Message from Dervman

"Maybe I was exaggerating *slightly*, but they are a hideously economical engine, and the Ibiza weighs f*ck all. Averaging 90-95mph it'd be averaging a good 50mpg easily."

How are you calculating this "average"?

Still can't see it averaging 50mpg easily at 90-95mph. Light it may be, but aerodynamic it is not.

50mpg @ *average* of 80mph is do-able, 80mpg @ *average* of 50mph is do-able.

50mpg at *average* of 90-95mph sounds like your're taking the instantaneous speedo reading as your average, and are basing the distance covered on the odometer...

"Maybe I was exaggerating *slightly*, but they are a hideously economical engine, and the Ibiza weighs f*ck all. Averaging 90-95mph it'd be averaging a good 50mpg easily."

How are you calculating this "average"?

Still can't see it averaging 50mpg easily at 90-95mph. Light it may be, but aerodynamic it is not.

50mpg @ *average* of 80mph is do-able, 80mpg @ *average* of 50mph is do-able.

50mpg at *average* of 90-95mph sounds like your're taking the instantaneous speedo reading as your average, and are basing the distance covered on the odometer...

"I hack across Europe at high speed fairly often, you have to be going some to *average* 50mph over a day."

Indeed...

"The longest section I do, 640 miles starting at Strasbourg takes 11-12 hours and two drivers with rest breaks. Maximum speed I've managed so far is an average of

57mph. And a rather disappointing 21mpg."

I've averaged (just!) 3 figures point-point on small sections whilst in Africa on the kind of road that requires zero steering input for 45 minutes at a time. That needs constant ~110 leptons/hr indicated even with zero traffic and in an (compared to an Ibiza/Golf...) aerodynamic german taxi* of 2004 vintage. The vehicle was NOT tuned for Euro-IV emissions compliance AND was running on (higher density) African grade fuels, both of which /increase/ fuel economy.(the latter by at least

5%, the former by an unknown margin). It /nearly/ got 30mpg.

In the UK/Europe you won't get those kind of roads. An Ibiza or Golf will have aero drag at least that of the taxi The taxi engine was an entire generation more up to date than teh Ibiza engines Still couldn't do it.

"30mpg, maybe 35" is *giving you the benefit of the doubt!*

*E270Cdi saloon with all the toys right up to vinyl seats...

Granted it did have aircon, which was used and working hard in the heat, but with the amount of throttle the thing needed to maintain cruise speed (5/6ths at least, everything on slight hills and about

3/4on slight downhills) I doubt the effect was huge!

Aha - you don't understand aerodynamics....

The Cd of the Merc may be lower, but the CdA of the Ibiza will be lower. Ibiza displaces less air therefore will use less fuel at high speed cruise. As drag increases as the cube of speed then it becomes very very significant.

in news: snipped-for-privacy@karoo.co.uk, "Tim S Kemp" slurred :

Sorry, couldn't resist....

Power increases as speed cubed, but energy required per distance (and therefore also approximately economy) only goes as speed squared.

The ibiza 1.9 TDi gets 56mpg@80, therefore at 100mph it should get about 56*(80/100)^2 = 36mpg

Or the concept of 'correct attribution', apparently...

-- JackH

My personal best in terms of half decent economy combined with rapid covering of distance is 172 miles covererd in 1hr 55mins equates to 89.7mph average speed... yes, going by the milometer of said vehicle, and my very own wristwatch... *cough*, all 'allegedly' of course.

48mpg was the achieved MPG - Mk3 Golf TDi (90).

-- JackH

"The Cd of the Merc may be lower, but the CdA of the Ibiza will be lower. Ibiza displaces less air therefore will use less fuel at high speed cruise"

I'd go find said "Cd" and "A" if I were you; euroboxes are pretty bad when it comes to the former and the latter is little different to a full-size car.

"

The ibiza 1.9 TDi gets 56mpg@80, therefore at 100mph it should get about 56*(80/100)^2 = 36mpg "

I hear you, but think plummeting engine efficiency at those loads/rpms and suchlike will also come into play, hence the even more pessemistic predictions!

Lots of work goes into economy at 75-80mph, provided it doesn't compromise the extra-urban cycle economy! 100mph economy receives less attention.

48mpg on mileometer and watch at an average of 89.7 leptons/hr.

Allow 5-10% odo inaccuracy as they generally over read.

45.6mpg to 43.2mpg Swap speed to the 100mph using relationship Albert posted: 36.6mpg to 34.8mpg

You can also:

Allow for fact engine is slightly less efficient at 100 leptons/hr than it is at 89.7 leptons/hr Allow for maintaining 100leptons/hr requiring more braking/overtaking than 89.7 leptons/hr Allow for experimental error etc

And I think the data *you* give supports the guestimate of 35mpg for an

*average* of 100mph and shows that 50mpg at an *average* of 100mph (as initially posted) is bollocks...

A speedo over-reads by design, but I think you'll find the mileometer part of it is far more accurate - what the speedo indicates, and what the mileometer clocks up, isn't like for like.

Yes, all these wicked car manufacturers... haul them up in court, because their claims for MPG must be vastly inaccurate as well, given I've bothered to work out the MPG achieved on most of my cars, at lower speeds, as well as the higher ones.

