Tire pressure - how much do we really need?

[northernlights]

Historically, my first instinct has been to inflate the tires to whatever maximum the sidewalls indicate! For a number of reasons, I now do not believe this is a good idea.

Early in April, I installed a set of four 205/55R16 Continental DW tires on my 1994 850 Turbo, and promptly inflated them to an ungodly pressure.

(Disclaimer #1 - the sidewall indicated 51 psi max, but I couldn't bring myself to go higher than 3 bar out of the gate!)

While the handling was quite good, the car did ride a bit....rough. Not quite 1980's WS6 suspended F-car rough, but close. And with all of that pressure in the back, I noticed very quickly that the new rear tires were barely scuffing the outside edges. So, over the past three months I researched the topic, started to experiment a bit with tire pressures, and thought I would share what I've found.

(Disclaimer #2 - I think JRL figured this out a while ago!)

Data point #1 - Volvo USA



Here in the USA, using the pre-recall 205/50ZR16 tire, Volvo recommended Front 36 psi Rear 36 psi for optimal fuel economy and/or a full load. I think. Maybe both.

Optionally inflation pressures of Front 34 psi Rear 31 psi are listed when you are only driving a pair of passengers. As I drive alone most of the time, maybe F36/R36 is overkill?

Data Point #2 - Volvo UK



Hmm. For some reason if I were in the UK, I could run F32/R29 in a non-Turbo without real risk of blowouts - at 99 mph! F36/R36 is now good for a full load in a Turbo at the same extra-legal speed, or top speed in a lightly loaded non-Turbo! For that matter F42/R42 is enough for top speed with a full load - about 155 mph is what I remember for the T-5R's. And fuel economy is not listed anywhere, because it's not like they were paying $5 per Imperial gallon. In 1994.

So unless you are bringing all of your friends with you to Bonneville, 42 psi is certainly too much for the USA. Maybe F33/R30 is actually a reasonable number for those of us with few passengers, 205/50R16 tires and a predisposition to keep the speedometer under three digits? Umm..is anyone still using 205/50R16 tires?

Data Point #3 - I don't use stock tires because Volvo told me not too!



Here's where it gets even more interesting. Volvo's recomendations appear to be based on 205/50R16 tires, not the 205/55R16's installed after the great Wheel Bending Tire Recall Fiasco (my emphasis). The old 205/50R16's are usually a load rating of 87. The new 205/55R16's are usually a load rating of 91.

Using the above load table (borrowed from Toyo) shows that a 31 psi load rating 91 tire can carry almost the same load as a 36 psi load rating 87 tire - 1197 lb vs 1202 lb. Therefore, even using the seemingly low USA F34/R31 pressures with load rating 91 tires allows a greater overall load than the stockers at F36/R36!

I have experimented as low as F31/R28 cold with my 205/55R16's, with no apparent loss of handling, and a much, much more pleasant ride. Keep in mind that with only me in the car, the rear load is nowhere near the 28 psi/1091 lb rating on these tires as the weight distribution is typical FWD poor. The sedans were around 60/40 F/R split, so each rear tire might have 700 lb on it with only a driver.

I think we get fooled when the rear tires are overinflated because the oversteer makes the car feel like it handles better, i.e. it works against the bad understeer. Unfortunately, overinflating the rear tires just unsticks the rear end easier, which doesn't actually make the car handle better, just slide more uniformly!

Back to my original question - how much pressure do we really need? I'm not sure, but I think the UK F33/R30 is plenty high, F32/R29 is actually recommended for non-turbos, and as I mentioned, F31/R28 is pretty nice.

Appendix I - Your vehicle will probably not explode when using less than 30 psi in the tires.

This is on my 1992 240. Front 26 psi, Rear 28 psi makes the grade, unless you need fuel economy. In the USA. Maybe.

[matthew1]

Good post, NL. I don't have any answers, and worse than that, I have an additional variable to add: insurance claim requirements. I've read that a claim can be rejected if tire pressure is out of spec. Which spec., tire brand's or car brand's, I don't know.

I really doubt claims rejection due to out-of-spec. tire pressure is common, FWIW.

I'm running 35/35.

[jblackburn]

38/36 worked beautifully for the tires I used on my S70. That dropped to 36/34 in the winter because the rear liked to chase the front in snow

[jimmy57]

You are absolutely right to not inflate tires to max pressure on sidewall. The times that is doen is when the weight of vehicle and its load approaches the max weight tire is rated for. Over inflation wears tread center and makes car ride quality worse.
The label from manufacturer is the start point. If you have a newer vehicle with tire press monitoring then that label is THE pressure with little wiggle room. TPMS pressure is at the high end because for them to not have system that senses vehicle loading and varying TPMS warning accordingly leaves them to use the max weight pressure as the warning system target pressure.
You can usually buy several different classes of tires in a size with different weight ratings. The door placard will be for the size and type fitted to car when built. The 33/30 is likely a good for two passengers. The car is forward weight biased so the rears have less weight to carry. You may find 35 front gives a bit more steering response and doesn't cause center tread wear.
If you drive 80 mph a lot on interstate you may find you need a couple less psi. The centrifugal force of speed will push center of tire tread out. If there is not enough weight to load the tire then this will cause more center wear.