Help with Space Saver Spare PLEASE

[The hairy angler]

:headbang: Hi all about to invest in a proper space saver spare.
After being ripped off by a Sheffield dismantler I really need to sort out the Space Saver Issue. My car runs on 225 50 x16's. diameter when new 630 mm, rolling circumference 1983mm. The recommended space saver 115 90 16 gives a diameter of 613mm and a rolling circ. of1927mm. difference on NEW tyres 17mm height, and more worryingly a rolling circ difference of 56mm. (worse case senario with new tyres)
If I change space saver to 125 90 16 the height and rolling circ. equal out on NEW tyres. Two questions....am I worrying unnecessarily, and will 125 90 16 space saver tyre fit safely on the existing wheel which has a 115 90 16 presently fitted.
Black circles had no idea and the Stealers trotted out their usual "advice"...BMW recommend that you do not change from runflats......
Thanks for any advice
John Duggan aka. The hairy angler.

 

[bcworkz]

The rolling circumference or height does not matter with fitment. Besides the inner diameter matching, the rim width needs to be within a certain range in relation to the tyre width, that's all that matters. I can't tell you if a 125 will fit or not, just to not worry about the height or circumference. It's quite common for spares to not match the height of the main tyres. Unless you have an LSD, the differential absorbs the difference easily with no harm, assuming you are not driving long distances at high speeds, in which case you would likely overheat the differential. As long as you don't drive huge distances and drive easy, there will be no issues.