Mark a point on the driveshaft and mark a point on the rear tires. Count the number of times the tires rotate per one rotation of the driveshaft.
That will roughly give you your gear.
If the tires turn three and a half times, for example, per one driveshaft spin then you probably have 3.55s.
2016 Comp Orange GT (current)
Aurora - 2002 Boosted GT! (Sold, sadly)