Yes, insurance would cost something, but you're literally saving $1500 a year in gas alone if you only drive it to work and back. If you drive it more, you save more. Not to mention you save miles and thus depreciation on your truck. Also, insurance won't be THAT much more because you will only get liability on the car, and insurance for the truck will be less because you aren't daily driving it.
Space... well that is a different issue. Money wise, it works out.
If you can't or don't want to swing the car thing, check out the 65+ mods link and the hypermiling tips. Adjusting your driving technique will be your biggest gain, especially if you're not going to go overboard on mods. Oh, and start with getting instrumentation too, it really helps tweak your driving. Good luck!
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