Time has to be accurate, the whole system depends on the time signal differences between the satellites. Altitude is the least accurate: the triangulation between satelites is generally at the wrong angle
Umm.. Low revving diesel. Probably running at optimum for engine performance.
Next time I am allowed to use it I'll zero the averaging fuel consumption readout and report back. My gut feel is that it is unlikely to use more fuel. You may have evidence to the contrary?
regards
In message , tony sayer writes
Not really. At low revs the engine braking effect is small so lifting the loud pedal foot doesn't slow the car very quickly, particularly on a slight gradient. Transiting from 40 mph to 30 is best done by changing down a gear. Also maintaining a gap behind another vehicle is easier in a lower gear. I blame the engine chip which is obviously designed to minimise CO2 production.
I'll report back when I get a chance to compare fuel consumption at
30mph in 3rd and 4th.regards
I've done the experiment on a turbo diesel BMW, and at 70, dropping from
5th to 4th made about 1% difference, as the increased volumetric efficiency from the turbo working harder counteracted the extra frictional and pumping losses due to the extra engine speed.Engine are designed for peak efficiency over a range of speeds, and as long as you're within the range, you won't get too much variation in fuel consumption. If there's no green band on the rev counter, the details will be in the manual. Peak efficiency on our non-turbo cars at work seems to be at about 2500 - 3000 rpm.
If 5th is top gear and it will pull that at 30, it's under geared. Unless some tiny engine with a top speed of 60.
Bullshit.
Nearly every car I had will run at 30 mph in fifth. Only one laboured. The current corsa is low geared, it only does about 20 mph /1000 rpm. The current astra does about 25 mph/1000 rpm.
1200 rpm is more than enough to keep a car going at 30 mph on the level unless you have some stupid power profile. You never need to accelerate quickly into danger at 30 mph either so changing down to second/third isn't a problem.
Which will be very difficult unless you do it over a considerable distance... The 'real time' fuel consumption guage on my car can show anything from 19mpg to 200mpg when cruising at 30 depending upon the degree of slope the car is on. You would have to drive over the same stretch of road, and be very precise with throttle settings, to get anything like an accurate comparison.
I have my 'multi function display' set to constantly show real-time fuel consumption - and it demonstrates very clearly just how town driving can wreak havoc with fuel useage. Accelerating away from rest, even gently, for example, will show 6 - 9 mpg until cruising speed is reached. Just shows how lots of stop/start driving afffects fuel consumption. Two drivers with identical cars and identical driving 'manner', but whose regular driving is different - ie a regular town driver compared with a regular long distance cruiser, will each achieve radically different fuel consumption figures for their cars.
depends on the car mate.. My supercharged XKR would just about pull from that sort of speed in top..if the auto box didn't kick down.
It would do 100mph in second..just.
I remember my Anglia doing 40mph in first going round the Hammersmith Broadway one way system.
Pug 205 1.9 GTI could do it :-) (8v engine so pulled from lower, somewhat light car)
Yes. This isn't likely to be scientific:-)
There is a long semi-rural 30mph stretch on the way to Luton. I had in mind to zero the gauge and do a couple of miles in each gear. I might just need to buy a router cutter from Screwfix tomorrow:-)
regards
It has a torque convertor. And any lockup won't operate at that speed.
My BMW won't accept 4th until 36 mph. 5th, 55. It would be near idle speed at 30 in top gear.
Yes - but in those days economy wasn't the main issue. And with that GTI performance was the priority. Most these days are very high geared to get the maximum mpg. If they aren't, they won't.
I doubt the ability to get any meaningful figure out of it. When driving on cruise control my MPG varies widly with gradient, even slight ones that are almost imperceptable by eye could be anything from 45 to 80+mpg.
I find if I forget to change into 5th gear at 30mph it is very noticable in the MPG average after a couple of miles.
I've looked, and failed to find a reference with 30MPH.
Here...
"Travelling at a constant 37 mph in fifth gear uses 25 per cent less fuel than in third gear". It also goes on to suggest that too high a gear is a bad idea too. I believe you want to be somewhere near peak torque - and don't forget that that moves with throttle settings.
My understanding is that Diesels are better at low power settings - they don't have to drag air past the butterfly - so you'll probably see less effect.
My car will run in any of its gears at 30. 1st is banging on the limiter, and 5th is for rolling at steady speed only - and not up any significant hill.
I've done the checks in the past, running on the same piece of road at the same speed on subsequent days, and seen differences. But this car doesn't have a computer.
Andy
Of course, the simplest answer (although not necessarily the cheapest!) is to get a car with a DSG type auto box. Unlike conventional torque converter auto boxes, DSGs are, in general, as economical as a manual (in some cases, particularly with the newer dry-clutch DSGs, - they are *more* economical than the equivalent manual). With a DSG, the inbuilt computer decides which is the optimal gear to be in and you can just forget all about that left-leg pumping and stick waggling...
Something must be fiddled. A DSG box has more moving parts than a basic manual so must also have more friction. It's also considerably heavier. Of course a computer controlled box may give better economy than a badly driven manual.
"Dave Plowman (News)" :
No.
Agreed, those are factors working against economy. But they're not the only factors.
A driver with a manual gearbox has only one clutch, so during a gear change the engine will be spinning to little or no effect. That's a disadvantage that no amount of skill can make up for. Two clutches give better engine utilisation than one, resulting in better economy despite the extra weight and complexity.
I'd be very surprised if it makes a difference in ordinary driving conditions. A good driver can anticipate the need for a gearchange by reading the road and traffic ahead - something no computer can.
It's sadly all too easy to fool official fuel consumption tests that the data is based on. And most makers are very adept at this.
I alos doubt whether the difference comes from the reduced neutral period between gears.
True, I can beat cruise control (by 3-4 mpg over a regular 50 mile journey) by knowing the locations of hills and not ploughing up & down them a strictly constant speed.
That's very true, I have to drive like a nun to even approach the official MPGs.
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