Mpg discussion and mpg/vehicle range records

[Adrian Wintle]

A great popular misconception is that speedometers are set to read 10% high. The allowable margin of error is actually much more than 10% but this is not the same as setting a speedometer to be inaccurate, it means that the speedometer can read high (but not low) and still satisfy requirements for speedometer accuracy and approval. Most are set well within this tolerance, when out Golf reads 50mph the satnav indicates 48, my Citroen is similar to the Golf.

 

My current Subaru speedometer is almost dead on the satnav speed with the stock tyres. With the winter tyres on it is less than 1% high (225/55-17 vs. 225/60R16).

 

Adrian

[jjb1970]

I find that iterms of how close you get to the claimed mpg figures in real life that powerful cars often do better. Their absolute mpg may still be poor but in normal driving they're very unstressed and the gear ratios tend to be closely spaced with makes for an easy and relaxed drive. Small econoboxes with gutless engines and widely spaced gears set up to get good results in the official test are often woeful in the real world as to make anything like acceptable progress the engine ends up getting thrashed. I found that with the Audi A1, the 1.6TDi is actually a pretty good engine and more than powerful enough for a car like the A1 but the gearbox was truly awful and just sucked the performance out of the engine with the result any sort of acceleration needed a heavy foot. The official combined for something like 74mpg (that's combined, not extra-urban), we were getting mid 40's and never got anywhere near the extra-urban figure on a steady A road run let along combined driving.

[jjb1970]

My current Subaru speedometer is almost dead on the satnav speed with the stock tyres. With the winter tyres on it is less than 1% high (225/55-17 vs. 225/60R16).

 

Adrian

 

Indeed, it is a dangerous misconception some people have that speedometers read 10% high as they end up driving to speed limit + 10% if they want to stay within the limit. Made worse by those who have decided that the speed limit is speed limit + 10%, so they end up driving at speed limit + 10% + 10% which becomes a significant excursion above the limit.

Clearly the speedometer is sensitive to a few variables such as tyre wear, but most of the cars I've had have been reasonably accurate and nowhere near the tolerance limit that is allowed.

[Dunsignalling]

There are many factors thatb affect fuel consumption, besides the driving technique, although that is probably one of the most significant.

 

I rent my car as part of my retirement package and change it about every nine months, so I get first hand experience of some of the changes brought about through legislation, technology improvements, etc. The best fuel consumption I consistently got was on a 1.6L diesel fitted - as part of its standard spec - with  "economy" tyres. My current car, although a later Euro 6 spec. diesel has low profile tyres (rubber bands on big wheels) and is about 10% worse. Mind you we picked it up in January and driving in the wet conditions we had in the early months of this year also has had a detrimental affect.

 

Interesting comments about very low speed fuel consumption, I don't think any of my cars give a reading below 20mph, including the Peugeot 206 my wife had.

Oddly, my 206 would do it but, as I mentioned in an earlier post, I always suspected it was somewhat non-standard.

 

Interesting what you say about Economy tyres because my 207 had them when I bought it and I noticed a significant improvement in mpg after I replaced the front pair with a good budget brand that make no such claims and actually provide some grip.

 

John

Edited March 18, 2017 by Dunsignalling

[Suzie]

Buying 'budget' tyres will often result in significantly increased fuel consumption that more than offsets the saving. Not just losses in the tyres themselves, there is the extra braking required to be slow enough to go round corners on the reduced grip. Goodyear Eagle F1 for me - expensive outlay but they last and are truly economical.

[jjb1970]

I also think that cheap tyres tend to be a false economy, the other thing easy to lose sight of is just what a difference tyre choice makes to ride comfort and noise. Not just the difference between high and low profile tyres but on a like for like tyre size basis. I made the mistake of going for cheap tyres a couple of times when money was tight and difference in ride and noise was very noticeable.

[georgeds]

I think modern engines are a lot more improved over even recent examples. Current supercharged 3.0 V6 petrol can do 41mpg on a steady run. Previous 2.0 "Ecoboost" 4 cylinder could not do that.

