How Fast.

[Ralf85]

Because any speedo is always set to read a good 5% fast. Bigger tyres mean more distance covered per revolution, so the speedo then reads proportionately lower i.e. closer to true road speed.

So why are speedo's set to be incorrect?

[silverbullet]

Fair point. Heavy van uses more fuel to accelerate.
[pedant v2.0]Tintops have an abrupt front screen frame/roof intersection, which makes for turbulence amd so more drag. A Dehler is the most aerodynamically efficient shape for a T3 generally available[/pedant]

[silverbullet]

Speedo always reads low because it allows for manufacturing innacuracies. No manufacturer wants to be sued for a duff speedo that results in excessive speed and the legal fallout that results

[CJH]

[pedant v2.0]Tintops have an abrupt front screen frame/roof intersection, which makes for turbulence amd so more drag. A Dehler is the most aerodynamically efficient shape for a T3 generally available[/pedant]

You could well be right. What about the low pressure area behind the van - is that any different with a Dehler roof?

Fair point. Heavy van uses more fuel to get to the same speed.

[pendant mode again] Fixed that for you
If a light van and a heavy van both accelerate with foot to the floor, then the heavy van won't accelerate as fast, but neither will it use more fuel at that moment - it'll use the same fuel as the lighter van to generate the same maximum force. It's the secondary effect, whereby it has to provide that force for longer to get to the same speed, that increases fuel consumption.[/pedant]

[silverbullet]

Pedantry is such fun!

The Dehler top gets the side profile a good bit closer to optimal (all things considered), the more gradual transition from screen angle to horizontal and then a slight reversal in gradient is more like that seen on current artic' trailers; the Dehler does have a step down at the back for the luggage rack but you get my point.
The Jaeger mid-top looks like quite a good shape but lacks the reverse taper as it tails off.
My Dehler/rooftent with its tonneau cover doesnt flap at any speed afaik which suggests good flow and minimal turbulence over the top.
What the wake must be like is anyone's guess