will 245/50R-16 96 B nitto 555r dr's throw off my speedo. if so how much in which direction?

 

u will be at least 10% too slow. i am doing the comparison based on civic sized tires

 

so based on that if my speedo says 20 im actually doing 18... and 30 i'm actually doing 27? basically like that?

 

actually, yes but then you are backwards..

if the speedo reads 60 you are actually going 66

 

Assuming from your username that you have a '94 Civic EX, then yes, they're WAY off. They will probably also rub big-time.

Your car came from the factory with 185/60-14 tires. 245/50-16 is almost 13 percent (almost three full inches) larger in diameter. If the speedo was accurate with the stock size, then if it says 60, you're actually going 67.67 mph with 245/50-16. The 16" sizes which are closest in diameter to your stock size are 195/45-16 and 205/40-16.

In the FAQ for this forum, you'll find a link to a tire size calculator where you can compare sizes to find out the differences in diameter.

 

http://www.1010tires.com/TireSizeCalculator.asp
here is a tire calculator. just enter your stock tire size then enter the size you want to use and it will tell you the difference.