Pressure for Tire Larger than OEM

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Currently I have 215/50-17 on the front and 245/45-17 on the rear of my S2000, the OEM sizes are 215/45-17 and 245/40-17. The diameter is about 3.5% larger and load index increase from 91 to 95 for the rear with the same W speed.

The placard on the door has 32 PSI for both front and rear, since the diameter increases 3.5% for both front and rear, the same pressure is not applicable for larger diameter, I have both front and rear at 32 PSI the performance/handling was terrible, when I went home I looked at the dust on the tires, the front had dust on 3/4 of the outside tread but the rear had dust only 1/4 of the outside tread, so I reduced the rear pressure to 30 PSI then 28 PSI then 26 PSI. The car is now very stable at highway speed(more than 70-75 MPH) and lateral acceleration is very good too. When I drove slowly on dirt road the dirt covered 3/4 of the outside treads which is much more than 1/4 when it had 32 PSI. I will try 24 PSI to see if performance/handling will improve or not.

Lesson learnt: When larger diameter(or wider) tires are used you need to adjust tire pressure for your particular vehicle, the lighter the vehicle the lower pressure may be needed, pressure on placard doesn't apply here.
 
You'll probably get an argument, but not from me.

Tire pressure even on a new set of same size tires for the same vehicle sometimes needs to be different.
 
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I had a similar experience with an F150 I owned. OEM tire size was something like 255/70/16 and I was given some GY Eagle tires in 285/60/16. First of all I'm pretty sure I was pushing it having a tire that wide on a 7" wide wheel. I ran PSI stated on the doorjamb of 32psi all around and in no time I had worn down the center rib on all 4 tires.
 
Originally Posted By: Kuato
You'll probably get an argument, but not from me.

Tire pressure even on a new set of same size tires for the same vehicle sometimes needs to be different.

I welcome all opinions, agree or disagree, because I like to learn from others too. This is what I learnt so far but I may be wrong, and if anyone has opposite experience(s) I like to know too.

I'm manly enough to accept that I am not always correct and I'm willing and ready to learn from others.
 
I play that same game with my Forester. Not only upsized the tires from stock, also chose a set with an insanely higher load index vs what was stock. Stock was 215/65R16 load index of 96 ([censored] Bridgestone Duelers). I upsized to 17" rims and a 225/60VR17 set of Nokian WRG2 in the heavier SUV version that has a load index of.... wait for it..... 103XL the XL meaning Xtra Load i.e. re-inforced sidewalls. I did get a deal on them but technically they're intended for vehicles weighing much more than my humble Forester. They look fantastic on the car and work really well in winter, but I have to run them about 4 - 5 psi lower than the frame sticker values or they'll skid in wet and beat me to death over RR crossings. Lowered my fuel economy a little too, even though Nokian calls them low rolling resistance. Next set will be the regular car version of the same tire or WRG3's if 2's are no longer made by then.
 
A 245/45/17 tire with a 95 load index is rated at right around 1200 lbs at 26 psi... ( the load table doesn't go any lower...)

Figure out your car weight / weight distribution... you will probably find the rear tires carry less weight...

On my Impreza, the fronts carry about 1000 lbs , the rears about 700 lbs. (with driver only...) My tires are rated at 1000 lbs at 26 psi , and 1250 lbs at 33 psi.

I don't run the rears that soft, but technically I could...
 
Originally Posted By: HTSS_TR
Currently I have 215/50-17 on the front and 245/45-17 on the rear of my S2000, the OEM sizes are 215/45-17 and 245/40-17. The diameter is about 3.5% larger and load index increase from 91 to 95 for the rear with the same W speed.

