I have an R172 too - fabulous thing. P30 package is important in my view for added body control and traction with the slippy diff, but as an all-seasons characterful motor car it fits the bill well. A DMS (Southampton) ECU/TCU dyno setup and 'optimisation' adds the icing to the cake and on a long run it's easy to get 35+mpg which is not to be stiffed at. Valved exhaust is clever in that it combines civility at normal low speed stuff and then sounds much more hardcore at WOT. Grab a Chrometec remote valve-flapper-opener box (technical term) and you can decide whether you want the valves Open/Closed/Stock as you wish. Roof down, valves open and gentle throttle is great for warm summer drives. Roof down, valves open and full beans is epic and don't believe anyone that says the 7G-tronic in this car is slow witted and dull - that TCU tweak in Sport/Manual modes is sharp as. Runs on 4-cylinders when it needs to (when you want the Stock valve control, as it sounds plain weird with the open pipes on half an engine) and that's how it makes the MPG numbers. Exhaust sound is a more complex mix than the M156 engines and I like the more 'sophisticated' sound if I'm honest. R172's run a totally different engine with a 12.6:1 CR and it's a really good power-unit.
I may be biased, but they're good cars and the very last of the N/A production AMG's ever built and more likely to hold their own in terms of future value and residuals for obvious reasons. I was tempted to chop both this and my Morgan 3-Wheeler in for a Series-5 Morgan Aero 8 (4.8l BMW V8, manual box, 367bhp/1100kg), but realised that I'd miss the magic mix of practicality, reliability and performance of the 55 and the madness of the M3W, so ditched that idea and will hang on to my now 'keepers' I think.
