Recommended tire air pressure for 275/40/17's & 315/35/17's?

[StangIt #1]

I just installed new deep-dish bullitt rims on my car about a week ago, and want to maximize the treadwear on the tires (they are factory rated @ 160 for treadwear...typical for a summer tire). I'm running a common staggered set-up, with 17x9's in the front, and 17x10.5's on the back (obviously ruling out tire rotation). I know that a bunch of folks run the Sumitomo HTRZ's with this arrangement, because they are the only reasonably-priced summer tire on the market in these sizes (other option is the Goodyear F1 supercar tire = $$$ ).

The sidewall lists the maximum pressure @ 44PSI. I don't drag/street race, but I do enjoy pushing the car in corners (i.e., exit ramps, backroads). I'm basically looking for feedback regarding what others with non-drag radial tires are running on the street for maximum treadwear. The last set of Sumi's on the car (245/45/17, HTR+ all-seasons, on the factory 17x8 wheels) showed signs of overinflation at the Ford recommended inflation specs (from the glove box). I currently have the new tires set at a cold PSI of 40, and they seem to stick pretty well; however, I'm not sure if a long trip or summer heat will push them over 44psi (BTW, the tires have an "A" temperature rating)! What do you recommend?

 

[Hipogt]

StangIt#1
Is this set up on a S197? And what are the factory specs? IMO, 40 seems way too high for the street and may lead to premature center wear due to overinflation. I would suggest to use the stock Ford specs and maybe add a couple of pounds. This may take away some of your "stick" or steering response, but should do wonders for your ride comfort and tire life. Also, if this set up makes the car understeer more, stagger the PSI more to the front by a few pounds. Hope this helps. If not, try the tire mfg.

 

[SaleenGT2001]

always go by what the car says on the tire placard, not the max tire pressure. it should be 32 psi front and rear.

 

[SVT32VDOHC]

I run 32 psi in all my tires. I only run less if I go to the track. I play between 22-28psi to get a good 60 footer. I put the fronts up to about 38-40 while at the track too....so they roll easier. Max air pressure for tires is usually 44-52psi. I just make sure I stay under that.

 

[StangIt #1]

StangIt#1
Is this set up on a S197? And what are the factory specs? IMO, 40 seems way too high for the street and may lead to premature center wear due to overinflation. I would suggest to use the stock Ford specs and maybe add a couple of pounds. This may take away some of your "stick" or steering response, but should do wonders for your ride comfort and tire life. Also, if this set up makes the car understeer more, stagger the PSI more to the front by a few pounds. Hope this helps. If not, try the tire mfg.

The car is an SN95 ('98GT). The tires were mounted & balanced (Hunter RoadForce balancing = sweet!), and shipped with 40psi. This pressure sounded pretty high to me, and the car is actually understeering (pushing through turns) quite alot at this psi. As for ride comfort...it doesn't exist at any psi. That's what you get with 600lb. front & 250lb. linear rate springs. It's worth it for the handling, though!

SaleenGT2001, I assumed that the signifcant increase of width over stock wheel size would require a different psi than the placard suggests?

 

[Mustang5L5]

Recommended tire pressure is a factor of weight on the tire and contact patch area.

With a stock 245/45/17, Ford recommends 30 psi.

Now with a wider tire, you have just increased your contact area, but have not increased the weight of the vehicle, so in theory you will require LESS tire pressure to support the same weight.

There is a way to figure out a ballpark range mathematically, but i'm too lazy to write it up. You will need to experiment, most likely with chalk, to find the perfect PSI. But i would start at factory 30psi and maybe work down a bit.

 

[StangIt #1]

Recommended tire pressure is a factor of weight on the tire and contact patch area.

With a stock 245/45/17, Ford recommends 30 psi.

Now with a wider tire, you have just increased your contact area, but have not increased the weight of the vehicle, so in theory you will require LESS tire pressure to support the same weight.

There is a way to figure out a ballpark range mathematically, but i'm too lazy to write it up. You will need to experiment, most likely with chalk, to find the perfect PSI. But i would start at factory 30psi and maybe work down a bit.

I'll try to Google a calculator for this...didn't know such a formula existed. The weight of the vehicle increased by about 14lbs. for the tires & approximately 15-19lbs. for the new wheels. I doubt this is that significant in the calculation, though, except for slowing the car down even more.

