OK, take one example of standard MB fitments from the factory.
W209 CLK with standard 16" avantgarde wheels -
225/50/16 on the front - diameter 631.4mm
Upgrade to the 18" AMG wheels from the factory -
225/40/18 on the front - diameter 637.2mm
That's a 0.92% change and that's just the first example I remember from my 209 research.
If it was so critical then MB would never deviate from the one wheel size that they determine to be ideal.
If you're on the track and using your car to 100% of it's potential then I can see the necessity to have the gearing absolutely spot on.
In the real world, on real roads, the effect is little to nothing.
Engine size is not irrelevant, a big turbo diesel engine has lots of torque and if it has to pull along at 20rpm lower to maintain the same speed, it's likely to have little problem.
As you say, a spectrum.
Ideally MB would set up our gearing for the UK market, but we know that just isn't going to happen. Most of our cars don't perform optimally at 70mph.
Add to that the fact that MB recycle drivetrains across a large range of vehicles, that includes engines, gearboxes and differentials.
Basically, what i'm pointing to, is the fact that the top gear will likely already be compromised in some way and if the OP wants to save himself a few pennies by fitting some tyres that are 1% larger and are the same size as his front tyres anyway...it's no big deal.
