Question about changing tire size and fixing speedometer

[hatcher1105]

Ok so i have a set of 315/70/17 and i have a programmer that lets me change my tire size to correct the speedometer reading. Well my programmer does not have my exact size to select it has one slightly taller and one slightly shorter (i thank they are 34.50 and 34.25) and my size is in between those. So my question is should i go with the bigger number or the smaller number? Thanks for the help.

[Mudvain]

check your tire brands website and see if they list revolutions per mile for that specific size. you can change that parameter on your tuner.