I will be changing my belts soon and that pelican parts and autohauzaz only sell 10mmx900mm replacement belts.
I currently have a 10mm x 925mm belt. Searching the forum yielded this post which also suggests that 10x925 is correct for an early 68/69 280sl.
https://www.sl113.org/forums/index.php?topic=3713.0Will a 10mm x 900mm work? What should be the correct size?
