Another tire question

I have a 2002 360 with the original tires. I bought it used a few months ago and the car has 11,000 miles on it now. My question is that the tires feel hard. I know they get harder with age but wondered if anyone knew what the hardness reading should be. They are the Bridgestones that came with the car. My durometer measures between 68 and 72 depending on whether measuring at the center or the edge. I tried the Bridgestone website as well as tire rack but can't seem to find anything.

I measured my viper tires, Pirelli's about 2 years old and they measure around 64 and have had a LOT of heat cycling.

Thanks

 

Thanks Carl. I appreciate the info. The tires just seemed to ride hard and the ASR activates before I expect it would. Maybe I need some track time in this car to get a better feel for the limits.