 

[Mustang5L5]

I'll try to Google a calculator for this...didn't know such a formula existed. The weight of the vehicle increased by about 14lbs. for the tires & approximately 15-19lbs. for the new wheels. I doubt this is that significant in the calculation, though, except for slowing the car down even more.

There isn't one. I was going to figure it out mathematically and see if it holds any weight


Assume weight does not change significantly.

You need to determine contact patch area. It's easily done by taking 4 peices of paper and putting them on all 4 sides of the tire. Mark and measure lenght and width of what part of the tire touches the ground. Then just multiple lenght times width and that's your area.

Do that for your old and new tire and then determine the percentage increase from the old to the new. That percentage increase should in theory be the percentage DECREASE in pressure.

For example if the contact patch is a 4% increase in area(don't know if that's an actual figure), you would decrease pressure by 4%. So 30psi would now be 28.8 psi.

Reason why writing up a formula is pointless is because everyone's contact patch area is going to be different. Different pressures on the new tire won't vary the contact patch too much, but the idea here is equal pressure all over the contact patch area pushing the tire away from the road.

Either way, wider rubber will require less pressure...more than likely only a few psi


BTW, Ford's recommended pressure on the 245/45/17 is 30 psi cold...not 40

 

[steve_stang2003]

There isn't one. I was going to figure it out mathematically and see if it holds any weight


Assume weight does not change significantly.

You need to determine contact patch area. It's easily done by taking 4 peices of paper and putting them on all 4 sides of the tire. Mark and measure lenght and width of what part of the tire touches the ground. Then just multiple lenght times width and that's your area.

Do that for your old and new tire and then determine the percentage increase from the old to the new. That percentage increase should in theory be the percentage DECREASE in pressure.

Forgot one thing, a neat trick to see contact area is to drive through a puddle, keep the wheel straight and stop.
Walk back and look at the tracks.
This is also a neat test to see if your car is crabbing, IOW front and rear wheels not aligned straight in line with each other due to an accident.
Hardly scientific, but interesting anyway


For example if the contact patch is a 4% increase in area(don't know if that's an actual figure), you would decrease pressure by 4%. So 30psi would now be 28.8 psi.

Reason why writing up a formula is pointless is because everyone's contact patch area is going to be different. Different pressures on the new tire won't vary the contact patch too much, but the idea here is equal pressure all over the contact patch area pushing the tire away from the road.

Either way, wider rubber will require less pressure...more than likely only a few psi


BTW, Ford's recommended pressure on the 245/45/17 is 30 psi cold...not 40

I have a 2003 GT with the stock Eagles 245/45/17 and I agree with you.
Never set the pressure to what the sidewall says, especially on high performance tires because the tires were designed for all kinds of cars and that is the absolute max.
Look at the door sticker as a guide and adjust VERY LITTLE up or down depending upon wider (down) narrower (up).
I tried my Eagles at 36 just to see what it did and the car handled horrid. It was like riding on marbles hopping all over the place.
I set mine for 30ish and it's fine for my 2003 GT Vert.

 

[BlackGT4.6]

stick with the placard

 

[Disturbed One]

After selling tires for the last 3 years and working with them.... and with my own car... 97 GT, ford recommends 30psi cold... I've kept 35 psi cold since I've gone though the 1st set and I'm on my 2nd set here. All have worn dead even across with no unusal wear. The guys that have worked in the back shop anywhere from 5-35+ years have said any low profile tires they've worked with have done anywhere from 32-25psi cold. I would recommend you stick with 32-35psi cold if you'd like to have even wear for a DD.

The sidewall maximum PSI is the MAX weight that the tire can hold cold. Obviously you don't need max pressure to hold the recommened weight.

Recommended tire pressure is a factor of weight on the tire and contact patch area.

With a stock 245/45/17, Ford recommends 30 psi.

Now with a wider tire, you have just increased your contact area, but have not increased the weight of the vehicle, so in theory you will require LESS tire pressure to support the same weight.

There is a way to figure out a ballpark range mathematically, but i'm too lazy to write it up. You will need to experiment, most likely with chalk, to find the perfect PSI. But i would start at factory 30psi and maybe work down a bit.

As for this statement here. That is not correct. You need to read the sidewall of the tire to see what the max weight of the tire is cold and adjust accordingly. Each tire is going to be a little different for it capacities. I'll have to grab a book I have that lists the size of the tire and the psi needed to maintain the correct weight.

