Compare stock sized tire to what you want to put on here
Jot down the difference is revs per mile, or as represented as a %.
Take the % and go to this
% calculator site.
Using % and the top converter, express the 'what is' as % difference (should be less than 100% if going to bigger diameter tire, more than 100% if going to smaller diameter tire) and bounce that off the '% of' side of the equation which should be your known gear ratio.
185/70~14 (stock tire) has a 4.06% difference to that of a 195/60~14 (after market tire)
'what is' 104.06 (because the 195 is smaller diameter) '% of' 4.10 (final drive ratio for my 85) = 4.27 corrected final drive ratio
To go with revs per mile instead of %, (slightly more accurate) just use the bottom calculator on the % calculator page first and punch in new tires revs per mile in the 1st box and stock tires revs per mile in the second, then carry that result up to the 'what is' box of the top calculator and finish with known gear ratio.
894 (your claimed revs per mile) 'is what % of' 859 (claimed revs per mile of stock from the other site) = 104.307
Carry the 104.307 up to the 'what is' box '% of' 4.10 (final drive ratio for an 85) = 4.28 corrected final drive ratio
For either to be truly
accurate, you'll need to know the exact revs per mile for both stock and aftermarket tire as measured with a tape and calculated out rather than relying on a tire suppliers pamphlet to account for manufacturing differences and such, then carry that to the % calculator.
The above system is just a 'get you pretty darn close' way of going about it.
Presuming you have a stock 4.56 diff:
894 revs p/mile yoko's 'is what % of' 859 revs p/mile for 185/70~14 = 104.307
'what is' 104.307% 'of' 4.56 = 4.77 corrected final drive ratio