The first number is the width of the tire in mm.
The second number is the ratio of width to sidewall height.
The last number is the diameter of the rim being used.
Restating the above + a little: Using P265/75/R16 and LT265/75/16
P or LT is whether the tire is "pmetric" or "light truck" (light truck as in 1/2 ton or > light truck)
First number = width in milimeters = 265 milimeters
Second number = sidewall height as a % of the width = 75% of 265 or 198.75milimeters
Last number = rim size in inches= 16" wheel
I have no clue why it was decided to mix metric and standard (milimeters and inches).
So in other words I would have to go to a 245/70r17 to keep the same ratio, correct? by doing this I would increase my height but have a thinner overall tire. A 255 tire would mean the speedo would be off by about 1.6%. I used the calculator located here:
Tire Size Calculator - tire & wheel plus sizing
Here is a tire size calculator that might help you with this: Tire size calculator
This one provides more of a visual of what is going on. Can view from side of vehicle or in front of/behind vehicle.
Using the stock Nismo size of 265/75/R16 and then changing to a 245/70/R17 you will have the following happen:
stock specs: Nismo @ P265/75R16
new tire spec: P245/70/R17
Speedometer reading with non stock tires is 3.6% too fast meaning that when the speedometer reads 60, you are traveling 57.8mph.
Keep in mind I went with stock Nismo vs your new tire size. If you have 17" wheels already then my numbers are slightly different.
You will also be changing your contact patch on the ground by (removing) 20 milimeters.