There aren't many genuine speed feats that you can find in Dragonball (wouldn't count ki blasts as being light speed considering lasers are visible to the naked eye), which has led many to have to resort to powerscaling with the few feats there are from DBZ characters. The best speed feat found within the series that I could come across was when Gotenks as a SSJ flew around the Earth several times in a period of minutes. So let us do some math:
Diameter of the Earth = 7,926.3352 miles
Now assuming it took Gotenks a minute to fly around the Earth one time, let's calculate how many miles he flew in a second. By taking the Earth's diameter and dividing it by 60, my result is:
132.10558 miles per second
I'm not too familiar with how to calculate Mach speed, so I had to take it onto this sitethat can measure units of speed. I add in the speed that I calculated onto the miles per second area to measure Mach speed and my result is:
And we take it to this site which allows me to convert mach speed into miles per hour. Here's the Mach speed converted to MPH:
In MPH, light speed is 671 million miles from what I'm looking up online, meaning that SSJ Gotenks and other characters weaker than him in DBZ are nowhere near close to light speed.
Obviously, this result is likely gonna get DBZ fans fuming and thinking they know better than I do. But take my little calculation however you want.
As for how strong the characters are, do you mean via ki blasts or the physical strength of the characters? Because there are more showings of destruction via ki blasts than with physical strength in DBZ.