How we estimate your annual fuel and tax costs

If you know your number plate...

To estimate your annual fuel and tax costs, we use your car’s vehicle registration mark (VRM) to look at its MOT history on the Driver and Vehicle Standards Agency (DVSA) website. Every car’s MOT history is publicly available, and it contains the car’s odometer reading at the time of the MOT. You can look this up yourself here. 

Having obtained your car’s annual mileage, we then estimated how much you spend at the pump. At the moment, our tool uses an average cost per mile to estimate your fuel costs. We’re constantly refining our methodology to make our estimates more accurate, but until then we assume that if you drive a petrol or diesel car, you’re spending £0.115 per mile. To work out what your mileage would cost in an EV, we use a similar average cost per mile of £0.038, which is based on doing 90% of your charging at home on an average domestic UK tariff. These figures are based on a February 2020 research paper by the RAC Foundation, which compared the cost per mile of different petrol, diesel and electric cars. 

To estimate your tax costs, we assume you pay £150 per year in Vehicle Excise Duty (VED), which is the standard rate of VED in the UK on cars that are old enough to have an MOT. 

If you don’t know your number plate…

To estimate the annual fuel and tax costs for the three different car types, we took three cars of different sizes and looked at their running costs. For the small car, we looked at a Vauxhall Corsa 1.2l which does 51.4 mpg, for the medium car we used a VW Golf 2.0 which does 47.1 mpg and for the large car we used a BMW X5 which does 38.2 mpg. We compared those cars with three different sizes of EV: a Vauxhall e-Corsa, a VW ID.3 and a Kia e-Niro. 

To estimate the annual mileage, we used the DVSA’s MOT database to look up the average miles done by small, medium and large cars. This told us that small cars do on average 6,000 miles per year, medium cars do 8000 miles per year, and large cars do 6500 miles per year. We assumed a fuel cost of £1.15 per litre for both petrol and diesel. That means we underestimate the cost of petrol and overestimate the cost of diesel slightly - something that we will fix as we continue to develop the widget.