just have someone ride next to you on a lonely highway and they can shout the speed at which they are traveling, then compare that to your speedometer.
Then, if you are industrious, you can take a pic of your speedo assembly, put it in photoshop and change the numbers to match the new data. Print it out then glue it over the existing numbers.
This is a constant problem for car buffs though. What needs to be invented is an unit that plugs in between the speedo cable and the speedo that has a variable adjustment mechanism which corrects for errors. Then you wouldn't have to play the guessing game every time you change the drivetrain variables.
