Spa trip 41 mpg! :-)
 

Only Leamington Spa! Drove down on Friday for a overnight stay at my dads. it's the first time I've done motorway journey and remembered to do proper Economy check 272 miles £39.70 of lpg at 63.9p.
most of the time I tried between 70-80 on the speedo (apart from the 50 road works). And a bit of town driving I think 50 mpg equivalent may be possible with all motorway and not using cruse control.
I've also been trying to find out if the car has a sweet spot where it's most efficient, by setting the cruse at different speed for 10min and reset the mpg average in the DIS here are my results
60mph average 29.6 mpg
70mph average 27.3 mpg
80mph. average. 26.5 mpg
I've only tried it once, but not on the same route! so I need to try it a lot more.
Anybody else tried this?
Cheers dave

Less speed=less air resistance and less friction=better fuel economy. Simples

Air resistance is insignificant at speeds below 20mph and you would want to keep car in as high as possible gear for better economy. You need about 40 mph to drive in fifth on D2 so you would most likely get best economy somewhere in 20-40mph range.

But is it simple?
Has a D2 got a speed it's happy cruse at in sensible speed on the motorway between 50-90 where it doesn't cost the earth.
Fuel economy at steady speeds with selected vehicles was studied in 2010. The most recent study indicates greater fuel efficiency at higher speeds than earlier studies; for example, some vehicles achieve better mileage at 65 mph (105 km/h) rather than at 45 mph (72 km/h), although not their best economy, such as the 1994 Oldsmobile Cutlass, which has its best economy at 55 miles per hour (89 km/h) (29.1 mpg-US (8.08 L/100 km)), and gets 2 mpg better economy at 65 mph (105 km/h) than at 45 mph (72 km/h) (25 mpg-US (9.4 L/100 km) vs 23 mpg-US (10 L/100 km)). The proportion of driving on high speed roadways varies from 4% in Ireland to 41% in Netherlands.
When the National Maximum Speed Law's 55 mph (89 km/h) speed limit was mandated, there were complaints that fuel economy could decrease instead of increase. The 1997 Toyota Celica got 1 mpg better fuel-efficiency at 65 mph (105 km/h) than it did at 45 mph (72 km/h) (43.5 mpg-US (5.41 L/100 km) vs 42.5 mpg-US (5.53 L/100 km)), although almost 5 mpg better at 60 mph (97 km/h) than at 65 mph (105 km/h) (48.4 mpg-US (4.86 L/100 km) vs 43.5 mpg-US (5.41 L/100 km)), and its best economy (52.6 mpg-US (4.47 L/100 km)) at only 25 mph (40 km/h). Other vehicles tested had from 1.4 to 20.2% better fuel-efficiency at 55 mph (89 km/h) vs. 65 mph (105 km/h). Their best economy was reached at speeds of 25 to 55 mph (40 to 89 km/h)
Officials hoped that the 55 mph limit, This was partly based on a belief that cars achieve maximum efficiency between 40 and 50 mph (64 and 80 km/h) and that trucks and buses were most efficient at 55 mph (89 km/h).

I don't know where you got those figures from.

Maybe we should ask Mythbusters to test it.

It's basic physics - higher speed has higher losses in friction and air and rolling resistance and they increase exponentially with speed. If you drive in top gear the slower you drive the better efficiency is down to the lowest rpm that engine is happy to with which would be around 1200rpm on A8 and 1500rpm on S8. The question is can you get better economy if you drive in lower gear as losses to drive engine and ancillaries remain the same even if air and rolling resistance drops. Please try and report back, I am sure it will be below 50mph and most likely 20-40mph range.

In my PF S8 it seemed to make little difference at speeds from about 45 to 80mph.
My FL changes into 5th at lower speeds, but given the different engine I don't know what rev range is the most efficient. Haven't driven it far enough to get a good feel on economy at different speeds but overall it's about 10% better. That may be due to a fair bit of 40mph driving where the PF would usually insist on 4th (even after I recoded the box) but FL is happy in 5th.

Wikipedia!
No need for mythbusters you have a 8 outside you can test it yourself! You might even save some money!
Mine doesn't get into top gear until 40mph,so I'd expect it so be above 40mph but I'm not driving at such low speeds on the motorway 60 is low enough!
but my engine is happy to pull from about 1100rpm In fourth gear but won't go below 1400 in fifth as it changes into fourth.
Cheers Dave

Ok so please go carefully on me because i'm only a counrty bumbkin and i are i little bit fik but my reckoning is,

29.6mpg is 6.5 miles per liter, 26.5mpg is 5.8 miles per liter over a 90 liter tank thats a wopping 63 miles, so at 29.6 just over 2 liters a massive £3 in your pocket.

It's just not worth worrying about, a 4.2 V8 has never been know for it's economy.

I've owned my 8 for a few years now and thrown a little over 80k miles on it and i've worked out that you can drive it pretty much however you like on UK roads and it really makes not difference much at all, when the tank is empty you have to fill it.

'Only' Leamington Spa, cheeky. I was born, raised and have lived here every year of my life. Hope you had a good visit. +++

Yes it was really good thanks its a really nice place I would recommended it to anyone. I had a good meal at the star and garter and the moorings. And a good walk round :-)

Both regular hangouts of mine, there are loads of others, too.