The placard on the door has 32 PSI for both front and rear, since the diameter increases 3.5% for both front and rear, the same pressure is not applicable for larger diameter, I have both front and rear at 32 PSI the performance/handling was terrible, when I went home I looked at the dust on the tires, the front had dust on 3/4 of the outside tread but the rear had dust only 1/4 of the outside tread, so I reduced the rear pressure to 30 PSI then 28 PSI then 26 PSI. The car is now very stable at highway speed(more than 70-75 MPH) and lateral acceleration is very good too. When I drove slowly on dirt road the dirt covered 3/4 of the outside treads which is much more than 1/4 when it had 32 PSI. I will try 24 PSI to see if performance/handling will improve or not.

Lesson learnt: When larger diameter(or wider) tires are used you need to adjust tire pressure for your particular vehicle, the lighter the vehicle the lower pressure may be needed, pressure on placard doesn't apply here.


Here's the short answer:

Stock
Front 215/45-17's @ 32psi = 1135 pound Load Limit
Rear 245/40-17's @ 32psi = 1224 pound Load Limit

In order to match the factory recommended Load Limits for your new tires:
Front 215/50-17's @ 26psi = 1146 pound Load Limit
Rear 245/45-17's @ 26psi = 1261 pound Load Limit

NOTE: 26psi is the minimum recommended pressure for both new front and rear tires.

HTH
 
Originally Posted By: HTSS_TR

Here's the short answer:

Stock
Front 215/45-17's @ 32psi = 1135 pound Load Limit
Rear 245/40-17's @ 32psi = 1224 pound Load Limit

In order to match the factory recommended Load Limits for your new tires:
Front 215/50-17's @ 26psi = 1146 pound Load Limit
Rear 245/45-17's @ 26psi = 1261 pound Load Limit

NOTE: 26psi is the minimum recommended pressure for both new front and rear tires.

HTH



Interesting. Wondering if you would show the math / calculation for Load Limit?
 
Originally Posted By: HTSS_TR
Currently I have 215/50-17 on the front and 245/45-17 on the rear of my S2000, the OEM sizes are 215/45-17 and 245/40-17. The diameter is about 3.5% larger and load index increase from 91 to 95 for the rear with the same W speed.

The placard on the door has 32 PSI for both front and rear, since the diameter increases 3.5% for both front and rear, the same pressure is not applicable for larger diameter, I have both front and rear at 32 PSI the performance/handling was terrible, when I went home I looked at the dust on the tires, the front had dust on 3/4 of the outside tread but the rear had dust only 1/4 of the outside tread, so I reduced the rear pressure to 30 PSI then 28 PSI then 26 PSI. The car is now very stable at highway speed(more than 70-75 MPH) and lateral acceleration is very good too. When I drove slowly on dirt road the dirt covered 3/4 of the outside treads which is much more than 1/4 when it had 32 PSI. I will try 24 PSI to see if performance/handling will improve or not.

Lesson learnt: When larger diameter(or wider) tires are used you need to adjust tire pressure for your particular vehicle, the lighter the vehicle the lower pressure may be needed, pressure on placard doesn't apply here.


There are 2 schools of thought with regard to inflation pressure when changing tire size:

1) Match the load carrying capacity. Gaijinnv gave you that answer.

2) If you can get enough load carrying capacity, match the spring rate by matching the inflation pressure. The problem with this answer is that it results in a smaller footprint which has implications on traction. The loss of traction may or may not be a problem.


Some other thoughts:

The way tires actually feel can vary quite a bit. Certainly you can change that feel by changing the inflation pressure. That is NOT the way to set inflation pressure unless you consider the load carrying capacity - which is THEE most important aspect of setting the inflation pressure.

Put another way, I would not recommend using less than the placard pressure (if you are using the placard size), and using more pressure than the placard may result in traction losses. It would pay to explore any changes you may be considering. I would hate to have someone injured just because what they thought was a good answer, wasn't!
 
The weight of S2000 is around 2850 lbs and weight distribution is 50-50 front and rear, so each end is 1425 lbs.

The OEM handling with OEM tire sizes is tuned for a slight over-steer, if the rear tires have less footprint then it will increase over-steer quite a bit. I have front tires at 32 PSI and rear at 26 PSI to dial in some under-steer.
 
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