Low Gear or No overdrive
 

I have just recently bought a 97 c3500 6.5 for a work truck and was wondering about the gears. I travel the interstate a good bit on a daily basis and when im going 70 mph im running close to 3000rpms. The truck does not come equipped with an overdrive button so i'm not sure if it is going into overdrive or not. The petal positioner sensor is going out in it and I was told that could cause the overdrive not to work. But I was also told that the truck has a really low gear ratio. Does a stock rear end have a gear ratio this low? Any input will be greatly appreciated.