How do I compare petrol and diesel running costs?

Enter the petrol car first, with the petrol price. For the second vehicle, enter the diesel car's economy and type the diesel price into Its fuel price; the price unit buttons appear when you do. Leave that price empty when both vehicles burn the same fuel.

Does 50% better mpg halve the cost?

No. Going from 30 to 45 mpg is 50% more miles per gallon but a third less fuel, because the cost follows gallons per mile, which is 1 divided by mpg. That is why the 45 mpg car saves 33.3% here, not 50%.

Can it compare an electric car?

No. An electric car is priced per kWh of electricity, which this calculator does not take. It compares vehicles that burn liquid fuel: petrol, gasoline, diesel, LPG, or a hybrid's fuel economy.

Can I compare over a month or a year?

Yes. The monthly and annual pages have the same Second vehicle switch, and there the Difference row is the saving per month or per year.

Fuel Cost Comparison Calculator

Price the same trip in two vehicles and see the difference in money and as a percentage. The page opens on 300 miles at 3.50 a gallon: a 30 mpg car costs 35.00 and a 45 mpg car 23.33, a saving of 11.67 or 33.3%. Each vehicle takes its own efficiency unit, and the second can have its own fuel price, so petrol against diesel or a US-rated car against a European one compares directly.

One way fuel cost35.0010 US gal over 300 miles

Distance

Efficiency

Measured in

Fuel price

Quoted

Sharing

Trip

Round trip

Compare

Second vehicle

Measured in

Its fuel price

Enter the price in whatever currency you buy fuel in. The answer comes back in the same one, because the arithmetic never touches an exchange rate. Leave the second vehicle's price empty when both burn the same fuel; give it a price of its own for diesel against petrol.

Fuel neededone way
10 US gal (37.854 L)
Total costfuel only, no tolls or parking
35.00
Per personone person
35.00
Per mile and per kmthe same money over two rulers
0.1167 per mile, 0.0725 per km
Second vehiclesame distance, 6.667 US gal
23.33
Differencethe second vehicle costs less
11.67 (33.3%)
The workingevery conversion, in full
300 mi ÷ 30 mpg = 10 US gal; 10 US gal × 3.50 = 35.00

Two gallons, and one unit that runs backwards. Choose the gallon used by your fuel price and efficiency figure. L/100km measures fuel per distance, so a lower number means less fuel used, and it multiplies where mpg divides. Every conversion the answer needs is written into the working above with the constant it used, so you can check which gallon went where.

Common questions

How do I compare petrol and diesel running costs?: Enter the petrol car first, with the petrol price. For the second vehicle, enter the diesel car's economy and type the diesel price into Its fuel price; the price unit buttons appear when you do. Leave that price empty when both vehicles burn the same fuel.
Does 50% better mpg halve the cost?: No. Going from 30 to 45 mpg is 50% more miles per gallon but a third less fuel, because the cost follows gallons per mile, which is 1 divided by mpg. That is why the 45 mpg car saves 33.3% here, not 50%.
Can it compare an electric car?: No. An electric car is priced per kWh of electricity, which this calculator does not take. It compares vehicles that burn liquid fuel: petrol, gasoline, diesel, LPG, or a hybrid's fuel economy.
Can I compare over a month or a year?: Yes. The monthly and annual pages have the same Second vehicle switch, and there the Difference row is the saving per month or per year.

Exact arithmetic on the numbers entered: distance divided by efficiency, times price. Real consumption varies with speed, load and weather, so treat the result as the figure your own inputs imply.