As with mightyjlr's 6 speed my auto is extremely smooth. I've had mine up to around 115 and just as smooth there as at 15.

I did have a back problem that was aggrivated by a car once. I've had 2 spinal compression injuries. One from a sprint car accident in 1972 and a second from a helicopter accident in 1981. In 1995 I bought a Ford Probe GT with a 5 speed manual. I started having some lower back pain but never thought about the car being the problem at first. Finally one day I couldn't even get out of bed. When I finally got to a Doctor he could figure out what was causing my problem so he sent me to a physical theropist. I would go get a treatment and feel much better for a few days and then slowly but surely the pain came back. Finally he went out to my car with me and had me sit in the car and go through the motion of shifting. He figured out that it was the hight of the seat combined with functioning the clutch that was causing my problem. I Traded the Probe for a Vette with an automatic and my problem was cured.

So, in conclusion.... A car can cause a back problem. Especially if you've had a history of back injury.

I hope this helps in some way.