Quick Question and Intro

you can get old muscle to have good mpg. if you wanted to do a 60's mustang you could put a newer efi 5.0 drivetrain in it with a 5 speed and get a good 20-25 mpg and still have good power and have a car that you can put money into a car that will appreciate in value and you don't have to smog!

Or... a newer mustang and do the basic bolt ons.

have you ever been in a mustang, new or old?
what do you like about your car? just mpg?

Are you talking about a Shelby GT500? Because going from a 25k new honda civic SI to a 70k Shelby GT500 is a huge jump. There night and day, the Shelby is a 500hp car and a Si is what 192hp? Not 100% sure on the SI. I think what you were looking at was a Regular GT. In which case it's really not that bad on gas. Of course the honda gets much better gas mileage, and will probably give you less problems down the road, but your talking about two very different cars.
Also you cannot test drive a Shelby unless you put a down payment on it.

They also have the hertz Shelby's, they are cheaper than the GT500, but they also dont have nearly the power.
You will get one answer on these forums, since it is a stang site. Personally I would not buy a regular Gt mustang, after owning and driving cobra's I will not get a regular one again. They are very nice, and i'm not putting them down by any means, but if you drive a cobra you'll know what I mean.
Also the new hondas are really nice cars too, I would opt for the honda if it's going to be your daily driver, especially if you commute a long distance to work.
Either way you go they are both nice cars, just decide what you want out of the car.