I had an '07 Access cab 4x4 V6, got 21-23 mpg on the highway, depending on what I was carrying (bike or not), and whether or not the a/c was on. Might drop to 20 with both. I hear that the 4cyl./5-speed isn't much better because the heavier truck and less power. I have a double-cab longbed V6 now and get 20 with bike in the back and a/c on. The older Tacomas were smaller and lighter and the advantage would be there in them. 2 bikes with gear bags and stuff for the day shouldn't be too much an issue, but you'll need to have the tailgate down and something to keep your stuff in. Tiedowns or bungee cords are the easy way, a bed extender is a little better, but more work on and off.
I wouldn't tie bikes down to the Toyota rail system, I was never all that confident in the security of it with just one bike, let alone two. If you watch it, you'll see it flex alot, and it is only plastic, with the standard tiedowns rated to 200#. I use a Yotarack, which attaches to the rail with it's own hardware, spreads the weight around the rail, and provides wheel chocks and tiedown rings for bikes.
www.yotarack.com There is also a means of adding tiedown rings to the front corner of the bed floor, it's a cheaper way to tie things down. The bed on the 06-11 Tacomas is a plastic or composite, to combat the rust issues they had in previous years (99-01??), so it is hard to drill and secure anything in it as well, plus it is weird shaped over the top of the bed sides, and no stake holes to mount tiedowns in. There is alot of info on
www.tacomaworld.com about everything to do with Tacomas.
The cab space behind the seats is good, I've had my adults kids sitting back there for short rides with no big issues, so storing stuff behind it is easy. Seats in back are forward facing, so leg room is limited. With the seat back comfortably, I could put my gearbag, toolbox (a big one), and other stuff back there, and it was below the window.
