Does my broadband plan speed and internet test speed mean anything?

My broadband plan is 8Mbps but when I use an online speed tester, the d/l speed doesn't even get near there. Infact, its still below 3Mbps. So if I change to a 3Mbps plan, will it reduce my speed, or will I still be fuctioning at the same speed as my 8Mbps plan since the test showed that the speed is below 3Mbps?