Well, that does make it a little more of a challange.
First off, remember that it is off on a ratio; not a fixed amount. The faster you go - the more off it is. In your driveway : it is dead on correct!
If a factory tire is about 28.5-29 inches in diameter, the 30 will go a little farther with each revolution, so you're going faster than the speedometer says.
The ratio of the 28.5 inch diameter tire to the 30 (probably really 29.5, most tires are like lumber, stated size is .5 inch larger than real) inch tire is 1.0351
So, whatever speed you're seeing on the speedometer; multiply it by 1.0351 and you have a closer approximation of your speed.
I'll leave you to run the speedo readings that work best for you. I live in Southern California, so I rarely have to multiply 2 digit numbers! :rattle:
I sometimes make better time off-road on big rocks that I do on the freeway to work! AND I HAVE MORE FUN ON THEM TOO!
When you get a GPS and want to figure out how to pick the correct speedo gear (number of teeth) I can help you with THAT too. Many people will point you links with tables they swear by, just as many people will point to to the same tables that they SWEAR AT. They fail just as many (probably more) than they work for.
Don't be afraid of a little math, it is your friend...
bburge