Originally Posted By: Craig in Canada
OK, I'll bite
The vehicle is a 1998 BMW E39 with M Sport I suspension package. Although there are many wheel packages considered "original", mine came from the factory with 235/45/17 94Y. There are two sets of pressures on the placard:
"normal" load up to 4 persons: 29 front 33 rear
max weight rating: 33 front 41 rear
..........
Whenever I do this sort of thing, I always check to see if the information supplied makes sense. Tire Guides is a publication that summarizes the vehicle tire placards for all vehicles sold in the US. There are 4 limitations to the book:
1) If the vehicle is not sold in the US, it will not be in the book.
2) If the vehicle is sold in the US under a different name, it will be listed as what it is sold as in the US. From a personal perspective, I am pretty good at figuring these things out, but not quite perfect.
3) If the vehicle has a different tire size when it is sold in the US, that is the size that will be listed.
4) The book only lists ONE pressure for each axle. It is not unusual for German cars to have 2 - normal and high speed/heavy load - so this presents a problem.
But the first problem is the vehicle identifcation. A 1998 BMW E39 is not listed in the book. I know from past experience that the "E39" refers to a particular model - in the same way a Mark 1 pre-facelift Capri refers to the models produced in the 1970 to 1972 range.
In this case E39 = 5 series. And the book lists 1998 to 2000 BMW 528i with 17" tires as 235/45R17 93W and the inflation pressure is listed as 29 / 33. That's a bit different that what was stated, (the load index and the speed rating) but it is not enough different to matter.
But, OK, that means the load carrying capacity of the tires is 1272# / 1380# / 1433#.
I find it interesting that a pressure of 41 psi is used, as the load carrying capacity quits increasing at 35 psi. I suspect this pressure is specified to maintain the front to rear balance.
Originally Posted By: Craig in Canada
.......A common non sport package size for this chassis is 225/55/16 which is what I'm running in the winter on a set of factory take-offs...........
In order to get the same load carrying capacities as a 235/45R17 gets at 29/33/41(35) psi, a 225/55R16 has to use 26/31/(33) psi. I put some of the pressures in paratheses to show that there is an issue that needs to be dealt with.
It is interesting that the 16" tire has a bit more rated load carrying capacity - and the result is a slight decrease in inflation pressure. Some folks think that maintaining a tire's spring rate is more important that matching the load carrying capacity, and they would opt (in this case) to using the original inflation pressures - and I'll go there! - and that makes the parathesis issue moot.
Originally Posted By: Craig in Canada
...... I've had a number of different snow tires of that size with load/speed ratings from ??Q to 95H. I presently have Hakkapeliitta RSis which are 225/55/16 99XL.
What's my placard conversion? Others may find the illustration informative and it would demonstrate the correlation of design pressures as size varies.
My dad's E39 came with 225/55/16 but he has a non-sport package and a different model which weighs more so I can't assume his placard pressures would necessarily apply to me. It will be interesting to see how the calculated results compare to his placard though.
So there you go. I think you'll find there is a difference between your Dad's car and what this calculation results in, but rather than discuss it now, I'll wait for you to post the info.
