There is an exact figure out there, somewhere, that shows the 'default' diameter or circumference of a tire that will maintain your final-drive ratio as it is on your current/actual rear gear ratio. I have used (diameter) of 26.75" as 'default' with very accurate results. It is easier to use circumference as that can be measured with a flexible tape as wrapped around center of tire. I also do both rear tires to ensure very much equal - when rear is a 'limited slip'.
Convert D to C by multiplying by 22 then dividing by 7 (Pi). So, 26.75 converts to 84 inches. Stock size 205-70-14's may have a D of 25 inches (C = 78.5) so doing the math, a set of 3.23 gears will convert to an actual 3.45 final drive ratio. The speedo pinion would be about a 33 tooth.
