This question has been up for six hours and no one has taken the opportunity to use the Ideal Gas Law??!!
PV=nRT
Let gauge pressure be 90 PSI, then absolute pressure is 105 PSI, which is 723 kPa. The volume of a 1.5 inch tire on a 406 rim is about 1.58e-3 m^3. The ideal gas constant is 8.3145 J/K at temperuture of 297 K (about 75 deg F). Solving for n, number of moles of gas we get n=0.46 moles. The atomic weight of CO2 is about 44 vs atomic weight of Helium of 4. So, the mass of the gas would be:
Helium: 1.85 grams
CO2: 20.4 grams
So, you can shave 0.65 ounces by using Helium instead of CO2
If you use start with good old air it might not save as much becuase CO2 has a lot of oxygen and air has a lot of Nitrogen which is slightly less massive.
My high school chemistry teacher would be proud...