PSI recommendation for new car tires

[Saturn_Fan]

Had four new Dunlop SP60 All seasons installed this morning. I looked at the door jamb and it says 30 front/26 rear (car is 2000 Saturn). Bought the SP60's based on their actual rating by TireRack, not just user ratings. They scored very well in light and deep snow for an all season.

I read somewhere on TireRack where it recommends that you set the pressure 2PSI above what the door jamb recommends. Does this sound about right?

I set the tires at 32 front/28 rear after the tires were cold and the temperature was 38F in the garage.

I am willing to sacrifice a little MPG's to obtain better snow3 performance this winter in WI.

With my old set of tires, I had them inflated at 33PSI all the way around, but it always felt too hard when it warmed up.

 

[bepperb]

Keep in mind that it's warm here now. You'll probably lose 2psi when the temp drops from 50 into the 20's.

 

[VK56VD]

I'd bump up 6 psi up from the original Saturn recommendation up front and 7 psi in the rear.
(36 psi front, 33 psi rear).

I did this on my ex's '97 SL2 and it worked out great.

 

[Craig in Canada]

It's generally advisable to start from the door jam and adjust from there. The behaviour depends a lot on the characteristics of the vehicle, not just the tire. Someone with a different model of car can't effectively give you tire pressure advice.

I had an enlightening conversation once with a regional manager for Toyo who talked about all kinds of things that are a big deal for tire companies and car companies that owner's often never even think of. One of these things was road crown, and how it's different in different countries. To get a satisfactory driver experience the manufacturer may have different door jamb pressures for different markets on an otherwise identical car.

You also have to be careful of the size. An otherwise identical car but with a different optional wheel package will have different door jamb recommendations. If you aren't using the door jamb size, the starting pressures may be different.

My sport package BMW E39 235/45/17 recommends 29F/33R for light loads and 36F/44R for full weight rating. My father's very similar but non-sport BMW E39 with 225/55/16 recommends 33F/35R for light loads.

I would be personally uncomfortable going lower than your jamb sticker of 28R as part of your tuning. I never go down to my sticker rating of 29F on my BMW because it feels too dead, but I retain the F/R split (usually 32-33/35-36, depending on the tires) as the split is designed to influence vehicle balance. Although one set of snow tires (Dunlop M3s 225/55/16) I had needed 34F/34R to be acceptable.

Remember - within reason (say 26-40psi?) - higher PSI = smaller contact patch = less grip = less rolling resistance

 

[eljefino]

Bear in mind snow tires have squishy sidewalls so the tread flexes and tosses snow out. By overinflating your all seasons you may negate this. But they'll have a smaller contact patch so they'll dig down better. But as wide profile tires you'll have a rectangular contact patch with the skinny side prone to sliding sideways.

I have also read a theory where overinflated tires make the struts turn bump energy into heat, creating drag. Properly inflated ones spring back while they're still on the bump, turning it into forward motion. Those lab tests with a tire on a drum where more PSI cut rolling resistance may not 100% translate in the real world where there are bumps. The best thing you can do is track your own mileage. Maybe more psi works on the freeway but not around town for example.

All that said I run my summer tire PSI even higher than you, 34/30 on my saturns, but I do it as much because I can't check them as often as I wish in wildly varying temps etc. And the car does handle better.

 

[IndyIan]

What kind of snow do you have on the roads typically?
If its hard packed I think having a bigger contact patch would be better so running them at factory pressures would be good.
If you get deep loose snow or lots of slush that isn't freezing to the road I think running the tires a bit harder would be better to cut through the snow/slush.

 

[Saturn_Fan]

Originally Posted By: IndyIan
What kind of snow do you have on the roads typically?
If its hard packed I think having a bigger contact patch would be better so running them at factory pressures would be good.
If you get deep loose snow or lots of slush that isn't freezing to the road I think running the tires a bit harder would be better to cut through the snow/slush.



It varies widely up here. It'll snow 8 or 10 inches but then warm up to the upper 20's then you're dealing with slush.

Sometimes, it'll snow 8 or 10 inches then drop like a rock to -10F or more. Pretty crazy weather here.


Thanks for all of the recommendations guys. After doing some more research on my specific model of car, I am going to do a +3 over what the door jamb says.

I'll try that out and adjust if necessary.

 

[PandaBear]

It really depends on the sidewall thickness of the tire.

I bump 5 PSI on the Sumitomo HTR 200, but only 1-2 over my other tires like Bridgestone HP50 and Dunlop SPA2.

