There are several good reasons to keep your tyres fully pumped. Unfortunately, saving massively on fuel isn't one of them
Potential Fuel Saving: up to five per cent
Essential for safety, moderately useful for consumption
A modern radial tyre with 25 per cent of the air let out of it looks like … a fully inflated modern radial tyre. In the absence of strange mental powers, the only way to tell if you tyres have the right amount of air in them is with a tyre gauge. Of course, you could just wait for a blowout (underinflation is the commonest cause of blowouts on the highway) or for your tyres to wear out w-a-y too fast ... but a tyre guage is the most prudent way.
In 12 back-to-back runs along 60km/h and 80km/h empty arterial roads in the dead of night we were able to clock up a repeatable five per cent variation in consumption between 36psi (the tyre pressure recommended by Holden for the Calais V and noted on the tyre placard inside the door frame) and 27psi (a 25 per cent reduction). So, if you don't check your tyre pressures, you could save five per cent on your fuel bill - and it might also save your neck if you avoid a blowout.
Frankly, the car felt terrible at 27psi, and was probably quite compromised in cornering and under brakes - although a 25 per cent reduction in pressure for just one tyre might not feel so bad, until you have to swerve hard or brake in an emergency, possible while cornering.
You really should do what most owner's manuals recommend and check tyre pressures once a fortnight (say, every second time you fill the car, seeing as the fuel and the air are on offer at the same place). The dollar saving – a maximum of a couple of dollars a fortnight – is bolstered by prolonged tyre life as well. Happiest consequence of all? Doing so it might prolong your own life as well.
The fuel at the servo could easily cost you $100, but at least the compressed air is still free.