Originally Posted By: Rolla07
Originally Posted By: CapriRacer
Originally Posted By: Rolla07
Been searching the web to find out what would be the ideal PSI for a trip when carrying a heavy load. I am running Pirelli P7 225/45/17's with XL load range, max tire inflation is 44psi, door placard is 30. I typically run 32 psi all around. I also should say I have a front passenger side leaking strut, which I want to go easy on til I replace it (soon). It also seems when I hit a bump that makes the car bounce the back tires scrape against the fender resulting in a round gouge into the sidewall of the tire. Only on bumps that make the car bounce, also wonder if my shocks are not as strong as they used to be.. wish I had an easy solution to fix that problem...besides replacing the tires with smaller tires like 215's... live and learn I suppose. So to clarify my questions:
1- What PSI to run with a heavy load? (or what PSI you would use & why)
2- Any solutions to the back suspension? (rolling fenders/other ideas)
Thank you for all suggestions!!
The first place to start is the vehicle tire placard, which is usually located on a doorframe or in the glove box. The vehicle tire placrd will list the original tire size and the proper pressure for that size.
According to Tire Guides, a 2007 Toyota Corolla came with P185/65R15 86S, P196/65R15 89S, or P165/55R16 87S - all at 30 psi.
A 225/45R17 XL has a 94 Load Index. (Standard Load = 91 LI)
So 30 psi for that XL tire would be more load carrying capacity that the original tires.
Please note that Tire Rack say the right tire size for a 17" upsize is 205/45R17 - so the current tire is 2 sizes oversized. That's why it is rubbing.
And some lessons on the way maximum tire pressures are done for P type tires.
All Standard Load P type tires max out for load carrying capacity at 35 psi (36 psi if they are not P metrics). It is permissible to use a higher inflation pressure for higher speeds (with no increase in load) and the prescribed max values are 44 psi and 51 psi.
Extra Load tires max out for load carrying capacity at 41 psi, and 50 psi is the perscribed max value for higher speeds.
Car manufacturers specify the inflation pressure on the vehicle tire placard based on a lot of things, but the most important is load carrying capacity. If someone wants to carry more weight than the vehicle was designed for, increasing the inflation pressure only deals with one thing of many that resulted in the vehicle's max load rating.
Plus, vehicle manufacturers do all their testing at the specified pressure - and that includes setting the spring rates, shock damping rates, and sway bar rates based on that pressure 9which results is the tire having a certain spring rate. Yes, spring rates vary a bit from tire to tire, but tire spring rates are strongly correlated to inflation pressure regardless of the tire's make and model.
So in theory, the 30 psi Toyota specified will carry the max load - and since we are dealing with a tire that is oversized, 30 psi would also be more than the max load of the vehicle.
So, No, starting at the max pressure is just inherently wrong.
Thanks for your opinion! Just wanted to add that using the diameter variance comparing to 195 65 15 as stock size..the 225 has a -0.03% diameter variance while the 205 45 17 you mentioned has a -2.87% variance..i was trying to stay as close to 0 as possible. I used 1010tires.com plus sizing tool to compare. I think tirerack that you looked at used 185-65-15 as stock tire size. Mine came with 195-65-15..so i decided to use 195 as the baseline.
So I found an entry in Tire Rack for a 195/65R15 for your car, and they recommend 205/50R17. You can be sure that if Tire Rack recommends an upsize, it will fit without rubbing - and I would suggest you go there on your next set of tires.
