The carmaker said it had identified a mistake in the Total Road Load Horsepower (TRLHP) of the affected vehicles. This unique resistance figure is based on track and wind tunnel testing, and is used to calculate fuel economy for vehicles on a dynamometer – a stationary laboratory test where only the wheels actually move.

Working with the United States Environmental Protection Agency, the entire Ford model range has since been reviewed and re-tested to correct the error, with a number of its most economical models affected. Both lease and purchase customers will receive goodwill payouts reflecting the additional fuel cost from the new ratings.

The new ratings affect hybrid versions of the C-MAX and Fusion, down from 43mpg (51.6mpg Imperial) to 40mpg (48.0mpg Imperial) and 47mpg (56.4mpg Imperial) to 42mpg (50.4mpg Imperial). Plug-in hybrid versions of the same car have each had a two-mile reduction in electric range, from 21 to 19 miles, which has reduced combined fuel economy, including the electricity use, from 100mpge (120.1mpg Imperial) to 88mpge (105.7mpg Imperial).

Owners of the Lincoln MKZ Hybrid will get the largest payouts, with the reduction from 45mpg (54.0mpg Imperial) to 38mpg (45.6mpg Imperial) corresponding to a $1,050 (£618) payment. Most of the Fiesta range has also been adjusted to reflect the error.

Alan Mulally, Ford president and CEO, said: ‘Ford is absolutely committed to delivering top fuel economy and accurate information. We apologize to our customers and will provide goodwill payments to affected owners. We also are taking steps to improve our processes and prevent issues like this from happening again.’

Ford has now altered the C-MAX Hybrid’s fuel economy ratings twice within 12 months. Last August, the company responded to complaints and lawsuits from owners about the inaccuracy of tests carried out to EPA guidelines.

Following an internal retest said to better reflect real-world driving, the company dropped the car’s official fuel economy figure from 47mpg (56.4mpg Imperial) to 43mpg (51.6mpg Imperial) offering a goodwill payment of $550 (£324) for purchase customers and $325 (£191) for leasing customers.