Okay, first we need to clarify the difference between "tire" size and "wheel" size. Tires come in various sizes but depending on the car's rim size you can end up with same exact wheel height by combining a large rim with a low profile tire as you would with a small rim and a tire with a wider sidewall to it, in which case they'd be no difference in speed since the wheel's diameter wouldn't change. Thus when we're talking about bigger wheels what we're really interested is the wheel's height, or diameter (think of it as the difference in size between a dime and a nickel). That said, if switch to bigger (meaning taller) wheels your car will go faster because the distance covered by spinning the rotor one revolution (say from 12 o'clock to 12 o'clock) is greater the bigger the wheel is, which means you cover more distance per rotor revolution. Of course this means that your speedometer will be skewed slower than your actual speed is when your driving down the highway (for example, if your speedometer reads 55 mph you actually might be doing 65 or greater depending on how much bigger the wheels are. For this reason, taller wheels actually save gas, not waste more gas. Think of it this way: If you compare two Dodge Caravans going down the highway both at 65 miles per hour - one with small wheels, and the other with bigger wheels - the one with the bigger wheels will require less depression of the gas pedal to make 65 mph than the one with the shorter wheels (since the speedometer will actually read less than 65 mph). And we all know the deeper you press your gas pedal the more gas you send into the engine and thus the faster you'll be burning up gas. Thus bigger wheels use less power than smaller wheels at any given speed when you compare two equivalent vehicles side by side.
