod,
If all you want to do is match the ohms of your new gauge to the old one, here's a way to do that. Go down to your Radio Shack or similar store and buy a cheap 90 ohm resistor. Disconnect your old gauge from the tank sender and wire it so the resistor is in the circuit where the tank sender would normally be. This gives you a known input the same as your tank sender would if it were at the level where it reads 90 ohms. On the 33-240 gauge that will show as about 2/3 full. On the 16-158 gauge it will show about half full, and completely empty on the 0-90 gauge. As most aftermarket gauges are of the 33-240 variety, that's the one I would expect to see.

BTW, if they don't have a 90 ohm resistor, you can wire two or more lesser resistors together in series as long as they add up to 90.