Topic: Inflation Pressure for +1 Size Tires?

[Angry Chicken]

I just replaced the tires on my 1989 Honda CRX Si.  I bought Falken FK-452, size 215/40-ZR16.  Here's the question.  My old tires were 195/50-VR15s and I inflated them to 32 psi.  Is there a rule of thumb for inflation pressure on these newer 40-series tires?  The sidewall is quite a bit shorter than the old tires.  They were inflated with nitrogen to 45 psi...I'm guessing it's up to me to decrease the pressure to something more reasonable.  The new rims are 7" wide if that matters.

Ideas?

Thanks,

Eric

[wing]

Just put them the same, 32. 45 is WAY WAY too high, geez when will tire shops learn?

[xviper]

Go by what the decal in your door jamb says.  Unless you've gone to a size that is vastly different than what's stock or what's normally meant for that size of rim, stick with the manufacturer recommended pressure.  Keep track of tread wear.  Typically, if you keep the same rims and go one or two sizes wider (as in 20mm), you are "pinching in" the bead ever so slightly and you would perhaps DEcrease pressures by a pound or two because you are causing the middle of the tread to be "bent" out.  A slight decrease in pressure will help to compensate for this, however, this will increase rolling resistence and hence, adversely affect fuel economy (but only very slightly since you didn't go excessively wide).  Keeping the same tire pressure as your stock size "might" mitigate middle tread wear over the long run.
45 psi is obsurd.

[capriracer]

On every vehicle sold in the US, there is a sticker commonly - called the vehicle placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

I've looked up a 1989 Honda CRX and it came with 185/60R14 82H's inflated to 28 psi front and rear.  If any of this information is incorrect, then what follows is suspect.

215/40R16's comes in both 82 Load Index (Standard Load) and 86 Load Index (Extra Load) and they need to be inflated to same as the the original tires - 28 psi front and rear.

However, considering how low the aspect ratio is, you might want to consider using 3 to 5 psi more than that to reduce the probability of impact related damage to the rim.

[Angry Chicken]

This is all good advice and confirms my gut feeling that 32 psi would be a good place to start.  You wouldn't believe how harsh 45 psi is!  :-)  Thanks guys.

I've lowered the car 1" so far, though 3.5" is possible with the coilovers and camber kits that I have installed.  It looks great at 1" and the handling is that much better though my goal in lowering the car is mostly cosmetic.  I want to avoid the wagonwheel look that you get with rubberband-width sidewalls and standard ride height.  I saw a Civic the other day with what looked like 18" wheels and 35-series tires.  It looked like crap...almost like a puppy dog with over-sized paws because the car wasn't lowered at all.

/eric

[xviper]

The premise is that if 32 psi is recommended for a stock tire/rim configuration, this is what it takes for any other tire size to properly "suspend" the weight of the car.  However, when tire size is altered too much, as in much wider tires and much lower profile tires (to maintain the proper diameter for speedo correctness, ABS and/or stability control function) then the appropriately wider rim should be used.  A further complication comes when you go very wide and very low profile in that daily street driveability suffers both in terms of ride harshness and resistance to road bumps and potholes.  Too little pressure and you risk compressing a low profile tire and impact the rim when encountering bumps.  To much pressure and you risk a very harsh ride and excessive bulging in the tread center, thus wearing that part of the tread prematurely.  On the other hand, if you're spending that kind of money to lower a car that much, tire expense may just be a normal consequence.
A 1" drop and an increase in 20mm of tire width is fairly minimal.

[Angry Chicken]

A 1" drop and an increase in 20mm of tire width is fairly minimal.

Yes.  The overall diameter is the same, and the rims are 25 mm (1 ") wider to compensate for a 20mm increase in tire width.
/Eric

[xviper]

A 1" drop and an increase in 20mm of tire width is fairly minimal.

Yes.  The overall diameter is the same, and the rims are 25 mm (1 ") wider to compensate for a 20mm increase in tire width.
/Eric

Ahh, this is important information that you left out originally.  By going with a wider rim, you did the RIGHT thing and the optimal thing.  Many people do not and must then make slight adjustments in pressure to compensate for tire bead pinch or stretch.  Since you've done this, for sure, stay with the OEM tire pressure to begin with, but monitor tire wear and performance and adjust a pound or two up or down as needed.

[Angry Chicken]

A 1" drop and an increase in 20mm of tire width is fairly minimal.

Yes.  The overall diameter is the same, and the rims are 25 mm (1 ") wider to compensate for a 20mm increase in tire width.
/Eric

Ahh, this is important information that you left out originally.  By going with a wider rim, you did the RIGHT thing and the optimal thing.  Many people do not and must then make slight adjustments in pressure to compensate for tire bead pinch or stretch.  Since you've done this, for sure, stay with the OEM tire pressure to begin with, but monitor tire wear and performance and adjust a pound or two up or down as needed.

Actually I did mention that the wheels were 7" in the original post.  ;-)

Thanks for the info everyone!
My CRX Site:  http://crx.unixlover.com

[xviper]

Actually I did mention that the wheels were 7" in the original post.  ;-)

Fair enough, but not knowing what width your stock rims are, I just had to cover all the bases.  EG, when you order after rims from TireRack, they will try to recommend ones that closely matches what the car had as OEM since they only guess that you may be using your stock tires.

BTW, keep that baby.  CRX's are getting quite rare and are highly sought after.

[Angry Chicken]

Actually I did mention that the wheels were 7" in the original post.  ;-)

Fair enough, but not knowing what width your stock rims are, I just had to cover all the bases.  EG, when you order after rims from TireRack, they will try to recommend ones that closely matches what the car had as OEM since they only guess that you may be using your stock tires.

BTW, keep that baby.  CRX's are getting quite rare and are highly sought after.

I'm keeping it into my dotage.  In another few years it will become a classic in the same way as MGs are but with one important difference....it's reliable!

My 18- and 15-year-old are already fighting over it.

/Eric