tire pressure vs. size

Zman · 01-21-2004, 01:21 PM

For any given tire, is there a difference in recommended pressure for a 225 vs. a 215? (same rim, same profile)

Suby gives numbers for 215/45/17's, but (technically speaking) are those valid/equal for 225's?

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Luke@tirerack · 01-21-2004, 01:53 PM

yes they are valid because the recmmended pressure is based on the vehicle weight and that is not changing

Zman · 01-21-2004, 04:03 PM

Quote:

Originally posted by Luke@tirerack
yes they are valid because the recmmended pressure is based on the vehicle weight and that is not changing

Why then, is there a separate recommendation for a stock tire (32/29) vs. 215/45/17 at 33/32?

If there is a noted difference between a 205/55/16 and a 215/45/17, why wouldn't there be a difference between a 215 and 225? I'd guess that (again, technically speaking) that a 225 might actually be 'lower' than a 215, due to more width/support from the wider tire - but if any one knows, from a techinical standpoint, if that is true, I'd appreciate some more info.

Thanks, Zman

rexxer · 01-21-2004, 04:27 PM

Remember that the car manufacturer wants to be known for having a safe car... so, they want the car to be very neutral for the average driver. With a narrower tire, dropping the pressure will help give more grip. 32/29 with the stock tires gives more grip rearward... less ability to rotate the car... 'safer' for the average driver. As the width increases, the contact patch increases and the ability to rotate the car decreases... so the average driver will be 'safe' with a higher pressure (which will give better gas mileage than the lower pressure).

Safety vs. gas mileage.

reks · 01-21-2004, 05:48 PM

I believe there are many threads which discuss the fact that the size of the contact patch does not change with tire width, only the shape of the patch.

Changing the tire pressure changes the contact patch size. So does the weight of the car as Luke stated above.

Zman · 01-22-2004, 05:56 PM

interesting..... I got an email back from Michelin, which said that since the 215's were recommended at 33/32, and the 225's were more volume, that the pressure would be lower, and they suggested 31/29 (which I think is too low) - but I did try setting down to 32/31, and the ride is smooooooth.... and the traction rocks...... response is down just a touch, but now I don't feel 'every' little bump in the road...... smooooooth.... and seems that the sidewalls hold up fairly well, no squirm.....waiting to see how much this will effect mpg.....

01-21-2004, 01:21 PM	#1
Zman * Banned * Member#: 26460 Join Date: Oct 2002 Location: Bay Area, CA Vehicle: 2003 WRX Wagon Silver	tire pressure vs. size For any given tire, is there a difference in recommended pressure for a 225 vs. a 215? (same rim, same profile) Suby gives numbers for 215/45/17's, but (technically speaking) are those valid/equal for 225's? * Registered users of the site do not see these ads.

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01-21-2004, 01:53 PM	#2
Luke@tirerack Tire Rack Moderator Member#: 1215 Join Date: Apr 2000 Location: The Tire Rack Vehicle: 800 -428-8355 ext. 4362	yes they are valid because the recmmended pressure is based on the vehicle weight and that is not changing

01-21-2004, 04:27 PM	#4
rexxer Scooby Specialist Member#: 27624 Join Date: Oct 2002 Chapter/Region: W. Canada Location: "Somewhere cheap... Red Deer" Vehicle: 2005 WRX STi World Rally Blue Pearl	Remember that the car manufacturer wants to be known for having a safe car... so, they want the car to be very neutral for the average driver. With a narrower tire, dropping the pressure will help give more grip. 32/29 with the stock tires gives more grip rearward... less ability to rotate the car... 'safer' for the average driver. As the width increases, the contact patch increases and the ability to rotate the car decreases... so the average driver will be 'safe' with a higher pressure (which will give better gas mileage than the lower pressure). Safety vs. gas mileage.

01-21-2004, 05:48 PM	#5
reks Scooby Specialist Member#: 9567 Join Date: Aug 2001 Location: Marlboro, MA Vehicle: 2005 Legacy GT Wagon Silver	I believe there are many threads which discuss the fact that the size of the contact patch does not change with tire width, only the shape of the patch. Changing the tire pressure changes the contact patch size. So does the weight of the car as Luke stated above.

01-22-2004, 05:56 PM	#6
Zman * Banned * Member#: 26460 Join Date: Oct 2002 Location: Bay Area, CA Vehicle: 2003 WRX Wagon Silver	interesting..... I got an email back from Michelin, which said that since the 215's were recommended at 33/32, and the 225's were more volume, that the pressure would be lower, and they suggested 31/29 (which I think is too low) - but I did try setting down to 32/31, and the ride is smooooooth.... and the traction rocks...... response is down just a touch, but now I don't feel 'every' little bump in the road...... smooooooth.... and seems that the sidewalls hold up fairly well, no squirm.....waiting to see how much this will effect mpg.....