Car audio purists, don't crucify this post. The point is to illustrate/conceptualize an example :). Typically this scenario applies to rear speakers in a car.
Almost everyone is familiar with amp ratings like 500 watts x 1 @ 4ohms when talking subwoofers. Well, the same applies to radios and interior speakers because guess what, your radio has an amplifier inside it :). Only in this scenario there is an additional factor that people always skip over.....speaker wire resistance.
Just like sub amps radios have a sweet spot where they work the best, and that typically is at 4 ohms of resistance.
If you yank out your radio, take your handy dandy multimeter and put the probes on the speaker wire at the harness, chances are you are going to see a number greater than 4 ohms.(Probably in the 5-6 ohm range. This does not apply if you have a premium sound system like Bose, Infinity, JBL, etc.) For the rookies, a multimeter is a tool used to measure the previously mentioned 4 ohms reading. Remember where I said radios like to work in a sweet spot? You guessed it, the higher number means your radio is working outside of it's peak performance range.
"How do I get the radio in it's peak performance range?"
Simple. Math. Chances are the speakers in your car have 4 ohms printed on the back some where. Remember that reading you took behind the radio? Lets say it read 6 ohms. That means if you take the number showing on your multimeter(6 ohms) and subtract the resistance of the speaker (4 ohms) that will leave you with 2 ohms. That means the radio has to push that audio signal through 2 ohms worth of additional resistance sending the signal to the speaker.
Now.....brace yourself.....if you were to install a pair of say Infinity Reference 6032si which so happen to be a 2 ohm speaker vs your traditional 4 ohm.....guess what, when you add the 2 ohms worth of resistance of the speaker wire(1 ohm + 1 ohm), BOOM 4 ohms will show on your multimeter, right in the radio's sweet spot.
Change speakers. Get more power from your radio. A car audio twofer!