I'm worried about my wife's safety in the car. She is a small lady, swears she is five feet tall, but won't let me near her with a tape measure to prove it. I think she's about 4-foot-10. When she sits at the wheel, she is less than six inches from the airbag, which I think is too close.
As it is time to replace her car, I want to find something that has the maximum amount of steering wheel adjustment so she can be positioned further from the wheel.
You are correct; her chest should be at least nine and preferably 10 or more inches from the airbag.
Even the new "smart" airbags, which deploy with less force when the seat is occupied by a small/light person, would likely cause some injury at the range you mention.
There are five possible solutions, in descending order of preference: 1) height-adjustable seat; 2) telescoping steering wheel; 3) adjustable pedals; 4) a proper cushion and 5) pedal extenders. Let's look at each.
Height-adjustable seats have become increasingly common. The seats in all vehicles slide on tracks that raise the seat as it is moved forward on the theory that if you need to be closer you are probably shorter. By raising a seat to its highest available position your wife might not have to move the seat as far forward and find a position a safe distance from the bag.
Telescoping wheel. Many new vehicles come with steering wheels that can be adjusted for both rake and reach. This is helpful for drivers with long arms and legs as it allows them to push the wheel out to the proper position - you should be able to rest your wrist (either one) on the top of the wheel with your back and shoulder firmly touching the seat.
But while a shorter driver may be able to pull the wheel out, bringing it within reach, that also means closer to the chest. A car with a height-adjustable seat and tilt/telescope wheel provides a good likelihood she can be accommodated.
Adjustable pedals are available in a couple of vehicles and would help, but like a telescoping wheel they are mostly beneficial for those with very long limbs. Even if she could move the pedals closer, she would still have to have the steering within proper reach - and proximity to her chest.
Proper cushions are available that provide an elevated position, allowing the driver to set the seat one or more steps back from the foremost position and thus a safe distance from the wheel.
But you have to be very careful here. You can't simply sit on any old cushion - you'd wobble around on top of it and the seat belts and sensors might not function properly. The cushions should fit the contours of the seat - both back and bottom and not interfere with the belts.
We used to use pedal extenders when teaching new drivers. But these were obtained from a professional and approved safety supplier, installed in a fleet of vehicles that were religiously maintained and inspected and had secondary controls for the instructor. They helped, but as with adjustable pedals, they don't necessarily help with proximity to the wheel. I'd advise against this approach.