What tire pressure are you using for your 20's
04-14-12, 01:52 AM
#16
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Thanks Toymota & others. I upgraded from OEM 225/45/17 91V to 225/40/18 92Y XL. Was wondering why Tire Rack told me to go up from 35/38 psi to 39/41 psi. This makes sense now thanks!
Oh any idea what happens when a value is off the charts? e.g. there is no 91 load index @ 38 psi for the OEM rear load value. Do I use the last number @ 36 psi?
Oh any idea what happens when a value is off the charts? e.g. there is no 91 load index @ 38 psi for the OEM rear load value. Do I use the last number @ 36 psi?
Last edited by sulfur; 04-14-12 at 01:56 AM.
04-14-12, 03:33 AM
#17
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Up front answer:
Fronts 41 psi
Rears 44 psi
The correct pressure is based on the load index of the tires compared to the stock spec. I'm assuming yours are 88 XL on the front and 97 XL on the rear.
The stock 18" on the front has a load index of 88 and recommended pressure of 35 psi so that's rated for a 1190 lb load, and the rear is 95 @ 38 psi so that's 1521 lb.
88 XL front tires will need a pressure of 41 psi to support the same 1190 lb load, and 97 XL rears will need 39 psi, but increase that to 44 psi to maintain the same 3 psi front-to-rear differential as stock.
Yes, your car will ride like **** with these pressures, but that's the problem with running 20s and super low profile tires--the sidewall is not strong enough so you end up having to run higher pressures to support the load.
An 88 XL tire @ 36 psi is rated at 1091 lb so the other guy is running 100 lb under spec on each front tire.
Fronts 41 psi
Rears 44 psi
The correct pressure is based on the load index of the tires compared to the stock spec. I'm assuming yours are 88 XL on the front and 97 XL on the rear.
The stock 18" on the front has a load index of 88 and recommended pressure of 35 psi so that's rated for a 1190 lb load, and the rear is 95 @ 38 psi so that's 1521 lb.
88 XL front tires will need a pressure of 41 psi to support the same 1190 lb load, and 97 XL rears will need 39 psi, but increase that to 44 psi to maintain the same 3 psi front-to-rear differential as stock.
Yes, your car will ride like **** with these pressures, but that's the problem with running 20s and super low profile tires--the sidewall is not strong enough so you end up having to run higher pressures to support the load.
An 88 XL tire @ 36 psi is rated at 1091 lb so the other guy is running 100 lb under spec on each front tire.
Just to correct you, the real reason why it rides rough its not because the sides walls are not strong enough.. its actually the side walls are too hard, the vibration transfer from tire to rim is more prominent, road vibrations has less tire to deafen the blow, more sidewall actually dampens the noise and sound , if you see the normal 55 to 60 series tires they balloon on the sidewalls and uses the air pressure for the cushion and has more surface area to dissipate noise and vibration. .
as for the OP, i would start at OEM air pressure and add till you are satisfied which comfort ,
but relatively if you go lower on the profile you usually have to add a little more
04-14-12, 05:53 AM
#18
Thanks Toymota & others. I upgraded from OEM 225/45/17 91V to 225/40/18 92Y XL. Was wondering why Tire Rack told me to go up from 35/38 psi to 39/41 psi. This makes sense now thanks!
Oh any idea what happens when a value is off the charts? e.g. there is no 91 load index @ 38 psi for the OEM rear load value. Do I use the last number @ 36 psi?
Oh any idea what happens when a value is off the charts? e.g. there is no 91 load index @ 38 psi for the OEM rear load value. Do I use the last number @ 36 psi?
Actually, I feel that there is a bit of leeway in the load capacity because the IS is speced for several different tire sizes including 16", 17" AWD, 17" stagger, 18" stagger, yet the recommended pressure is always 35/38 psi. The lowest load capacity on the front is the 18" 88 @ 35 psi = 1190 lb, and on the rear is the 16" 89 @ 38 psi = 1279 lb. The 16" rears have a much lower load capacity than the 18"-20" tires we run, so mainly keep an eye on your front tires if you change size and make sure you set the pressure to meet at least 1190 lb load capacity.
04-16-12, 07:47 AM
#20
Up front answer:
Fronts 41 psi
Rears 44 psi
The correct pressure is based on the load index of the tires compared to the stock spec. I'm assuming yours are 88 XL on the front and 97 XL on the rear.
The stock 18" on the front has a load index of 88 and recommended pressure of 35 psi so that's rated for a 1190 lb load, and the rear is 95 @ 38 psi so that's 1521 lb.
88 XL front tires will need a pressure of 41 psi to support the same 1190 lb load, and 97 XL rears will need 39 psi, but increase that to 44 psi to maintain the same 3 psi front-to-rear differential as stock.
Yes, your car will ride like **** with these pressures, but that's the problem with running 20s and super low profile tires--the sidewall is not strong enough so you end up having to run higher pressures to support the load.
An 88 XL tire @ 36 psi is rated at 1091 lb so the other guy is running 100 lb under spec on each front tire.
Fronts 41 psi
Rears 44 psi
The correct pressure is based on the load index of the tires compared to the stock spec. I'm assuming yours are 88 XL on the front and 97 XL on the rear.
The stock 18" on the front has a load index of 88 and recommended pressure of 35 psi so that's rated for a 1190 lb load, and the rear is 95 @ 38 psi so that's 1521 lb.
88 XL front tires will need a pressure of 41 psi to support the same 1190 lb load, and 97 XL rears will need 39 psi, but increase that to 44 psi to maintain the same 3 psi front-to-rear differential as stock.
Yes, your car will ride like **** with these pressures, but that's the problem with running 20s and super low profile tires--the sidewall is not strong enough so you end up having to run higher pressures to support the load.
An 88 XL tire @ 36 psi is rated at 1091 lb so the other guy is running 100 lb under spec on each front tire.
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