Yes they are lighter. A gallon of gas only weighs a little more than 6 pounds. An extra fifty or sixty pounds isn't going to make any difference. Besides the cost of the small amount I will be spending on gas will be offset by me not buying the inevitable mountain dew. I am not going to put gasoline in the car. One mishap and the smell would never come out. I just seems a little dangerous. Tanks are strapped on so I should be alright as long as I use the straps from the accord I take it off of.

I have a 11.9 gallons tank with my VX (coming from a DX probably) and I get like 625 miles and more (1000km).

So 13-14 should be more then enough...

Will an accord fuel pump being of a different size and pressure output matter if the crx pump doesn't just slide right in. I like the idea of a 90-93 accords 17 gallon tank if it's a direct bolt in or the 16 gallon tank from the previous gen.

I was mistaken, I thought your car already had a 12 gallon tank. Rockauto.com lists a 12gallon tank as the correct one for a 1991 Honda CRX HF, and says it cross-references with several civics. It is the same tank as the civic hatchback of the same year.

rockauto.com is kinda cool, for most parts you can look at a picture of the particular part, and they will tell you every other car that part fits on. I had been looking at both model numbers and the pictures of the tank shapes when comparing what would likely fit from other '91 Hondas.