Tyre Pressures

4.3.5 Tyre Pressures
Another popular topic for debate often revolves around tyre pressures. Drivers' experiences show that the Impreza is sensitive to minor differences in tyre pressures (you can easily feel a drop of 2psi), so it's worth checking them regularly. For the '96 model turbo, the recommended tyre pressures for the 205/55 VR 15 tyres are 33psi front, 32psi rear. Some dealers have suggested that these pressures are adhered to when fitting larger 16 and 17 inch wheels, however Prodrive suggest 33psi front / 30psi rear (up to 32psi on the rear for sustained high speed or heavy load) for all wheel sizes, while MRT suggested 35psi on both front and rear results in better grip and handling, at the slight expense of ride quality. Prodrive even suggest leaving the tyres at the pressures for trackday use, although increasing the pressure will stiffen the sidewalls somewhat. An owner in Milton Keynes (probably the highest roundabout density in the world!) reports that to combat excessive front tyre wear, Continental tyre engineers recommended that the MK police increase front pressures by 10-15%. The owner reports that while standard pressures wear the fronts in 8,000 miles, running at 36psi provides significant reduction in wear.

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Tyre pressure and gas-mileage.
For the first two years of our new life in America, I'd take our Subaru for its service, and it would come back with the tyres pumped up to 40psi. Each time, I'd check the door pillar sticker which informed me that they should be 32psi front and 28psi rear, and let the air out to get to those values. Eventually, seeing odd tyre wear and getting fed up of doing this, I asked one of the mechanics "why do you always over-inflate the tyres?" I got a very long and technical response which basically indicated that Subaru are one of the manufacturers who've never really adjusted their recommended tyre pressures in line with new technology. It seems that the numbers they put in their manuals and door stickers are a little out of date. I'm a bit of a skeptic so I researched this on the Internet in some of the Impreza forums and chat rooms and it turns out to be true. So I pumped up the tyres to 40psi front and rear, as the garage had been doing, and as my research indicated. The result, of course, is a much stiffer ride. But the odd tyre wear has gone, and my gas-mileage has changed from a meagre 15.7mpg (U.S) to a slightly more respectable 20.32 mpg (U.S). That's with mostly stop-start in-town driving. Compare that to the official quoted Subaru figures of 21mpg (city) and 27mpg (freeway) and you'll see that by changing the tyre pressures to not match the manual and door sticker, I've basically achieved their quoted figures.

So what does this prove? Well for one it proves that tyre pressure is absolutely linked to your car's economy. I can get an extra 50 miles between fill-ups now. It also proves that it's worth researching things if you think something is a little odd. It does also add weight to the above motto about not trusting forecourt pressure gauges. Imagine if you're underfilling your tyres because of a dodgy pressure gauge - not only is it dangerous, but it's costing you at the pump too.

What's the "correct" tyre pressure?
How long is a piece of string?
Seriously though, you'll be more likely to get a sensible answer to the length of a piece of string than you will to the question of tyres pressures. Lets just say a good starting point is the pressure indicated in the owner's manual, or the sticker inside the driver's side door pillar.I say 'starting point' because on every car I've owned, I've ended up deviating from those figures for one reason or another. On my Subaru Impreza, as outlined above, I got much better gas mileage and no difference in tyre wear by increasing my pressures to 40psi. On my Honda Element, I cured the vague handling and outer-tyre-edge wear by increasing the pressures from the manufacturer-recommended 32/34psi front and rear respectively, to 37psi all round. On my Audi Coupe I cured some squirrelly braking problems by increasing the pressure at the front from 32psi to 36psi. On my really old VW Golf, I cured bad fuel economy and vague steering by increasing the pressures all-round to 33psi.
So what can you, dear reader, learn from my anecdotes? Not much really. It's pub-science. Ask ten Subaru Impreza owners what they run their tyres at and you'll get ten different answers. It depends on how they drive, what size wheels they have, what type of tyres they have, the required comfort vs. handling levels and so on and so forth. That's why I said the sticker in the door pillar is a good starting point. It's really up to you to search the internet and ask around for information specific to your car.

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