bigrob

Boy I am sure making good use of this forum. I belong to one for my C5 Vett & it rocks as well. Anyway, can someone tell me the correct tire pressure? I have the 225/50/16 tires on the flat wheels. Inside my fuel door it says 43 rear & 36 front. Inside the door jam it says 36 rear & 32 front! The 43 sounds too high. Right now I am doing the latter, but I want to know which is correct!

Once again Thanks!!

Rob

RicerSchnitzzle

I run 36 up front and 38-40 in the rear.

Owners manual recommended the second settings you mention for use where speed is limited to 55mph (USA?) Higher speed of the autobwaaahn require the higher pressures, Of course you will really feel american potholes at higher pressures.

heinrich

32

anonymousagain

It's a Tire specific measurement:

1) max load / max psi = load per psi

2) total vehicle weight (with you and typical cargo) / 4 (fortunately 928s are ~50/50 bias) = load per tire

3) load per tire / load per psi = tire pressure

4) fine tune - make a fat chalk line across the tread (kids sidewalk chalk works great) and drive for a 100yds or so on flat ground. "Read" if there is LESS chalk wear; outside/outside edges = over inflated; center only = under inflated; only outside or only inside = alignment

Good luck !!

bigrob

If weight distribution is basically 50/50 or 49/51, why so much more in the rear? I had all 4 at 36 & boy the road comes alive with imperfections! What do these cars weigh? My manual is in german & I am too lazy to convert the metric weight.

Rob Edwards

The weight depends on what year your car is, auto vs 5-speed, etc.

But you could take the stated weight in kilos and multiply by 2.205. Yell if you need help with that.

heinrich

Because Porsches have the engine in the rear, all weight is there, thus rear tyres need insane pressure. Something the 928 folks missed, so they slapped a 911 specification on there.

danglerb

32 is easy to remember, same as my waist size.

bigrob

I have an auto! I think I may go with 36 rear & 35 front & see. The 32 front feels a little sloppy in the front!

I like the Ferris quote! " It still does'nt change the fact that I don't have a car!"

dr bob

I've played with rear pressures a bit, mostly with an eye toward keeping the wear somewhat even across the whole tread width. With the original 16" 245/45 rears, the recommended 44 PSI caused slightly more wear in the center of the tire. I had previously noted this on other 928's and wanted to avoid that problem, so I reduced to 40 for a while and it seemed to help. Tires were replaced as part of a wheel upgrade with about 30k on them, abd they were just getting close to the worn-out bars. The outside ribs were worn a little extra from hard cornering but otherwise they were fine. I'd try 40 in the rears as a starting point.

Changed to 17" wheels (255/40) and tried 38 PSI for a bit before going up to 40 again as I watched the wear pattern. These were Pilot MXX3's. 28k Later they were worn to the wear bars pretty evenly. New Pilot PS-2's went on a couple weeks ago, with 44 for a few weeks of break-in before going back to 40. The PS2's are UTQG 220 vs the 140 on the MXX3's, so I should get 40k+ if the factories are telling the truth on the ratings.

The fronts are a different story. At 38, the 225/50 PS2 tire rolls onto the outside and chatters during slow cornering. At driving speeds, they do best at 36 or so. I may add a little more camber and a little less caster to see if I can get the footprint squared up a little better when cornering. It's a delicate balance there with the camber, and depends on how you drive the car most. I have a very long curvy driveway, so I can take a little more camber I think.

Of course, your results will be different at different temps, with different brands and under different driving conditions.

IcemanG17

Spec is 36 front 44 rear.......front is fine for most drivers....44 in the rear is quite high.....you'll get better traction and a softer ride with less pressure......I ran 35-6 all around on my 16"s

BrianG

Interesting theories. Never heard it laid out quite like this but it sounds reasonable.

Certainly, tire presure is tire and load specific. An excellent way to determine if the current tire pressure is correct for the current situation is to get a temperature reading across the tire tread, once the tire is up to operating temperature. One of those hand held iR thermoscan units is excellent for this. Ideally the temp will be uniform across the tire. Higher center temp = too high tire pressure. Lower center temp= too low tire pressure.

heinrich

Is there anyone who can tell me why you think rears need more pressure than fronts? Because I disagree.

anonymousagain

H - manufacturers assume a high likelyhood of addt'l passenger(s) weight, plus some amount of "stuff" to be carried in the cargo area/trunk. Easy to accumulate ~50 or so pounds in the back, to mandate an increase of ~2-4psi in the rears. Conservative literature and probable untimely customer tire pressure checks, requres lawyer speak to direct a higher rear pressure recommendation in the manual --> CYA guidelines.

Since each vehicle/tire/useage/cargo combination varies by owner, best to calculate pressure, read the chalk line wear, or <good detail> follow the IR tempurature indications.

Funny too, because same size has nothing to do with load rating or tire compound, so mfg recommendations can only be a loose "ballpark" value...or close for exact OEM (not OEM options).

Namvet

After a bunch or tests and hard runs...., I find 36F and 32 rear pressure to work the best..So Far...LOL

TOYO Praxes4 tires..1984S