 

[Disturbed One]

http://www.bfgoodrichtires.com/specs/g-force-t-a-kdw/22.html

Right off BFG Website:

BFG KDW
245/45ZR17 95Y Load index, 1521lbs@44psi cold = 34.63lbs/1psi x30psi =1038.9lbs. You need 31.7psi to = 1100lbs

275/40ZR17 98Y Load index, 1653lbs@44psi cold = 37.56lbs/1psi x30psi = 1126.8lbs. You need 29.28psi to = 1100lbs

P315/35R17 1433lbs@44psi cold = 32.56lbs/1psi x30psi = 976.8 lbs. You need 33.78lbs to = 1100lbs.


Ultimatly it's up to you to run what you'd like on the street. I really recommend 32-35psi cold if you want to wear the tires evenly. You can run less if you have a gutted car or not carrying as much weight.

 

[StangIt #1]

You need to read the sidewall of the tire to see what the max weight of the tire is cold and adjust accordingly. Each tire is going to be a little different for it capacities. I'll have to grab a book I have that lists the size of the tire and the psi needed to maintain the correct weight.

Disturbed One, the last tire I had on the car was a Sumitomo HTR+ in 245/45/17 (on the factory 17x8 rims). From Sumitomo's website, the maximum PSI was 51 for this tire. For the 275/40/17's & the 315/35/17's, Sumi says that the max PSI is 44 (both sizes are showing the same).

Based on your calculations, should I run different pressures (MORE pressure in the rears)? For example, 30psi in fronts (rounded up from the calculations) & 34psi in the rears? I know tire pressure isn't a perfect science, and certainly tire compounds, driving habits, road conditions, weather, and other variables will affect treadlife...but these tires are expensive. I would like to get the best bang for my buck(s)!

Thanks for everyone's responses...I appreciate them all!

 

[Hipogt]

Duh. Next time I'll read everything before I ask the obvious. Disturbed One is pretty much right on. Run the same psi front to rear or more in the front to reduce your understeer. If you now have more understeer than with the 245's and this was the only change to the car, then the 315's are doing a better job of fighting oversteer but increasing your understeer. This is the same effect a larger rear bar will cause. Running more psi in the rear will increase your understeer. Also look at where most of your weight is. The 275's are carrying more weight, need more psi. Start adjusting and enjoy your new treads.

 

[joshheat25]

I like to run my fronts at about 34 and the rears at about 30-32.

 

[Disturbed One]

Disturbed One, the last tire I had on the car was a Sumitomo HTR+ in 245/45/17 (on the factory 17x8 rims). From Sumitomo's website, the maximum PSI was 51 for this tire. For the 275/40/17's & the 315/35/17's, Sumi says that the max PSI is 44 (both sizes are showing the same).

Based on your calculations, should I run different pressures (MORE pressure in the rears)? For example, 30psi in fronts (rounded up from the calculations) & 34psi in the rears? I know tire pressure isn't a perfect science, and certainly tire compounds, driving habits, road conditions, weather, and other variables will affect treadlife...but these tires are expensive. I would like to get the best bang for my buck(s)!

Thanks for everyone's responses...I appreciate them all!

Well if your doing the Sumitomo htr z tires... I'd probably start off with around 32psi and see how it feels to ya there. The more air you have in the tires the firmer they are going to be and spin a little easier in the rear with more air pressure. I think 32psi cold would give you a good starting point. Not too stiff, not too soft. That's 1041.92 lbs carrying capacity.

If you go with anything lower then 32psi you'll probably start to wear the shoulders from being under inflated. anything more then 35psi cold and you'll start to wear the center of the tire pretty bad from over inflation. I've run 35psi cold on 2 sets of tires here and have had them dead even across. I put a set of 275/40/17's on a friends mustang at work and set them at 35pis and they've worn perfectly across so far.

 

[Twista]

35

 

[StangIt #1]

I like to run my fronts at about 34 and the rears at about 30-32.

How even has your treadwear been with these psi specs (any shoulder wear, other than from hard cornering)? I see that you are also running a staggered set-up. Also, what brand/model tires are you running?

I also set mine at 34 in the front & 32 in the rear. Based on all of the feedback, this seems to be a good starting point!

 

[BurningRubber]

Damn I need to fix my pressure. Im running 42psi warm in my Sumitumo 315's!!! No wonder the sides look brand new, damn.

 

[jeffnoel]

I'm on my 3rd set of HTRz 275/315 combo and run them at 30psi cold. They last me about 25K miles a set.