Working out correct tyre pressure
Discussion
My car has much bigger wheels on it than were fitted as standard. As such I have no idea what the correct tyre pressures for the application should be.
The car is a 1975 Jaguar XJC and as standard it should have 15 inch wheels on it but my car has much bigger and wider 17 inch wheels on it (245/55R17)
How do I work out what the correct pressure should be?
The car is a 1975 Jaguar XJC and as standard it should have 15 inch wheels on it but my car has much bigger and wider 17 inch wheels on it (245/55R17)
How do I work out what the correct pressure should be?
There is no hard or fast rule apart from watching the tyre wear. Its not just the diameter that affects the required pressure...
If you suffer wear mainly on the outer edges of the tyre your pressures are low, and the opposite to that is wear in the middle of the tyre only means your pressure is too high. But by the time you notice the chances are your tyres will need replacing. This is assuming all allignments etc are correct and some cars naturally may wear the inside edges a little more due to camber and general car setup.
Just found this..
http://www.procarcare.com/includes/content/resourc...
If you suffer wear mainly on the outer edges of the tyre your pressures are low, and the opposite to that is wear in the middle of the tyre only means your pressure is too high. But by the time you notice the chances are your tyres will need replacing. This is assuming all allignments etc are correct and some cars naturally may wear the inside edges a little more due to camber and general car setup.
Just found this..
http://www.procarcare.com/includes/content/resourc...
Edited by Mikey G on Sunday 7th February 19:23
I'm basing this on nothing other than gut 
The pressure in the tyres is probably related in some way to the weight of the car. Unless the car has changed I'd stick with whatever the manufacturer stated for the original wheels and use that in the larger wheels. Remember pressure is not volume, (though it's related of course) so you will get less air in a low profile tyre but the pressure exerted on the structure will be the same.
The pressure in the tyres is probably related in some way to the weight of the car. Unless the car has changed I'd stick with whatever the manufacturer stated for the original wheels and use that in the larger wheels. Remember pressure is not volume, (though it's related of course) so you will get less air in a low profile tyre but the pressure exerted on the structure will be the same.
richs2891 said:
Google 4 psi rule
interesting......After you inflate all tyres to either the pressure recommended by the manufacturer, or the maximum pressure-10%, drive for say a distance of 100 klm. We are assuming the ambient was cool and hence it is called a “cold tyre” reading.
Upon stopping, immediately recheck your tyre pressure while the tyre is still warm.
If your tyre pressure reading is greater than 4 psi from the “cold tyre” reading you took at the beginning of your trip, then the tyres are getting too hot and your starting tyre pressure was too low.
If the tyre pressure reading is less than 4 psi from the “cold tyre” reading you took at the beginning of your trip, then your starting tyre pressure was too high.
You will need to run the test again (once the tyre has completely cooled) with less pressure until you find the right balance.
Not sure we can really rely on this 4psi rule, ambient temps and road conditions can make a difference.
It is true though that an underinflated tyre will overheat and cause the air inside to expand and vice versa.
I just read my tyres, works for me as ive even found the manufacturers specs can change depending on brand of tyre used.
It is true though that an underinflated tyre will overheat and cause the air inside to expand and vice versa.
I just read my tyres, works for me as ive even found the manufacturers specs can change depending on brand of tyre used.
TooMany2cvs said:
Mikey G said:
I just read my tyres
That tells you the maximum pressure, to be used if the tyres are at their full load rating. Which, of course, doesn't tell you anything about what the right pressure is for them on a specific car.
An alternative to tire wear is tire temperature. Go for a decent run, motorway best. Then measure the temperature in the inside, middle and outside of the tire patch. If middle is warmer, too much pressure, if edges are warmer than middle, too little pressure. If the inside is warmer than the others it's probably right but check camber is within spec.
off course all changes with the load, if you then carry much more luggage/people, will need a tweak. I would start with advice given before, depends how bothered you are with it, on the road, providing is close enough is probably fine, better to be slightly over inflated than under inflated.
off course all changes with the load, if you then carry much more luggage/people, will need a tweak. I would start with advice given before, depends how bothered you are with it, on the road, providing is close enough is probably fine, better to be slightly over inflated than under inflated.
