For most modern cars, sustained high speed means flat out on the unrestricted Autobahn - I treat 70 mph on UK motorways as normal so only increase pressures for load or towing.

I have never adjusted tyre pressures to reflect changing loads or speeds.

But I suspect there are theoretically good arguments for doing so:

• flexing of the tyre as it rotates creates heat. Higher pressure = less flexing = less heat = less tyre wear = reduced risk of "catastrophic" failure
• reduced fuel consumption as less energy is wasted in tyre flexing and reduced rolling resistance

However most tyres are likely to be rated at 120-150 mph. 70 mph is hardly putting them under great stress. And as even fairly modest family hatches weigh in at 1200-1500kg, the weight of an extra one or two people (+5-10%) is hardly likely to materially change handling or tyre characteristics!

We have a small hatchback at the moment and I usually keep the tyre pressures at half-way between the maximum and minimum recommended. It works OK for me and all 4 tyres, which are now quite old, are worn evenly across the treads.

Edited by Sofa Spud on 28/03/2021 at 14:29

On the subject of tyre pressures, if I inflate my tyres to the recommended pressure (32psi) how long should that pressure be maintained before needing to be topped up given that the car is doing about 200 miles per week with 50 of those being motorway and the balance urban travel on uncongested roads?

It really depends upon the car, what combo of wheels and tyres and the age/condition of all three, as well as usage pattern. Older wheels will likely lead to small tyre leaks due to corrosion/misshapen rims, even when running new tyres. Having the proper sealant (or a bit more) used when fitting can help reduce leaks when tyres are fitted on older wheels.

Play it by ear, really - check them weekly (when cold, before use [they will result in higher readings if left in direct sunlight), then if they don't drop, leave it for half a week more and then do so again, until they do drop, then monitor and pump back up to the recommended level that often.

For your usage (about average), I'd say 1-2 weeks between checks will be fine, assuming the wheels and tyres are in reasonable condition and your driving pattern isn't made up of lots of high speed turns on twisty roads.

I'm currently only using my car once every 1-2 weeks (about 25 miles round trip) for shopping and can get away with a check once a month, needing pump the tyres up slightly every couple of months, but then both tyres and wheels are still quite new (I had them replaced 3 years ago and have only covered around 6k miles in that time).

On the old (12yo) rims and 6yo tyres, I was having to check them weekly (using it for about the same mileage as you) as the corroded alloys were starting to engender small leaks around the tyre sidewalls. I forgot to check them for a month and 3 had dropped from 32psi to 28 and the worst affected one had dropped to about 24psi.

Underinflated tyres can be dangerous, as they are more susceptible to blowouts, which obviously isn't good at high speed. better to check more often than not - it only takes a couple of minutes, so keep the tyre pump in the boot and pressure gauage handy.