Petrol vs diesel and the variance from claimed figures.

Ben Cole · 23 Feb 2019 at 06:14

My question, do they vary from claimed figures by the same margin?

We all know that the official figures published for cars are not realistic but merely serve as a comparison mechanism between models. Is the error the same for petrols as diesels?

I’m trying to figure out whether an efficient petrol is viable for 30k miles a year.

paradigm · 23 Feb 2019 at 06:45

I’d imagine (just like with diesels) it varies from car to car and usage case.

The wife’s 1.4 Petrol Polo actually beats it’s published combined cycle figure for her usage most of the time.

My old A4 2.0 TDie came within about 5% most of the time.

My S3 (when driven carefully/economically) is probably a good 20% off the claimed figure.

So even within the same group of brands I’ve seen variance.

Matthew.M · 23 Feb 2019 at 07:10

It definitely varies. I find that non-turbo cars achieve closer to claimed figures. I don’t know if that’s because they’re easier to drive gently without accidentally using too much boost.

I used to do 30k miles a year and used a 2.0 petrol SKYACTIV Mazda 3. It never really went below 50mpg and 55-60mpg was achievable (as long as the journey was longer than to the local supermarket!).

It’s definitely viable.

DarkHorizon472 · 23 Feb 2019 at 09:42

The manufacturers figures tend not to represent real world driving of any kind and some are a long way off. If you find owners forums etc. they will have more accurate mpg or websites like this I found were accurate.

https://www.honestjohn.co.uk/realmpg/volkswagen/tiguan-2008

mccarf · 23 Feb 2019 at 10:03

For me it's been my most powerful cars that have come the closest to the official MPG figures. I'm guessing the fact I don't have to rev their nuts off to make them move at a decent rate is probably what helps them most.

Semple · 23 Feb 2019 at 10:29

30k is quite a bit.

Is that a lot of motorway driving? That's normally where diesels excel over petrol.

Ben Cole · 23 Feb 2019 at 10:52

80%+ motorway. That's why I've only looked at diesels until now.

Nasher · 23 Feb 2019 at 11:15

mccarf said:
For me it's been my most powerful cars that have come the closest to the official MPG figures. I'm guessing the fact I don't have to rev their nuts off to make them move at a decent rate is probably what helps them most.

I've noticed that too.

Small, turbo engine figures are way off I find. I never got anywhere near the 47mpg combined when I had my mk7 Fiesta ST (more like mid 30s). The mk2 Focus ST I had before that was pretty close to the official figure though, even when tuned.

robj20 · 23 Feb 2019 at 14:16

I've never had a car I couldn't get the published figures in and exceed. But some are definitely easier to achieve than others.

cuke2u · 23 Feb 2019 at 16:46

My A3 1.4 matched or exceeded the quoted figures, with my A4 2.0tfsi I can match both urban and highway...

ShiWarrior · 23 Feb 2019 at 17:04

for me, i look at the "urban" figure on Auto Trader and thats accurate to my car within 2 mpg
but of course if i started to use the car for motorway trips then its way off the Urban figure

Ben Cole · 23 Feb 2019 at 20:49

robj20 said:
I've never had a car I couldn't get the published figures in and exceed. But some are definitely easier to achieve than others.

What cars?

Rroff · 23 Feb 2019 at 21:10

A lot will depend on the condition the car is in, tyres, fuel, way it is driven, etc.

My current vehicle (Navara) is rated:

MPG Urban 22.2 mpg
MPG Extra Urban 37.2 mpg
MPG Combined 29.7 mpg

Tends to get around 26-28 mpg on mixed trips involving around 50/50 sitting in slow traffic through town then on reasonably flowing country lanes, same trip when quiet 33-35 mpg - get it out on decent roads and it can just about hit 40 mpg.

robj20 · 24 Feb 2019 at 06:55

Ben Cole said:
What cars?

Ford Focus mk1 1.6 petrol
Mazda Rx8 230
Audi A3 1.9tdi
Ford Mondeo mk5 2lt diesel 180

paradigm · 24 Feb 2019 at 07:12

robj20 said:
Mazda Rx8 230

We’re you quoting fuel or oil consumption with that one?

jpaul · 24 Feb 2019 at 07:46

aren't the new wltp regulations fixing this diparity ?
prior to that just use the german spirit site to find true mpg

DarrenM343 · 24 Feb 2019 at 07:58

Over the years I think I've found with BMW sportier models, ie, Z4 3.0 Si, Z4 M Coupe and even an two old E30 M3's, all the cars achieved higher than the manufacturer combined figure. The X1 that I currently own doesn't, 44.5 currently, 48 I think is the combined. I seem to remember an old 320 diesel compact exceeding the combined figure too, but not entirely sure, but I have 54MPG in my head vs 49 combined. Also have a Yaris and that's under too (44.7 vs 50.1) Note that these are all according to the OBC - I've never done the maths. I've done similar driving over the years or if not I've reset the OBC and tested under similar conditions.
I think BMW test their more sporty cars under conditions the car could be used, harder driving.

[TW]Fox · 24 Feb 2019 at 08:25

DarrenM343 said:
I think BMW test their more sporty cars under conditions the car could be used, harder driving.

Every car is tested under exactly the same conditions - that's the point in the old NEDC and now new WLTP testing procedures.

DarrenM343 · 24 Feb 2019 at 08:34

[TW]Fox said:
Every car is tested under exactly the same conditions - that's the point in the old NEDC and now new WLTP testing procedures.

Very true.
I'd like to try an M2 vs a 220i. My suspicion is that driving both cars the same way, conservatively, the M2 will over achieve on MPG vs the 220i which may struggle to reach it's combined figure.

Simon · 24 Feb 2019 at 11:16

If you drive them both on the NEDC they will achieve what is stated.