The real question here is,
"Is your speedometer accurate?"
Chances are, no, it is not. Speedometer accuracy changes depending on the age, the design, and the tire size. Everything is engineered to be accurate with factory components. Older YJ's have a mechanical speedometer setup with a cable and a spring inside of the gauge itself. Over time this spring gets worn out, so accuracy is altered, even with the factory 215/70/15 tire size (which is about 28"). I had 235/75/15 tires on my YJ (about 29.5" tall) and my speedometer was perfectly accurate on the money. My spring was worn out inside of the speedo itself. Newer YJ's with electronic speedometers do not have this issue.
Case in point, any change to the factory specified components alters the speedometer reading. 33's would throw it off quite a bit, so you might actually be going 75mph while the speedo only says 60mph. You need to verify with a GPS unit, preferably 2 separate ones (your phone, and a traditional GPS unit typically works well).