It doesn't say on the window and in the owners manual I should have a 23 or a 35. The most I've put in my truck is 29.somthing and I had 3 miles till empty and the needle was in the red. I'm thinking I have a 30 gallon tank but how can I find out for sure.

I have a 2015 F250 6.2L CC SB FX4

 

It's 35 gallons. Not sure where you're seeing the 23 gallon option, but all 2015 SD pickups with gassers have 35 gallon tanks.

All other size options, and there are a few, are either for incomplete trucks (chassis models, no bed installed) or for diesels.

 

Thanks I wonder why when I still have 5-6 gallons left in the tank it shows I have single digits miles till empty. I know they usually give a couple of gallons when it's on 0 but 5-6 gallons?