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I recommend you head to tirerack.com and read up on tire tech. Ot give you a little of the basics I'll use the tire size you have provided.
The 205 in the 205/40R17 refers to the width of the tread face in millimetres. The 40 refers to the height or profile of the side wall as a percentage of the width of the tread face. So a 205/40 tire has a tread face width of 205mm, and a side wall height of 205mm x 40% = 82mm. And the last number is the rim diameter the tire is to be mounted on.
The (most common) original equipment sizes for the 850 are 195/60/15, 205/50/16, and 205/45/17. If you work out the diameters of the wheels 61.5cm, 61.14cm and 61.63cm respectively. As you can see, there is little variation. However, a 205/40/17 tire probably further off than you would probably like. The outer diameter of the 205/40/17 tire is 59.58cm. And when you factor in the difference in circumfrence or rotations per mile, this difference becomes significant. Basically, it will change the gearing of your transmission to a certain extent. You will basically have to run a higher rpm at any speed. This could improve your acceleration, but in my opinion, this tire size is not appropriate for your car.
This brings us to the reason why the 215/40/17 is offered as a suitable replacement (at the far end of the spectrum as I wouldn't even run this size). A 215/40/17 tire has an outer diameter of 60.38cm. This isn't as far off as 205/40/17, but I'd prefer to sick with OE sizes, but that is just my opinion.
When someone goes to far off from original specifications in terms of wheel diameter bad things can happen. Beyond changing the gearing ratio of the tranny to ranges that may not be desired, you will also change the suspension geometry of your car. And going too far often leads to problems that will adversely affect handling unless appropriate changes to the suspension are made.
BTW, the Konig Tantrum does come with the appropriate offset for your car, but be careful. I know many people that see a nice set of rims, buy them, and then realise the offset is totally off, which leads to rubbing, and wheels that stick out awkwardly.
If you like these rims, see if the distributor will put 205/45/17 or 215/40/17 tires on it.
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