The pneumatic powered car has been around for several years now. Compressed air has the same problem as hydrogen though; low volumetric energy density. To get usable range out of an air car requires the car to be really light. The heavy use of light weight materials raises the car's cost, and I don't think any prototype met US or European safety regulations. There might have been a try at commercializing it in India where it would compete against the Tata Nano.

Using liquid nitrogen would help with the energy density issue, but introduces others. One being that the car couldn't be filled at home for the majority of people.

Hydraulic hybrids would seem a good fit for trucks, and I wonder why it appears Ford has stopped developing them. Diesel and electric hybrid seems to be their chosen path for the F150. Could noise be the issue. Now the pressures were obscene, but the high water pressure equipment I've worked with weren't exactly ear friendly.

Most of a car's fuel economy is ruined with moving 2+ tons from a dead stop. At the other end of the spectrum, the average car only needs 7hp to keep moving at freeway speeds.

A pneumatic propulsion system to give a car a catapult-like start from a dead stop would be a MPG advantage. Air could be recompressed into the system when braking.

In a hybrid, electric, pneumatic, and hydraulic systems can help with stop and go fuel economy.

I just think a car running on just compressed air will have limited applications and market.

I think the cost to compress air would be much greater than an electric vehicle. Also the weight of a tank would add way to many pounds/kilograms to a car and take up way to much space.

Years ago there was a movement to have a flywheel connected to the braking system and then it would help get the car moving