[mememe]

For better fuel economy, you need a tire that changes pressure the least during temperature changes. When you fill your tire up cold compared to after driving, the psi changes.

You will need a gas that conducts heat the least. A gas that is dense so it will leak out of the tire the least over time. A nobel gas that doesn't react to the remaining air in the tire and doesn't react to the chemicals in the tire.

Since it is probably not a good idea to fill your tires with Radon, the next best choice would be to fill your tires with XENON.

Thermal Conductivity of gases comparison:
Helium___0.142 (W/(m·K)
Neon____0.046 (W/(m·K)
Nitrogen__0.024 (W/(m·K)
Argon ___0.016 (W/(m·K)
Krypton__0.0088 (W/(m·K)
Xenon____0.0051 (W/(m·K)
Radon____0.0033 (W/(m·K)

Density of gases comparison:
Helium____0.1786 g/L
Neon_____0.9002 g/L
Nitrogen__1.251 g/L
Argon____1.7818 g/L
Krypton___3.708 g/L
Xenon____5.851 g/L
Radon____9.97 g/L

So please fill your tires with xenon and tell me how it goes.


But if you want your gas to be very dense, denser than Xenon so it doesn't leak out and you want to do the opposite by conducting heat more rapidly than Helium, why don't you try Sulfur hexafluoride.
Thermal Conductivity of 0.0121 (W/(m·K) and density of 6.17 g/L

Fill one tire on your vehicle with xenon. Fill a second tire on you vehicle with sulfur hexafluoride. Drive for a while. Measure the tire pressures after driving. Compare the difference.