I have an 05 reg cab Prerunner with the 2.7 5-speed. I never get less than 20 mpg even when pulling my little pop-up trailer (1700 lbs). My average is probably around 24 mpg. The four has adequate power. (But coming from a Tundra you might be disapointed with the performance. I came from a 97 2.4 Tacoma so the performance seems alright to me.

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I still agree with the other comment. I also prefer driving a pickup, but if my wife and I need to travel any distance we use her 05 Corolla. It averages about 36 mpg. I also would suggest a Corolla or Yaris.
I love my Tacoma and I usually try to avoid driving the Corolla. But, the price of a second truck kind of outweighs the the small savings in gas or the wear on your Tundra. Today I had to drive a round trip of 70 miles to work instead of my usual 18. I should have had the wife take the Tacoma to her work and drove her Corolla instead. Would I have enjoyed it as much? No, but I would have saved gas and wear and tear on my truck... If gas prices go up much more I might consider a Prius to replace the Corolla.
Since the V6 Tacoma almost gets the same mileage as the 4 (2-4 mpg less - read the mileage threads on this board) and if you don't tow or haul anything big, why not replace the Tundra with a V6 Tacoma. You would gain some mpg, still be able to tow, have performance similar to your Tundra and only have one insurance bill. Unless you need two vehicles this sounds like the most economical plan.