Try different PSI and see what feels comfortable, and pick the highest you can tolerate reasonably well.

Or you can use the chalk test: spread chalk around the trend and go drive for a little, find the highest PSI that will wipe out all the chalk on the trend, that's the PSI with the best contact surface.

 

[wander2039]

Are they the same size/weight rating as the old tires? If so keep them the same. On both the Ranger and Escape if don't run the recommended psi it'll wear out the middle faster than the rest and the tires cup a bit.

 

[mechanicx]

GM almost always recommends 30 psi all the way around on their FWD cars but sometimes recommends a staggered 30 or 29 front and 26 rear. 26 psi kind of makes since for the rear on a FWD since the load is a good 12% less or more at least with the car unloaded, plus that should reduce understeer which FWDs have in abundance. Generally I think it's best to stay with the manufacture recommendation or at least within 2 psi of it.

 

[Craig in Canada]

Originally Posted By: mechanicx
GM almost always recommends 30 psi all the way around on their FWD cars but sometimes recommends a staggered 30 or 29 front and 26 rear. 26 psi kind of makes since for the rear on a FWD since the load is a good 12% less or more at least with the car unloaded, plus that should reduce understeer which FWDs have in abundance. Generally I think it's best to stay with the manufacture recommendation or at least within 2 psi of it.


Lowering pressure in the rear increases the contact patch, increasing rear grip, increasing understeer/decreasing oversteer.

I consider 26psi getting down to the territory where the tire is no longer being properly supported. I know it's on the door sticker, but I personally wouldn't run that low no matter who told me to. I'd be looking at 28-30 as my personal minimum. Starting at 26psi you've got no safety margin for sudden drops in temperature or new slow leaks in order to detect them before you're in truly critical low pressure territory.

 

[rszappa1]

This stuff is not really rocket science... Just follow the sticker on the door jam and you will be fine.........

 

[38sho]

just put 35psi in it and go, I would personally never run 30 psi or less on a tire. I see some cars even say 26psi! thats crazy

and you know what I notice on cars running that low of air pressure? The tires wear down more on the inside and outer edges. depending on your wheel width and tire size, you will find an optimum pressure that wears the tire more evenly. If you have a narrower wheel with a wider tire, and run too much air pressure, you will begin to wear out the center of the tire.

its not rocket science, adjust until you find a good wear pattern and at a psi that is less or at your tires maximum inflation pressure. I really wouldn't beat yourself up at 1 psi here or there and trying to tune it that way. Unless you run some kind of time attack, or run on a road course, you are really just waisting time on something you will probably never feel a difference with when just driving down the interstate!

if your that OCD on tire pressures and adjusting them in small increments for nothing..... waiste your time on keeping them properly inflalted, instead of just setting them once, and having your tire pressures adjusted again when you get your next oil change(like the rest of the heard out there) Then you can feel good with yourself that you are doing a positive thing for your car. Also, having a really nice, accurate air gauge is worth obtaining.

 

[rszappa1]

Yea those 98 cent pen type guages are a waste of money.

 

[mechanicx]

Originally Posted By: Craig in Canada

GM almost always recommends 30 psi all the way around on their FWD cars but sometimes recommends a staggered 30 or 29 front and 26 rear. 26 psi kind of makes since for the rear on a FWD since the load is a good 12% less or more at least with the car unloaded, plus that should reduce understeer which FWDs have in abundance. Generally I think it's best to stay with the manufacture recommendation or at least within 2 psi of it.


Quote:
Lowering pressure in the rear increases the contact patch, increasing rear grip, increasing understeer/decreasing oversteer.


It's could but not necessarily does a lower pressure increase grip. It can lower grip. If I had to guess maximum grip for this application would be where they set the fronts at-30psi, but perhaps maximum for the rear is met at a lower pressure. Too high or two low from either way from optimum will decrease grip.

Quote:
I consider 26psi getting down to the territory where the tire is no longer being properly supported. I know it's on the door sticker, but I personally wouldn't run that low no matter who told me to. I'd be looking at 28-30 as my personal minimum. Starting at 26psi you've got no safety margin for sudden drops in temperature or new slow leaks in order to detect them before you're in truly critical low pressure territory.




Keep in mind we are talking about lightly loaded rear tires on a FWD and were only talking 3-4 psi lower than GM recommends for the heavily loaded front. The rear tires have a long way to go before they're under inflated at least at normal speeds. Having sad that I tend to like to run the rears the same pressure as the front.