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If the tire fits, wear it. I've never liked running on old mis-matched tires, just doesn't sit well with me in the safety department. Ditch the mixed 14" tires, but keep the rims. Run the 15" set until you find a better more suitable 14" set of tires. I'm assuming the fiero has wider tires than your neon, so that'll hurt, but the taller tire may offset to put you back where you started.
to address the air pressure statement, the sidewall max is rated at the tire's weight max, something I doubt most people run within 70% of. Do contact patch tests first however, before jacking the pressure above and beyond. All vehicles are different, the blanket statement of "pump those to 50psi" doesn't fit.
Safety. New tires are the reason you should change, regardless if they drop your mileage by 1-2. Safety of you, your passengers, and others on the road. Never sacrifice safety for better mileage.
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