Tire pressure increase for lower-profile tires?

Messages
10,843
Location
Nokesville, VA
My 1988 Mustang, which came with 225/60/15 tires, suggests 31PSI all around. I now have 245/50/16 tires on it, which aren't listed on the sticker. It wasn't until today that I considered that the tire might need more pressure due to being a lower-profile tire. I put 40PSI in them as a start, since some suggest using 80% of the max pressure which on these tires is 52PSI. So far I've noticed better gas mileage (on the trip computer) and better handling. They were at 31PSI, which I now believe is way too low for these tires. Anyone have any thoughts on the subject?
 
Messages
12,385
Location
Northern CA
quote:
Originally posted by brianl703: My 1988 Mustang, which came with 225/60/15 tires, suggests 31PSI all around. I now have 245/50/16 tires on it, which aren't listed on the sticker. It wasn't until today that I considered that the tire might need more pressure due to being a lower-profile tire. I put 40PSI in them as a start, since some suggest using 80% of the max pressure which on these tires is 52PSI. So far I've noticed better gas mileage (on the trip computer) and better handling. They were at 31PSI, which I now believe is way too low for these tires. Anyone have any thoughts on the subject?
The tires are wider cross section, so being on the same wheels, the tread will tend to bow out more in the middle. Fortunatly they are radials which are more tolerant of different wheel width than bias plys were. If it were my car I would go for what felt best to me and keep an eye on tread wear. If they start wearing more in the center, less pressure. You mentioned fuel economy and handling, so higher than stock is a good starting point. My guess is that 40 is bit high, but not a concern. Since you have some experiance with 40 in them, why not continue a bit with 40, then drop to 35 and see which you like the best. By experianceing both pressures for a while you should be able to choose one or the other or be able to make an educated gusess about what 3rd pressure would be right for you.
 

brianl703

Thread starter
Messages
10,843
Location
Nokesville, VA
I did get different wheels for it (went from 15" wheels to 16" wheels) and the new wheels are 7.5" wide, compared to the old ones which are 7" wide. (I was expecting the new Ford OE-style wheels to be 7" wide, like the OE Ford 16" wheels used on the 91-93 Mustang, but was pleasantly surprised to discover that they're 7.5" wide). I guess I need to get a tire tread gauge, since by the time the wear becomes noticeable it's probably a bit too late. I guess that'll be my determining factor..if the center wears too fast, then I'll drop the pressure.
 
Messages
12,385
Location
Northern CA
quote:
Originally posted by brianl703: I did get different wheels for it (went from 15" wheels to 16" wheels) and the new wheels are 7.5" wide, compared to the old ones which are 7" wide.
The wider wheels change things a bit. If my quick calculations are right, your old tires had a cross section 1.27 times the rim width and the new ones 1.28 times the rim width. That's close enough to recommend starting with your previous pressure if you did some serious evaluation to find an ideal presure with the older tires. If you didn't, 40 is cool.
 
Messages
3,933
Location
Somewhere in the US
A couple of thoughts: Knowing the thought process behind what inflation pressure gets printed on the sidewall of a tire - there is no connection to that and the proper inflation pressure for a vehicle. It all has to do with load capacity and tire spring rate. So forget the "80% rule" - it's Boooogus! Using higher inflation pressures for lower profile tires is a good idea because the sidewall height determines the amount of deflection an impact to the tire can absorb before there is damage. You will always get better fuel economy and more response (quicker steering) with higher inflation pressure, but what gets compromised is ride quality, dry traction. It's also possible to go too high and wet traction starts to get compromised, too. Hope this helps.
 

oli

Messages
61
Location
new england
These two tires have almost identical load ratings at 31 psi with the new tire being rated for about 50 pounds additional weight per tire. So there is no need to increase pressure.
 
Messages
2,051
Location
Jupiter, Fl
I have the same question but for different sizes I went from a 265/70r16 at 32 PSI on a toyota tundra to a 295/45r20. I have started at 36PSI but don't know if this is too much. My last set of tires this size scallopped the shoulders running 32PSI. The stock size wore perfectly. The rim size went from 16x7 to 20x9.5 They are an OE option rim and tire size. Any suggetions?
 
Messages
3,933
Location
Somewhere in the US
Some thoughts on the Toyota Tundra with the 295/45R20's. 1) My book doesn't list 32 psi as the proper inflation pressure for a Tundra with P26570R16's. Are you sure about the pressure? Look for the vehicle placard which will list the original tire size and the proper inflation pressure for that size. The placard is usually located on a doorpost or in the glove box. 2) Your 295's need to be inflated to 35 psi to get the same load capacity as the 265's get at 32 psi. 3) It's probably a good idea to use even a bit more pressure to decrease the chances of an impact damaging the wheels. 4) The problem with the scalloping wear is probably due to the wider tires and the increased offset.
 
Messages
11,342
Location
Florida, Cape Coral
This might be a question I would ask of a TireRack garu. I'm guessing that capriracer has the correct answers as it must be related to the load carrying capacity difference. I also know that load capacity is somewhat linear but not near the minimum or maximum pressures. Great question 703. ed
 
Messages
12,385
Location
Northern CA
CapriRacer. Is you source of tire information something available on the web or something you can share with us? Or is it some $$$$ tire industry book?
 
Messages
3,933
Location
Somewhere in the US
XS650, It's an industry standards book and unfortunately, it is not available on the web as it's the only source of revenue for the standards organization. Besides it's at least US$50. Not to mention there are other parts of the standard that affect what pressure ought to be specified, and it's very tempting to look at only the load table. But if you have a question like CRB had, I'd be glad to answer it.
 
Messages
2,051
Location
Jupiter, Fl
Thanks guys - I guess I will stick with the 36 psi and see what happens. the 32 PSI is what I found worked best for my driving and loads with the 16 inch wheels. at 32 psi I was getting about 50K out of each set of cross terrains. I think the book calls for 26 or 29. I don't remember. running the stock pressures the back tires always looked flat with a load, and the front shoulders wore too fast. I do a lot of highway driving and also a lot of mountain road driving. Honestly I am tired of driving this truck, and putting on the big wheels and tires at least makes it look cool. I would be happy if they performed as well as the stock ones.
 
Top