I got a low tire pressure warning yesterday so I stopped at nearby gas station to fill the tire up. I found out that I got around 25-30 psi (cold) while my max psi for all 4 tires are at 51 psi.

So, I filled it up to 40 psi (cold) and I hoped it's okay.

1. Can anyone tell me what 40 psi (cold) will be on hot summer weather??
2. My max psi is 51 on all 4 tires (After market tires), what's the best psi to put on for better gas millage and performance wise?? (ie. 40(cold), 35(hot))

 

The rule of thumb that I have always used is a +/-5-degree shift will result in approximately a +/-1 PSI change in tire pressure. As far as tire pressure versus gas mileage, I was always under the impression that more pressure you run, the better mileage you get.

If I am incorrect, please let me know.

 

The best tire pressure is the pressure listed on the placard located on the driver's side door.

 

The OP is running aftermarket tires, so depending upon what they are, the 35 PSI all around might not be suitable.

bacquerel, what size tires are you running?

 

i wouldnt put 40 during cold weather and playing wit the "max 51 psi" rating..
use whatever is written on the side of ur door ... + 5psi if ure running wider tires..
usually i put 38psi in my 235/40/18..

 

every single car that comes into the shop here gets 35psi during service. this is also after the car has been driven in from wherever and then driven around the building into whichever bay it goes to for work, so I'm sure they drop a bit once home and cooled off. the threshold for that tpms light is right around 27-29 psi, placard numbers, just in case you were wondering.

 

i find the factory rating of 35 psi still a bit soft on the sidewall and go up to 38.

my friends who compete in SCCA auto-x set their tires 41-44psi for the street, and upwards of 45-50 (warm) for auto-x. the road feel in their cars is amazing! but not very comfortable.

one of my friends has gone and tried doing 5-10psi OVER the max rating on the sidewall. he hasn't blown out any tires at auto-x. yet.

 

Running the 'correct' spec tire pressure will yield the best tire life.

 

correct. although a touch over will yield a bit better gas mileage and still retain great wear characteristics. for example, i have 17k miles on my stock yokos and thanks to regular tire rotation and proper maintenance still have 8/32" on all 4 tires and see 20+ mpg around town and upwards of 30+ mpg on the highway. and i'm tuned with the mods listed in my sig

 

40psi is kind of high. I would think the car is going to feel too twitchy with that high of pressure.

 

what psi (cold reading) would you put for a tire that is 225/45/18?

 

THIS

My friends ran...I think 48psi?....on his RX-7 he used to autoX

Personally I run 38psi cold on mine and I prefer the feel (my nokians have soft sidewalls to begin with, anything less and it feels like they're ready to roll over on themselves).