My personal experience is that the newer versions of buses are being strangled by the increasingly strict emissions regs.
Our best single decks are the 07 and 57 plate B7RLEs, which are properly quick. The newer ones are noticably slower, which is a total pain when you need to be able to pull away quickly, particularly at certain roundabouts. The ones I drove on the 49, there's two distinct flat spots in the performance - one right at the bottom of the rev range, as you're pulling away, then another further up. There's a bit of decent acceleration in between, but it's still nothing on the older buses.
I've only driven the deckers a couple of times, but the flat spots are very noticable on them, too.
at Western Greyhound there were two different types of Trident, 4 fitted with a ZF 'box, 2 with a Voith 'box. The 5-speed ZF meant a top speed of between 50-60mph and reasonably quick progress, the 3-speed Voith meant a top speed of 48mph and an eternity to get there due to the wide spacing of the gear ratios.
The majority of Lothian's fleet is Voith boxes, but there are two B7RLEs with ZF boxes, and the new B5Ls have Volvo's I-Shift box. (No idea what the Solos have).
The Voith 5 speed boxes are great, the ratios are long enough to get a decent speed out of them, but at the same time they're quite quick.
The I-Shift ones are also pretty good, as they're 12 speed but only go through the gears you need to - they're not the smoothest in the world being an automated manual rather than a proper automatic, but my only real criticism is that they very occasionally change up just when you don't want them to. It'll be interesting to see what they're like a few years down the line.
The ZF 6 speed boxes, on the other hand, are rubbish. I think they must be designed for small HGVs, as first and second are incredibly short - by the time you're at 10mph, you're already in third. The changes are also quite jerky, and mean the two buses are impossible to drive smoothly. They also seem to be quite dim, changing up and down at set points, regardless of how you're driving. What bugs me most about them is changing from fourth upwards - they accelerate alright, because they have good engines, but as soon as you're halfway through the green band on the rev counter, suddenly you change up, and there's a noticable drop in acceleration as it pulls you down out of the power band. It's incredibly frustrating heading out towards Ratho on the 12, where you get a good run along the A8.
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