Cummings's Journeys (April 2020)

Range Rover SUV 3.0l SDV diesel: 31.5 mpg

• source: https://www.carbuyer.co.uk/reviews/land-rover/range-rover/suv/mpg

It has an 86 litre tank:

• source: https://www.landrover.co.uk/vehicles/range-rover/specifications.html

Convert to gallons: 86 / 4.546 = 18.92 gallons

• source: https://www.which.co.uk/reviews/new-and-used-cars/article/driver-calculators-and-tools/litres-to-gallons-converter

Range of the vehicle on a full tank: 31.5 mpg * 18.92 gallons = 595.91 miles

The journey distances we have been given by Cummings are:

London to Durham 260 miles
Durham to Barnard Castle 30 miles
Barnard Castle return to Durham 30 miles
Durham return to London 260 miles

Total mileage: 580 miles

However, Durham to Barnard Castle is only just under 27 miles, though could be 30 miles if taking the A68 via Piercebridge.

• source: https://www.google.co.uk/maps/dir/Durham/Barnard+Castle/@54.6568854,-2.0341542,10z/data=!3m1!4b1!4m14!4m13!1m5!1m1!1s0x487c2aa525d8b1c1:0x678a46d5f522452b!2m2!1d-1.584852!2d54.77525!1m5!1m1!1s0x487c235f994d8231:0xa0e235a2aa664382!2m2!1d-1.923741!2d54.545284!3e0

And London to Durham is 263 miles or 270 miles (depending on route, via A1(M) is 263 miles, via M1 is 270 miles).

• source: https://www.google.co.uk/maps/dir/London/Durham/@53.1300904,-3.1414574,7z/data=!3m1!4b1!4m14!4m13!1m5!1m1!1s0x47d8a00baf21de75:0x52963a5addd52a99!2m2!1d-0.1277583!2d51.5073509!1m5!1m1!1s0x487c2aa525d8b1c1:0x678a46d5f522452b!2m2!1d-1.584852!2d54.77525!3e0

Changing 30 miles for 27 miles is offset directly by changing 260 miles for 263 miles. The total mileage of 580 miles is reasonable, and depending on route may have been slightly more.

The total mileage may have been driven on one full tank.

But is it more likely that at some point the vehicle was at least partially refuelled?