[Dunsignalling]

Buying 'budget' tyres will often result in significantly increased fuel consumption that more than offsets the saving. Not just losses in the tyres themselves, there is the extra braking required to be slow enough to go round corners on the reduced grip. Goodyear Eagle F1 for me - expensive outlay but they last and are truly economical.

I went from full price Continentals to their "budget" brand (around £55 a corner) several years ago and didn't notice a reduction in grip or service life. I have a fairly heavy right foot and do a lot of driving on country roads that feature longish straights interspersed by twisty sections. They are as good as anything else I've ever used.

 

I fitted the same in place of the allegedly fuel-saving "Eco" tyres (can't remember the brand) that came with my current car and they do everything better, grip, life and mpg. 

 

John

Edited March 18, 2017 by Dunsignalling

[Horsetan]

...I fitted the same in place of the allegedly fuel-saving "Eco" tyres (can't remember the brand) that came with my current car and they do everything better, grip, life and mpg.

 

Most likely a Chinese-made brand such as Nexen.

[jjb1970]

Most engines are perfectly good, the issues are application. Engine performance, including efficiency and emissions, is sensitive to a range of variables, including ambient conditions. That's why the official emissions testing produces results that are different from what you'd measure when driving and also why testing under controlled conditions is essential. Engines are also set up for a particular duty, automotive engines are extremely flexible but they're still optimised for certain cycles, hence why some do very well in official tests and not in the real world. For all that, driving style, drag, gear ratios, tyres and lots more are important. The question is not whether an engine is good but whether it is right for the application. As with any tool it about how it is used. The official tests are a problem. Clearly there needs to be some sort of standardized test and approval regime. When those tests are distorting real world performance and manufacturers are setting up cars to get marketing rights based on a meaningless figure it is not good. This is a bit like the difference between educating children and getting them through exams. We all want our kids to be educated but when society places so much value in exams what do we think the education system will do? Now manufacturers are between a rock and a hard place. They know fine well the value of official figures but they don't want to suddenly have to advertise a car as achieving 55mpg that they've previously claimed could do 75mpg. Easier to maintain a charade however these are statutory requirements so if politicians were serious about transparency and meaningful data they could change the tests.

[Kickstart]

Clearly the speedometer is sensitive to a few variables such as tyre wear, but most of the cars I've had have been reasonably accurate and nowhere near the tolerance limit that is allowed.

A fair few years ago we had an Alfa 155. The speedo on this normally read about 5mph high at 30mph, and about 6mph high at 80mph. Inaccurate but remarkably consistent. But strangely on a long journey on a hot day it would become dead accurate. Landed up running 2 sat navs at one (one was a high frequency sample performance measuring unit, the other a basic normal one) as I was concerned about the sat nav doing odd things.

 

All the best

 

Katy

[peanuts]

 when out Golf reads 50mph the satnav indicates 48, my Citroen is similar to the Golf.

 

much to the chagrin of HGV  drivers through road works someone plodding along in lane 2 at an indicated 50mph when its actual 46/7 with a que of hgvs  in lane one unable to pass within the speed limit as they have speedos that are calibrated to the tachograph 

[jjb1970]

 much to the chagrin of HGV  drivers through road works someone plodding along in lane 2 at an indicated 50mph when its actual 46/7 with a que of hgvs  in lane one unable to pass within the speed limit as they have speedos that are calibrated to the tachograph 

 

The difference in journey time between going through a temporary speed restriction at 46mph and 50mph is trivial. One of the worst driving habits I see on a regular basis is trucks tailgating dangerously to try and bully cars into going faster in these temporary speed restrictions. Even if the cars are going slower than they need to there is no excuse for tailgating and aggressive driving to try and force people to speed up. I tend to stay in the inside lane in these situations and drive according to my speedometer, if that is a couple of mph high then so be it, it is the instrument I've got and the difference between 48 and 50mph is pretty much irrelevant.

