Ballpark Gear Ratio

Use this link> ET/MPH From Horsepower Calculators

Add in your estimated fwhp/and the rest of your parameters/weight ratio's tyre size to get an ET etc. etc., add converter slip to the trap rpm which can be 4>10%?. Play around with it and you'll see the differences. Been doing it since 1990, was quite accurate for all my cars back then and still is today.