Don’t be so sure, older petrols still way better than diesel. MotherT has now flogged her 2014 EClass, with a 45mpg 2.1 diesel, that was liable for the ULEZ. (Not reason for selling)
My 2002 3.2 V6, doing 24mpg, still lists as no charge. 
Its tricky, because it seems every city is making up its own rules as to what is and is not chargeable. 
Birmingham, everything that isn't Euro 6 (diesel) or Euro 4 (petrol) is chargeable apparently. As always though, I'll bet they don't apply it to all the sh!t chucking diesel busses they clog the city centre up with. 
Don't get me started, they announced it, fixed a date, and then decided lets have a consultation with Joe public, and carried on regardless.
Before I retired, I used to travel into the centre, lovely duel carriageway, cut outs for each bus stop, traffic used to flow.
Then they filled in the cut outs, put down a bus lane, fined everyone that used it, traffic jams all the way to the centre.
Added nearly 3/4 of an hour to my journey!
Several big car dealerships in the 'zone', I notice the Audi one has put up a 'For Let' sign.
I feel sorry for the people who live there and are trapped, most from what I've seen from driving through, drive pre Euro 6 diesel/Euro 4 petrol cars.
I live north side of Birmingham, a lot of my relatives live South side, will mean a 20 mile plus detour to visit them.