[coolhandcountry]

well you divide the pinion in to the spur and do the same for the diff gears and then multiple them by one another then take the new diff you are running and divide that into number you got by multiplying the old stuff.
70 divided by 14 give you a 5 to 1 ratio. with the diff ratio 37 / 13 give you 2.84 x 5 =14.23 divide that by the new diff you have 43 /13 =3.3
14.23 / 3.3 = 4.31 for you new spur gear ratio. Since you have 70 tooth spur just divide 70 by spur gear ratio 4.31 =16.21 so a 16 toothpinion