Air pressure in upsized wheel and tire?

Question

A while back I changed the wheels and tires on my car from 205-60r15 to 205-40r17, a smaller than recommended size to replace the originals. After installing the tires I noticed the recommended tire pressure of 32 lbs looked fairly low, a few thousand miles later the outside of my tread confirmed it. So for the remaining life of the tires I inflated to 45 lbs. Now I have new tires the correct size replacement for the originals, 215-40r17 would I go back to 32 lbs?

Answer 1

The recommended tire inflation pressure is listed on the tire sidewall. If you change tire sizes, it is quite likely ( as you have discovered) that the door sticker tire pressures are off. However, inflation to 45 psi is dangerous. Generally speaking pressures should range in the 30-35 psi range, when checked COLD. Looks are deceiving, and sidewall design can make a tire appear under-inflated.

Answer 2

Strictly following RMA guidelines when plus sizing the two most critical factors are to find a tire that matches as closely as possible the overall diameter of the OEM tire size and also its load carrying capacity.

So far you are not doing well at either but I can help.  Are you ready for a crash course in tire sizing?  

When shopping for plus size tires you absolutely have to look at the complete tire size, not just the numbers that represent the physical dimensions.  For example, your OE tire size is not "205-60r15".  It is probably "P205/60R15 90H" or something similar.  The "P" tells us the tire conforms to "Passenger Metric" load and inflation tables and the "90" tells us how much weight the tire can carry at specific inflation pressures. There's a chart for this and if your really keen on knowing I can provide a link, but if you read on I'll tell you everything you need to know.  The physical dimensions, the "205/60R15" part of course tell us the overall diameter of the tire - about 24.7 inches - so all of this information is crucial to making a good tire choice.

205/40R17 was really a bad choice, especially if the tire was a Standard Load and not an Extra Load size.  In a Standard Load the tire size might read something like:

205/40R17 80H

In an Extra Load it would read more like:

205/40R17 XL 84H

Dimensionally these tires are identical, they differ in how much weight they can support and how much inflation pressure they can take.  Notice there is no "P" in front, that means the tire uses European Metric load and inflation tables which are different than P-Metric tires of the same dimensions. Euro-metric sized tires are more common in these plus sizings. The Extra Load tire carries a greater maximum load at a higher inflation pressure (41 vs. 36 psi) than the Standard Load tire and will have a higher maximum inflation pressure (usually 51 psi vs. 44 psi).

At 32 psi your stock P205/60R15 90 tires can carry a load of 1,246 lbs.  When upgrading to a different tire size this is our target number.  Notice also your OEM tires have a load index of 90 - much higher than the 205/40R17's.  

At 32 psi a Stanard Load 205/40R17 80 can only carry a load of 893 lbs. and the Extra Load 205/40R17 XL 84 is actually slightly worse at just 882lbs.!  This explains why you were getting so much edgwear.  Quite litterally this is about the same as running your stock 15-inch tires at about 15 psi!!!!!!  Edgewear is the least of your problems.  You are lucky they didn't overheat, fail and blow out on you.

Could these sizes have worked with more air?  Nope.  The Standard load tire has a maximum load capacity of only 992lbs. at 36 psi - clearly not enough.  The Extra Loads even at their peak 41 psi only carry a load of 1,102lbs That just will not cut it.  That would be the same as running your original tires underinflated by 3 psi.  The overall diameter of that size is also only only about 23.5 inches when what you want to reach is 24.7 inches.  This will result in huge speedometer and odometer reading errors and a big change in final drive gearing.  

The next size you chose is an improvement but still isn't right.  Again this size is available in Standard or Extra Load varieties, 215/40R17 83 or 215/40R17 XL 87 and we don't know which you have.  Look at those Load Index numbers: 83 or 87 vs. 90 on your stock tires.  At 32 psi the Standard Load tires lload carrying capacity is just 970 lbs. and peaks at only 1,074 at 36 psi.  That is just not enough.  

If you got lucky and managed to get an Extra Load version of that size your in better shape but only if you pump them up to 41 psi where you will get a load capacity of 1,201lbs.  This is close - it probably won't kill you - but still shy of what we would like to see.  The other problem is, this tire size is still too small in diameter - about 23.8 inches - so for those reasons is still not the right choice.

But I know what is.

You'll do better with a 215/45R17 which like the other sizes comes in either Standard Load (215/45R17 87W) or Extra Load (215/45R17 XL 91W).  Dimensionally this tire will nominally come in at 24.6-24.7 inches, or in other words nearly identical to your OEM tire size so the speedometer and odometer will be accurate and the gearing the same.  With the Standard Load tire we still come up slightly shy on the load carrying though which is still 1,201 lbs at 36 psi.  Better would be an Extra Load size "215/45R17 XL 91W" which is a bit more rare but can be found.  At 38 psi this tire size has a load capacity of... wait for it... 1,246 lbs.!!!!

Thank you Jesus we have a winner!  

215/45R17 91W has the exact diameter we are looking for and when inflated to 38 psi exactly matches the load carrying ability of the OEM tires.  This is what you want.