What tire pressure after market rims?

Question

I have an Audi TT 225hp that came with 225/45/17 and recommended 33psi in the rear and 30psi in the front. I now have 235/40/18. Should I keep the recommended tire pressure like the place that put them on said or should the pressure be higher as I was just told. Like 40 psi all around.

Answer 1

Ready for a crash-course in the finer points of plus-sizing?  Good, lets start with the fundamentals.

The correct way to express your Audi's OEM tire size is depending on the year either "225/45R17 91Y" or "225/45R17 90Y", not "225/45/17".  ALL of the information in the tire size is important, not just the physical dimensions, especially when you are talking about plus sizing.  You'll see why in a moment.

When plus sizing there are two critical criteria you want to meet.

1. The replacement tires should be as close as possible in overall diameter to the OEM tire size.

2. The replacement tires need to be able to meet or exceed the load carrying capacity of the OEM tires at their recommended inflation pressures.  The amount of air the tire can hold determines how much load it can support. To a point the higher the pressure the greater the load. Tires of different sizes have different internal air volumes and sometimes when switching sizes one has to increase air pressure to compensate.  There are tables specifically formulated to allow you to do that.  Lucky for you I keep one at my computer.  This is where knowing the complete size and not just the physical dimensions is critical.

The original 17-inch tires on your Audi have an approximate overall diameter of 25.0 inches and will rotate 833 times per mile when new.  The Load Index is either 90 or 91 depending on the year which tells us that at 33 psi the tire can support a load of either 1,235 lbs (90) or 1,268 lbs (91) respectively or at 30 psi 1,146 or 1,179 lbs respectively.  

Those are your targets.

A tire sized 235/40R18 91Y or 91W will have an approximate overall diameter of 25.4 inches and rotate 820 times per mile.  This is slightly larger than the OEM tires but acceptable.  A better choice would have been the size Audi went with as an option on these cars: 225/40R18 XL 92Y.  This size has a diameter of 25.1 inches and rotates 830 times per mile - nearly perfect!  Anyway, since you have the 235's already we'll run with that.  

In a standard loading (235/40R18 91W or 235/40R18 91Y) they have the same Load Index number as the OEM tire = 91.  This means at 33 psi this tire carries the identical load of 1,268 lbs and at 30 psi it is 1,179 lbs.  Thus, no inflation pressure changes are necessary.  You can still run 33/30 with no worries at all.  The same holds true even if you have Extra Load (often referred to as "Reinforced") 235/40R18 XL 95W sized tires.  At 33/30 psi the load capacity is the same.

Simple, right?

PS

Stangman is regurgitating a widely held but totally baseless myth.  Plus sizing and proper inflation pressure is all very scientific and laid out in detail by the Rubber Manufacturers Association (RMA)

Answer 2

Proper tire pressure is based upon the tires first and foremost.  Then you should take into consideration what the vehicle is being used for, if it just a daily driver, I would keep the pressure at the level the tire dealer recommended.

Answer 3

you need to check the sidewall of the new tires for their max PSI.  when you change tire and wheel size, the manufacturer's recommended pressures go out the window.  if your new tires say 50psi max, then you should run them around 40psi cold for a mix of comfort and fuel economy

Answer 4

I would stay will the Audi recommendation and if you do add additional air, no higher then 2 pounds more.