The last three tanks of petrol in my Focus have been absolutely consistent at 34 mpg each with the usual 30 psi in the tyres. I've just pumped them all up to 40 psi to see what happens. Wearing the tyres out in the centre of the tread a bit quicker isn't an issue because I get all mine free from the local tip anyway and my mate has a tyre changer so none of it costs me a penny. An improvement in the mpg would be very welcome though. I'll report back in due course.
You'd actually be using those sorts of hot tyre pressures on a race track (club racing or track day perhaps) simply by virtue of the pressure increase as the tyre gets stinking hot. Pottering about on the road the pressure won't increase much from the cold setting.
For example given that pressure is roughly proportional to absolute temperaure (Kelvin) a tyre set at 30 psi at 15c would reach approx 30 x
363/288 = 38 psi at 90c which is not an unusual temperature for a tyre under race conditions. 40 psi hot seems to be a common target figure for race cars on road tyres.
Anyway apart from the steering being quite a bit lighter (which I rather like) it seems happy enough and the cornering balance is the same. Sadly from both my own calcs and info I've gleaned from the intergoogles it seems there is little benefit to mpg from increased pressures. However this will depend on average speed. At high speed the tyre drag is much less than the aerodynamic drag. At low speed the situation reverses somewhat.
Running the numbers on my Focus through my vehicle drag/bhp spreadsheet at
40 mph, of the total 7.3 wheel bhp required to propel the car 4.0 bhp is overcoming tyre drag and 3.3 bhp for aero drag.
At 70 mph the numbers are 7.0 bhp tyre and 17.8 bhp aero.
My spreadsheet also calculates mpg using estimates of brake specific fuel consumption (BSFC) and suggests that if I can achieve a 5% reduction in tyre rolling resistance I might gain at most 1.5 mpg at 40 mph and 0.5 mpg at 70 mph. Not really much to chase then it seems as per other people's findings.
I'll leave it as is for one tank then and set them back to normal pressures if mpg doesn't change much.
Of equal interest it appears that fitting narrower tyres might not reduce rolling resistance as one might first surmise but possibly even increase it. Wider tyres at the same tyre pressure under the same vehicle weight have a wider shorter contact patch with less tyre wall deformation and it is this deformation which seems to contribute considerably to rolling resistance. Hmmmm. Food for thought.
Finally, I wrote another spreadsheet to evaluate the net cost implication of better mpg versus worse tyre life. The results somewhat surprised me. Tyre cost per mile is only about 1/10th of petrol cost for the average car it seems. That means that it would take a 25% reduction in tyre life to outweigh just one extra mpg on my Focus. However 1 extra mpg is probably the most I'm now expecting and the annual cost saving would be miniscule. Given that 10 extra psi in the tyres might well create a 25% faster tread wear rate in the centre of the tyre anyway it all looks very pointless.
I'll continue to plough through the test but the maths appear to suggest that running anything other than stock pressures (as a minimum of course) is largely a waste of time and no doubt something that plod would pounce on gleefully if you were unlucky enough to have a prang. Not of course that plod would understand any of the wider implications of higher tyre pressures other than "it ain't the manufacturer spec so it must be bad n'kay".
Possibly more of an issue with your insurers, and of course if grip *was* compromised, the cost of even a tiny accident would soon outweigh any savings.
Car makers usually have 2 pressures for each vehicle loading. One for low speed and one for higher sustained speeds. Both assume some distance will be traveled. They don't have pressure settings for unladen, driver only going down the shops 2 miles away for a one/two bag shop.
After a 10 mile legal max drive to work including 2 miles on 70mph DC and 3 miles of NSL, my tyres are still only warm to the touch. For sheet metal too hot to hold is around 60°C, it's higher for non conductive material like rubber.
Depends on your definition of 'much'. 25% would not be out of the question.
In the Spring and Autumn, with colder nights but sunny mornings, if I roll my Focus out of the garage to check lights, fluids, and tyres, I do the tyres first. Otherwise, in the 10 minutes or so it takes to do the other things, the tyres on the side that is in the sun go up by 2 or 3 psi.
Yebbut a tyre at 40psi and 30c won't have the same level of grip as one at 40psi and 90c.
I've never seen even close to this in normal road use. That's the sort of change we're talking about in race use.
Road tyres are made from compounds that produce optimum grip at relatively low tread temperatures - in fact the temperatures that they will normally see in road use which I suppose is kind of obvious. Above about 40c to 50c tread temp most road tyres will actually be losing grip. By 90c they'll be cooked.
Race tyres are optimised for much higher temperatures and usually have a narrower optimum temperature band. Road legal race tyres (track day specials) like the Yokohama A048 are optimised for between about 60c and
90c. That gives you an indication of how much lower the temps are that pure road tyres will be optimised for.
Agreed. I always check my tyre pressures before the sun gets on them.
After buying a car, the pressures always differ side to side where it has sat in the sun before pressures were adjusted by 'technicians', or the previous owner.
David wrote in news:k2n735$7sq$ snipped-for-privacy@speranza.aioe.org:
As an aside, I've been using All season tyres on my car. V70 with 15"
195/65 tyres. I've been running them at the recommended 29 psi for a while, then on the recommendation of a car building friend, stared to run them at 38 psi. The max is someting like 41 for high speed or toowing etc. Instead of getting about 400 miles tops out of the 70 litre tank, I'm getting 520+ out of the tank. No difference in comfort or tyres wear or noise. Used to squeal a bit on roundabouts when very warm, but quieter now.
Well it's definitely too squirly to run at these pressures long term. The front end is just about ok but the rear skips out if I press on really hard although it's quite progressive and easily catchable. If I drop just the rear pressure I'll generate oversteer which isn't very driver friendly so I think I'll have to try 35 psi front and rear as a maximum safe limit and work from that. That isn't too much higher than the book 32/32 psi front/rear for unladen use.
Shame really as I prefer the lighter steering and I'm fairly sure I can feel the reduced drag as longer coastdown distances and marginally reduced pedal position at cruising speed.
Yep complete waste of time as I surmised. I didn't run through a full tank given she's a bit squirly under hard cornering on 40 psi but after 113 miles and 16.0 litres I got only 32.4 mpg which on such a short mileage has a large margin of error but clearly isn't at least noticeably better than my normal average. I'd have needed to have filled up with 14 litres or less to be sure of any statistical improvement. Given I always fill up at the same pump facing the same way until the tank can't take any more fuel any major discrepancy in filling procedure should be obviated. Back to stock tyre pressures for me.
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