I'm sure we've all heard the propaganda "every mile per hour you go over 70, you lose 1mpg in fuel economy.."

That being said, I know some engines run at a "sweet spot" where they're at their peak efficiency. I would assume most companies engineer their power units to be at peak efficiency in the rev-range that equates to traveling at the legal speed limit on motorways. In my '05 HSE, 75mph = 2,500rpm and 70mph = 2,200rpm. Today I drove from Cincinnati, Ohio to St. Louis, Missouri, a trip of about 375 miles, and averaged about 16.5mpg keeping a fairly steady 75mph clip. On my last highway journey of approximately 200 miles, I kept it around 70 and got 18.6mpg (this is US gallons obviously). Tomorrow, I have to head further west, from St. Louis to Kansas City, a trip of about 250 miles and it should be mostly flat, as today's drive was (although with a motherf*cker of a headwind!). Anyhow, was just wondering if anyone has discovered the BMW M62's sweet spot, because if 80mph means I'm running at better efficiency, I'll happily bump the clip up a notch to cover more ground on these long hauls my job forces me on.

Oh, and for posterity, here's a pic of the famous St. Louis arch. "The Gateway to the West" as it's called...

Click image to enlarge

Admin note: this post has had its images recovered from a money grabbing photo hosting site and reinstated

it is not over 75, I can say that for sure.
Drove from Colorado to Pennsylvania over Christmas break.
70 or below, I was seeing 16+ mpg (US).
Set cruise at 80 or 85, it dropped to 15 or less. Jim
2000 NAS HSE
2005 NAS HSE

Nice to see the arch again, looking down from the viewing windows made me ill, I had to sit on the floor before I could walk again.

We were told 'You should always take your girlfriend to the arch and kiss her between the legs'.

Back on track now.

On the odd occasion when I used my 4.4 petrol solo on long runs I found that 80 returned slightly better MPG than 70.

Not had a chance to find out what the 4.4 diesel does but have seen 34 mpg on the OBCD crusing at 80.

Ian

I reckon mine is about the 70 mark and it'll drop a couple of mpg when pushed upto 80.

I think what aaffects me more is the effort in driving concentration when driving at the higher speeds, I get more tired, or more to the point the general stress level sits at a slightly higher amount which makes you more tired towards the end of the drive.

I used to drive lorries which were restricted to 56mph. They were actually great to drive on long runs. It was a much more relaxed drive ... you couldn't go any faster, so you sat back and enjoyed the view.

I now find I do that more often, I've seen myself kicking back and setting the cruise to 65mph and chilling out rather than blazing along at 80+ V8 or else ...

I'd say mine is also 70mph, but I've only really taken mine on one long-ish trip so far.

I would assume that maintaining a truly constant speed is the key, though.

Therefore, cruising at 80mph in mixed motorway traffic would mean more slowing down and acceleration, whereas 70mph would be a smaller difference and therefore consume less fuel.

With empty or clear roads, maybe as others have said, there isn't that much difference at 80 if the speed can be kept completely constant?

I've got a rather nice conundrum coming up in late June. Do we take my V8 RR on holiday to Scotland (round trip somewhere around 1,500-2,000 miles or our (slightly more economical, FAR quicker and a lot more fun but slightly less cosseting) 5.4 E55? I know the E55 would get around 27mpg on the motorway, but am really curious how well my RR would do on a largely clear route. It's probably going to be a rock-paper-scissors decision. Si. <This is my name.

I eat rat poison.

A man ain't truly been insulted until he stands buck naked in front of a woman and she didn't even notice. Or care.

If it's Scotland it has to be the FF so you can enjoy the scenery of the small back roads without worrying if the E55 may hit a pothole or ground. The FF also allows you to see over the hedges and walls. 2010 3.6TDV8 Stornoway Grey/ Black interior - gone
2005 4.4V8 (M62) Bonatti Grey/ Parchment interior - gone
1998 4.6HSE Blue/ Cream interior (can't remember exact colours) - gone
1977 White - gone (should have kept)

IanV8 wrote:

Nice to see the arch again, looking down from the viewing windows made me ill, I had to sit on the floor before I could walk again.

We were told 'You should always take your girlfriend to the arch and kiss her between the legs'.

I think worse than looking out the windows was the damn ride up in that tiny capsule! I did it a few years ago when I had to come here on a business trip, and once was enough for me. Neat piece of design though. Surprised a tornado hasn't taken it out yet  Cheers,

Aaron

I did two trips in my 4.4 v8 2004 RR from glasgow to manchester. literally there and back no town driving except for 3-4 miles. averaged 20.5mpg. done this twice and both times got this mpg. this was mostly at 70-75 never over 80 at all.

i used to average 16 mpg mixed so general MPG was great and it was literally 50/50 mixed driving.

i dont think i ever came close to 20mpg even at a steady 60mph, 15 and 16mpg,18 tops if the wind is behind me.. V8 facelift Range Rover Autobiography 22" wheels.
1976 Harley Davidson Iron Head Sportster

Just as a matter of interest when talking about MPG. An Imperial gallon contains 20% more liquid than a US gallon therefore 17.5 MPG in the States would be about 20.5 in the UK.

Daveo is right, I rememebr it all so clear now. I thought the RR WOULD DO 20mpg easy on a run however to my shock topped out at exactly 17mpg on the obc and also when i done my manual brim to brim checks LOL.

I'm now in Kansas City. My drive across Mizzou today was at a (roughly) 80mph clip and lo and behold....18.9mpg  Cheers,

Aaron