just put on my Goodyear GT II P285 60 R16. My old tires were P275 70 R16. the tire were 31.1 tall, my new ones are 29.4 tall. will this affect my speedomiter any, and if so, how do i fix it.

 

It will read high by 5.75%
meaning that after 100 miles the speedo will actually read 105.75 miles.
a hypertech programer, ls1edit or superchips(not positive) will allow you to change the tire size to get the speedometer accurate.

go here to see more info on the differences in mph and total miles etc.
you can also put your gear ratio in and it will tell you what it went to after installing the somewhat smaller tires.
a 3.42 will be a 3.62
hope this somewhat helps.

 

Your stock tires were 29.5-30" tall so when you went with the 31.1" tires, you actually slowed your speedo down UNLESS you corrected it with a HPP3. With the 285/60s it will be accurate as stock again. You can take a overall diameter measurement and plug that in the HPP3 to get the speedo more accurate and alter the shift points but its probably not worth messn with it.

 

is there any way to change the speado without a hpp3