90 Chevy extend cab dually gas tank size?

I been doing searches trying to find out what size gas tank is in my truck and came up with lots of different tank sizes, I haven't ever filled it up to know. I'm planning to make a long trip and want to calculate fuel expenses. Anyone know what size the tank is in my 90 extend cab dually?

Have another question, whats the best mph to drive to get fair fuel economy and make it to my destination with out taking too long. It has a 454 Gen VI with TH400 trans and 3.73 gears. Im guessing around 55mph to 65mph. Im planning to go about 410 miles.

Shortbed trucks (rare in GMT400 1 tons, anyway) get the 25 gallon tank; anything larger will not fit.

Longbed trucks like yours get the 34 gallon tank. It's been rumored the 25 was optional but I have no idea why anybody would choose that, and have never seen one equipped that way.

Richard

Ahh, cool thanks for the reply Richard.
Must be right my tank does look like it could be the 34 gallon tank, since its a long bed and it looks kind of like all the tanks i seen in pictures.

If you were wondering how much it takes to fill up, gas is about $3.04 for mid grade at my favorite station, it takes me aover $80 to fill my tank up if its on E.

I can't imagin how much it would cost to fill up, gas price have gone to $4.25 for 87 octane (lowest grade).

That trip I made little over a year ago cost me $90 to fill up. Everything on the trip went well, but recently the Th400 is starting to act up. For some reason when the truck is warmed up it doesn't seem to up shift. I have to put it in manual 2nd then back to D to shift up, happens intermittently. Still haven't fixed it.

Installed new vacuum modulator, and governor still same issue.