Etrto rules give the minimum pressure. Car manufacturers determine the recommended pressure, which will be between this minimum pressure and the maximum pressure shown on the sidewall.
Usually lower air volume requires a higher pressure. So if the width and the outside diameter remains the same and you increase the wheel size, you would normally have to increase pressure.
Usually lower air volume requires a higher pressure. So if the width and the outside diameter remains the same and you increase the wheel size, you would normally have to increase pressure.
Kawasicki said:
So if the width and the outside diameter remains the same and you increase the wheel size, you would normally have to increase pressure.
The OPs tyres are wider though. I think if you change size within the normal permitted ranges (ie those that maintain speedo accuracy) you basically stick with the same pressures. If you knew the corner weights of the car you could look up the pressures on the charts of weight vs pressure vs load index. You'll see there's no reference to tyre size in the tables - ie this one:
Be careful if the tyres are XL as they can require slightly higher pressures for normal loads.
Similar to some of the other comments above about measuring tyre temp, in "the olden days" they used to put a chalk band across the tyre and then adjust the pressures until it wore off evenly.
I'd say you want the same contact patch size. Contact patch size depends on PSI and weight, for a given tyre.
So if you go wider, you use less pressure. Cyclists understand this very well, road bikes tyres are often inflated to 120 PSI whereas MTB's not commonly more than 40 PSI, as the narrow tyre needs more pressure to stop it from squashing flat.
Ignoring the diameter of the rim as assume you have kept a similar rolling radius.
Obviously subject to fine tuning.
So if you go wider, you use less pressure. Cyclists understand this very well, road bikes tyres are often inflated to 120 PSI whereas MTB's not commonly more than 40 PSI, as the narrow tyre needs more pressure to stop it from squashing flat.
Ignoring the diameter of the rim as assume you have kept a similar rolling radius.
Obviously subject to fine tuning.
996TT02 said:
I'd say you want the same contact patch size. Contact patch size depends on PSI and weight, for a given tyre.
So if you go wider, you use less pressure.
For a given pressure if you go wider the contact patch should be shorter - for the exact reasons you state.So if you go wider, you use less pressure.
So you need to be careful reducing pressures that you don't drop below the load capacity needed.
Sheepshanks said:
996TT02 said:
I'd say you want the same contact patch size. Contact patch size depends on PSI and weight, for a given tyre.
So if you go wider, you use less pressure.
For a given pressure if you go wider the contact patch should be shorter - for the exact reasons you state.So if you go wider, you use less pressure.
So you need to be careful reducing pressures that you don't drop below the load capacity needed.
Ultimately the size of the contact patch (area) is that size in sq.in. that is a multiple of the tyre's pressure that corresponds to the vehicle's weight on that tyre (yes I've confused myself too...)
Anyway, e.g. 400 lbs, 40 psi, contact patch 400/40 = 10 sq.inches. Yes it will always be the same size irrespective of width.
One could (taking things to extremes just for understanding's sake) imagine fitting double wheels to the rear, thus effectively doubling the width of the tyres. 2 wheels are capable of taking the entire weight at the previously recommended pressure, but you now have 4 wheels, so the pressure can be reduced greatly (won't say halved as perhaps not so simple) for the same load carrying capacity at recommended tyre distortion.
If you did not reduce the pressure then ride quality would be reduced.
Look for a car of similar weight and layout fitted with the same size wheels and tyres, use the recommended tyre pressures as a starting point.
I did this with my kit car using settings for an Elise running the same tyres and have trimmed them to 1 psi less all round over time as that feels right when driving the car.
I did this with my kit car using settings for an Elise running the same tyres and have trimmed them to 1 psi less all round over time as that feels right when driving the car.
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