Potential Fuel Saving: up to five per cent

Verdict: Essential for safety, moderately useful for consumption

A radial tyre with 25 per cent of the air let out looks like … a fully inflated radial tyre. The only two ways to tell this are either with strange mental powers inaccessible to most of us or with a tyre gauge. In 12 back-to-back runs in 60km/h and 80km/h empty arterial road running (read: middle of the night) we clocked up a repeatable five per cent variation in consumption between 36psi (the placard pressure) and 27psi (25 per cent reduction). Also, the car felt like mush at 27psi, and was probably quite compromised in cornering and under brakes. Do like the manual says and check tyre pressures once a fortnight. The dollar saving – a maximum of about one latte a fortnight – is bolstered by prolonged tyre life as well … and it might prolong your own life as well. And while the fuel at the servo could easily cost you $100, the compressed air is still free.