[peanuts]

The difference in journey time between going through a temporary speed restriction at 46mph and 50mph is trivial. One of the worst driving habits I see on a regular basis is trucks tailgating dangerously to try and bully cars into going faster in these temporary speed restrictions. Even if the cars are going slower than they need to there is no excuse for tailgating and aggressive driving to try and force people to speed up. I tend to stay in the inside lane in these situations and drive according to my speedometer, if that is a couple of mph high then so be it, it is the instrument I've got and the difference between 48 and 50mph is pretty much irrelevant.

i agree full well tailgating by anybody is idiotic unfortunatly with the amount of telematic monitering that goes on especialy with fuel and smooth driving you are pushed a lot by your managers to to "push on " so that 2mph differance may seem to mean nothing to you to the driver being pushed it does

[Guest]

On that subject. Van driver is obviously displeased with the biker keeping to the 30mph limit.

[cheesysmith]

i always thought that the speedometer reading 2or3 mph high was to make sure that the manufacturer could not be responsible if you were caught speeding. if they underreported the speed and you get caught you could be innocent but the manufacturer would be liable, but if they overread the only one responsible is the driver.

[The Nth Degree]

My best mpg performance was 94mpg at a constant 50mph.

 

I've lowered my Golf estate by 5cm, fitted smaller mirrors, overinflate my tyres to 40-45psi and service regularly with synthetic oils and grease. I have a calibrated ScanGaugeII to monitor my driving, which reads the throttle position sensor, among many other things, through the OBD port.

 

Typical driving at 65mph+ will get approx 70mpg+, depending on ambient temp and traffic conditions.

[Kickstart]

i always thought that the speedometer reading 2or3 mph high was to make sure that the manufacturer could not be responsible if you were caught speeding. if they underreported the speed and you get caught you could be innocent but the manufacturer would be liable, but if they overread the only one responsible is the driver.

Maybe, but they MUST not read low. As things can affect the reading it is safest for the makers to make them read a bit high to give a bit of leeway for slightly larger tyres, etc

 

All the best

 

Katy

[How about a Dictator Loco Class?]

My best mpg performance was 94mpg at a constant 50mph.

I've lowered my Golf estate by 5cm, fitted smaller mirrors, overinflate my tyres to 40-45psi and service regularly with synthetic oils and grease. I have a calibrated ScanGaugeII to monitor my driving, which reads the throttle position sensor, among many other things, through the OBD port.

Typical driving at 65mph+ will get approx 70mpg+, depending on ambient temp and traffic conditions.

Does your OBD device show current/instantaneous mpg higher than 99.9mpg?

I'd love a device that goes vastly higher to see what cars are really doing on in gear over run.

[Horsetan]

My best mpg performance was 94mpg at a constant 50mph.

 

I've lowered my Golf estate by 5cm, fitted smaller mirrors, overinflate my tyres to 40-45psi and service regularly with synthetic oils and grease. I have a calibrated ScanGaugeII to monitor my driving, which reads the throttle position sensor, among many other things, through the OBD port.

 

Typical driving at 65mph+ will get approx 70mpg+, depending on ambient temp and traffic conditions.

 

Interesting. As far as over-inflation of tyres is concerned, I'm not entirely sure about the accuracy of air pump readings. The Bora has a "fully-loaded" pressure guide of 2.2bar front, 2.8bar rear. I generally inflate to 2.3 front, 2.9 rear, as it allows the tyres to settle back down to the recommended maximum.

 

Did you have your ECU remapped as well?

 

When I put in a remapped ECU in the diesel Bora, I found that fuel consumption for in-town and suburban driving improved by about 2-3mpg, but it was noticeably more difficult to breach the average 80mpg line for longer distance trips. The best I have had so far out of the remapped ECU was 81.2mpg, compared to 86.4 for the original standard one.

[Ian J.]

My old (2001) Skoda Octavia showed 999 mpg as its highest current consumption. The Alfa Mito I'm currently driving has 99.9 mpg as highest.

Edited July 20, 2017 by Ian J.