Rear diff

I just bought a 87 B-2200 and the rear seems to be geared way to low for the highway. What is the stock gearing for this truck, and how many rpm's are you guys turning at 65 mph?

According to the mazdatruckin spec page, the stock ratio is 3.909

I tried jackimg the truck up and spinning the drive shaft to see if the ratio was stock and to get 2 full turns out of the tire I had to spin the shaft 3.75 times. Does this sound right to you guys, or am I doing something wrong?