Using your logic, at the lower speeds, they've grossly misquoted achieveable figures, because using your methods, I'd be calculating I was getting that many less miles to the gallon than the calculations showed... which funnily enough in the case of both my Golfs, have, normally tallied up with the official figures for them.

See below for further confirmation of the actual mileages covered...

You've made a lot of, what I believe to be inaccurate assumptions I'm afraid - A clue... plotting the route taken by me on that specific trip on Multimap, proves my mileometer was actually quite accurate, and if anything on that specific car, was under-reading.

-- JackH

In my experience, odo is usually roughly accurate, maybe 1% either way even in the 1960's although tyre pressures, etc, vary this (not to any huge extent unless something is significantly out of spec.)

Marco's right Jack - the figures don't add up.

You've driven a bunch of cars - you know full well that 50mpg at 100mph is crazy talk ! If you could obtain 50mpg at 100mph, then said car would be looking at

*well* over 70mpg on a 70mph cruise !

I'm not claiming 50mpg at 100mph average, I'm *stating* I achieved 48mpg at an average speed of 89.7mph.

I clarified the mileage covered by comparing the miles covered on the mileometer, and that Multimap stated on multimap on the route I took... it tallied up to within 2 miles, with my mileometer actually giving the lower total...

That aside, who exactly am I to claim such things... I was only the one that carried out said journey!

I know I achieved 48mpg at an actual average of 89.7 mph, with the latter calculated by distance / total time x 60 (minutes) - my watch isn't *that* badly out. ;-)

I'm not the one saying I got 50mpg at an actually achieved average of 100mph

Different cars are geared differently / develop their peak power and torque at different points in their rev range - a big barge will be more efficient on a long steady run in relation to the MPG it achieves around town, than a small car, which is having to be thrashed more on a run, to maintain the same speeds.

FWIW, the VAG TDI lumps I've had, have all been much more efficient at higher speeds than the more common non DI stuff I've had in the past, as in the MPG achieved when being driven hard vs the MPG achieved on more sedate runs, isn't as far apart as that that I was getting with some of the older 'skool' stuff I've had in the past, and you can put this down to the engine being more than man enough to power the vehicle it's planted in, and vastly superior induction techniques meaning you don't have to hammer the crap out of them to maintain post motorway speed limit speeds.

HTH

-- JackH Fazer 600 - Golf TDi

Yes, but that's just using physics equations. It doesn't take into account that wind resistance might not follow a simple pattern that can be expressed in a simple equation relative to speed.

This is how I worked out the economy of mine, and why it's roughly accurate, but could be a bit out - Tim picked up my Ibiza for me in Hull, and put £15 of derv in it somewhere around Doncaster, with the needle just below the red mark on the gauge, resetting the trip computer. I met him at Leicester Forest East Services on the M1, and then drove it back to my mate's place just past Dorking in Surrey. For the first half an hour or so I was doing only about 60-65 down the M69 as the weather was crap, but once it cleared up, I was doing a fairly steady 90-95mph down a private test track parallel to the M40. I then went around the M25 to Leatherhead, slow-ish up to the end of the roadworks, then around 70-80, then down the A24 past Dorking, doing about 70-80 down there. Tim caned the f*ck out of it on the section he drove it.

On arriving at a petrol station the needle was, judging from what Tim said, around the same place as just before he put £15 in it. So roughly speaking, that £15 of derv had got me 218 miles (IIRC). I worked out what mpg would be equivalent to 6.66p/mile (i.e. £20 for 300 miles), and it worked out at around 57mpg. A similar pence per mile rate would have got 225 miles from that £15 of derv, so it was therefore a little below 57mpg - probably nearer

55mpg. And the average speed for that run was probably an *easy* 80-85mph, possibly 90mph. So that's how I reckon it *would* do around 50mpg on a run at 100mph - they are very economical engines.

I worked mine out by brimming the tank at both ends of the journey in question... some pumps might have slightly more sensitive cutoff points than others, but then I normally keep dribbling it in anyway, and regardless, they're not *that* out to make a significant difference when the fuel burnt off covered 179 miles...

-- JackH

No, that was me, but it was semi-tongue-in-cheek, which can be a bit tricky to grasp from the written word.

I've explained in the other post that it was a rough calculation, but on that journey (particuarly the Hull-Leicester bit of it) it was driven *very* hard, and still achieved excellent economy figures, and continues to do so (sold it to a mate of mine).

Yeah, what he said. Put it this way, in general driving, whether it was pootling at 50-60mph on motorways/main roads everywhere, or doing 90mph everywhere, I'd bet that there's next to bugger all in it between an Ibiza

1.9 TDI-90 and one of the more modern commonrail efforts, like a Fiesta 1.4 TDCI (68bhp?), and I'd choose the VAG product any day for proper long term (as in running up to 300-400k) reliability (call me cynical but I don't trust those high pressure commonrail fuel pumps).

Aye, that's a much more accurate way, don't get me wrong, but my guesswork wouldn't have been *that* far out - I mean it would have been *at least*

50mpg, and most probably nearer 55 than 50.

I don't know what you mean - was just trying to keep up with my mate... who was in my volvo...