Power units at high load cost less fuel than at low loads. That is shown by bsfc maps, and that's the reason why p&g works.
Lets just make the calculation from the TDI bsfc map.
We will compare steady state at 1800 rpm in 5th (~55 mph) to 1/3 pulse and 2/3 glide in neutral between 1500 and 2200 rpm in 5th for an hour.
Assumptions/approximations:
- The idling engine requires 2 hp
- Steady state at 1800 rpm requires 15 hp
- Pulsing 1/3 from 1500 rpm to 2200 rpm and gliding back 2/3 the distance back to 1500 rpm leads roughly the same speed as 1800 rpm constant.
So, steady state is easy, 15 hp (11.18549808 kW) at 250 g/kW-Hr is 2.79637452 kg of fuel used to drive an hour at 1800 rpm.
Pulsing and gliding for the idle part we have 2 hp (1.491399744 kW) for 2/3 Hr at 500 g/kW-Hr which means we'll use 0.497133248 kg of fuel for the idling parts of the P&G segment.
Now for the pulsing part between 1500 and 2200 rpm we'll average bsfc to 200 g/kW-Hr and hp to 45.
So we have 45 hp (33.55649424 kW) for 1/3 Hr at 200 g/kW-Hr which gives 2.23709961 kg of fuel
So in the end:
2.79637452 kg of fuel used for an hour cruising at 1800 rpm (~55 mph). That makes roughly 64.34 mpg
2.734232858 kg of fuel used for an hour pulsing and gliding at a speed average roughly the same, but probably just a tad higher than steady state 1800 rpm. That makes 65.81 mpg.
There are a lot of approximated assumptions which could have a relatively important impact on the results in this calculation, but at least you get the idea.
Personally I've been able to increase mileage by ~10% while P&G at highway speeds.