To be precise, the 229.5 specs requires at least 1.8% fuel saving.
Since we're being precise, I have no idea where this 1.8% number came from? The MB229.5 spec requires testing in the M111FE, with different minimum limits for xW-30 and xW-40. Neither limit is 1.8%. Furthermore there is also an M111FE limit for MB229.3, but it is lower than for MB229.5. MB229.5 also requires testing in a chassis dyno FE test, whereas MB229.3 doesn't.
Source? Well, MB don't publish their specs publicly, but you can get an idea here
. It's a bit old and the MB spec have been updated since then, but they are similar to this.