I was 18 when I graduated from High School, and my grandmother offered to pay my tuition, room and board if I decided to go to her alma mater, which I had been planning on doing anyway, so the decision to accept her money was easy.
I was all ready to leave for school and the car all packed to the brim when grandma mentioned that she would like to come with us to see the old campus and the dorm I would be living in. Things had changed since she was there, she reminded me: Men were not allowed in the women's dorm. Now room assignments were made without regard to gender, and bathrooms were often "co-ed". There was actually still privacy in the bathrooms because everyone knew the opposite sex might be there, or come in. Even so, it was new and different to imagine taking a dump while some pretty girl was on the other side of a partial wall also taking a dump.
The car was too full. We had packed a lot of stuff in the back seat, thinking there would only be 3 of us in the car. Mom explained that she and grandma could squeeze in the back together and we did that. The drive was 200 miles and after about 2 hours, my mom suggested I move to the back seat and grandma could sit on my lap- she had been sitting on mom's lap for the entire trip so far and it seemed like a good idea. So we switched places after stopping to gas up the car and take a pee.
It was only going to be another hour or so, but after only a few minutes of having grandma's soft tiny butt bounce on my lap, I was developing an erection. I should mention that my grandma is very small- under 5' tall and weighs less than 100 pounds. She looks normal, just very small. This meant that her weight was not a problem for me, but my erection was getting to be a problem. She must notice it, and eventually she did offer to try to shift position and did that by sliding forward onto my legs more. But as she did that, her skirt slid up some and I realized I could smell her pussy. When I wondered if she had on panties, it just made my cock really stiff. When I say grandma, you may think "really old"- but actually she was only 60 and really looked nice, especially when she dressed up.
She looked down at my cock bulging up my pants and whispered "that looks really uncomfortable". My dad was busy driving and my mom seemed to be sort of sleeping, but even so I was surprised when Grandma unzipped my pants, reached in my fly and pulled out my erection, which was straight up.
"That feels better, doesn't it?"
How could I say anything except "yes"? It was no longer bent in half, but even so having a stiff cock sort of demands that action be taken. I wanted to ask Grandma to look away so I could jerk off but instead I slid my jacket over to cover my cock, and hoped it would detumesce. That's a word I rarely use!
Grandma whispered, "I have a much better idea that will make the rest of the ride nicer for both of us." She lifted up enough to reach under my jacket and bend my stiff cock toward her, so that when she slid back, the tip of my penis found her already wet cunt and slid into it easily. I tried to be quiet as Grandma sort of gave a whispered low moan, then whispered "you are a lot bigger than grandpa...". She paused a few seconds then added, "or your dad". I was trying to hold still but hearing her compare my cock to my dad's cock confused me a little at first- then I realized that she was not talking about his cock when he was little, but about his erection when he was my age (or older). I would later be very curious and ask her about the family history of incest, but right then it felt so hot and slick in Grandma that I was holding still and enjoying every irregularity in the highway. So was grandma, whose breathing was getting faster, and whose pussy was getting my very wet! I still had my pants on and they were getting wet around the fly where my cock emerged. I realized that when I shot my load, that mess would probably end up on my pants also.
As you might expect, thoughts like that do not increase sexual excitement.. but just then Grandma retrieved two handkerchiefs from her purse and told me to "put them where they are needed".