[UK Don]

Hi - I just bought a Mazda 3 2018 sport automatic and I love it, but... Fuel consumption for me driving only in central London is around 19.5 MPG (or 12l/100km, or 8km/l).
Is this normal?
I had read reviews with figures almost twice as good...

Which engine do you have?

 

[UK Don]

Your MPG does seem low. I wouldn’t be expecting 40 MPG but I’d hope for around 30.
But…..you’re driving in central London so will be mainly start/stop driving at an average of only around 12 - 15 MPH. It’s winter so you’ll be using fuel to run a lot of electrical items, heater, lights, wipers etc.
Accurately check your MPG by filling the tank to the brim, driving for 25 - 30 miles, refilling to the brim & dividing the miles driven by the fuel used. (Example: miles driven = 30, fuel used = 7.6 litres = 3.9473 miles per litre x 4.5461 to give an MPG of 17.9447).
If the above confirms your MPG at around 20 then take the car somewhere less urban & see what MPG you get.

 

[UK Don]

You would need proof that there is a fuel consumption problem which will be difficult to get. It’s a long shot but could you borrow an identical car to compare it against? I’m getting around 34 MPG in a 2020 saloon with the 178 BHP Skyactiv X engine. That’s short journeys in Sheffield. In better weather I get around 42 MPG for similar driving.
There are a lot of reasons for poor fuel consumption from a faulty thermostat (is the car getting to temperature in a reasonable time & staying there?) a faulty O2 sensor is a common cause as are faulty injectors.
A dyno (rolling road) fuel consumption diagnostic would be a good place to start. Taking a dyno diagnostic report to the seller would help your case considerably if it shows that you have a consumption issue. You’ve plenty of options for dyno centres in London.
It could be that there isn’t a problem with the car. The cost of a dyno report could be a price worth paying for peace of mind though.

 

[UK Don]

Thanks guys - all very useful points of view and tips. In all honesty I have been (rather embarrassingly) doing more like 6x1 mile trips a day than 2x3 miles ones,,.so it might be all down to the impact of continuous stop and go, and driving until the engine has sufficiently heated up is. I will start ignoring continuous requests from the kids to give them a ride to school and keep an eye on the data on longer drives.

Your fuel consumption is actually good considering the amount of short trips you’re doing.
It isn’t good for your engine as It’s running constantly cold & using the richest fuel mixture possible most of the time. Over time it will use even more fuel. The injectors will gum up & you’ll get excessive carbon build up in the engine.
Have you considered an electric vehicle. It would suit your usage a lot better.

 

[UK Don]

Thanks again. I think you are absolutely right, but once every 2/3 months I drive to a small town 200 miles north. And once a year I do another holiday trip in the countryside or to France... And while I would possibly save money with an electric car for the test of the year and rental cars for these trips, overall petrol seemed the better choice. Perhaps in a couple of years I will see things differently

Most modern electric cars have a 200 mile + range so you’d only really need a rental car when you go to France or on longer trips. I’m not a fan of electrics, I think the future is hydrogen, but they have a lot fewer moving parts & can cope with short journeys without risk of critical component damage. I hope I don’t cause offence by giving an honest opinion but I think that at the moment you’re spending a fortune on fuel (& future maintenance) running a car that doesn’t suit your usage needs.