BRYAN MCILWRAITH  Bsc. (Hons) Ost.Med.  DO.




There are great difficulties in measuring the interior of a car since there are no straight lines or levels to measure from. The system used in this survey established a datum line across the window openings using a rigid plastic profile. From this the centre of the seat back could be measured, and the Wheel Offset (WO) calculated from this in cm (4, Fig2).  The angle the steering wheel made with this profile (and hence with the driver) could also be calculated (5).   The figures for Wheel Angle (WA) column give a range of  0 (good) to 5 (awful).  see pictures

Pedal offset is extremely difficult to measure in a way that gives any real meaning, but is quite obvious to the experienced eye, and here is based on a visual assessment  (6)

Headroom was measured in two ways; the total interior height  (H1 Fig1), and the height from the back of the seat squab to the roof with the seat in its lowest position  (H2). Headroom was also assessed by the author sitting in the seat. H2 is the more important figure for taller drivers. Models marked * had sunroofs fitted; the same model without will have approximately 2 – 5 cm more headroom.

A comparative measure of Kerb Height was obtained by using a spirit level to measure from the top of the author’s thigh to the ground with the seat in the lowest position (KH).  A low Kerb Height of less than 60 cm may cause difficulty getting out of the vehicle, especially for taller drivers. A higher Kerb Height figure is generally better, but once it exceeds 75cm (as in many 4x4s) it again causes difficulty when exiting the vehicle.

Seat Score (SS) is based on the amount of adjustable features available, and the presence of an adjustable lumbar support, and other features such as lateral support and the firmness of the foam. The higher the figure the better.

In all cases the measurements were rounded in favour of the manufacturers.

In general, car design has steadily improved since my first major survey in 1993. The layout of many models is significantly better, with pedal offset largely disappearing. However many models still feature noticable wheel offset and wheel angle. In this area there  some surprises, with some of the manufacturers previously known for attention to ergonomics apparently taking retrograde steps. Many cars have increased in size generally, and this has allowed for greater headroom.




         Fig 1, H,1 H2,  & Kerb Height          








  Fig 2, 4=Wheel offset, 5=Wheel angle, 6=Pedal offset



Link to tables of car measurements  click here


Further Reading

McIlwraith B. An Analysis of the Driving Position in the Modern Motor Car. British Osteopathic Journal Vol XI pp27-34 1993

McIlwraith B. Seating and Associated Back Conditions.  Automotive Interiors International - Seating Review 1994. ISSN 0967-0386  pub Turret Group plc.

McIlwraith B. Loss of the Lumbar Curve in the Driving Seat. British Osteopathic Journal Vol XIX pp19-23 1996 ISSN 0263-5194

Keegan J J. Alterations of the Lumbar Curve Related to Posture and Seating. J Bone & Joint Surgery Vol 35A No3 July pp589-603  1953

Kelsey J L .  Hardy R J. Driving of Motor Vehicles as a Risk Factor for Acute Herniated Lumbar Intervertebral Disc. American J. Epidemiology 102 (1) pp63-73 1975

Porter J M, Porter C S, Lee V J A. A Survey of Driver Discomfort. Contemporary Ergonomics.pp262-267.  Pub Taylor & Francis 1992

Andersson G B J, Murpy R W, Ortengren R, Nachemson A L. The Influence of Backrest Inclination and Lumbar Support on Lumbar Lordosis. Spine Vol 4(1) pp 52-58. 1979.





                                 Pedal Offset

                                 Wheel Offset

                                  Wheel Angle






                                 Higher Kerbheight

                                 Sufficient Headroom

                                 Adjustable Lumbar Support

                                 Power Steering

                                 Servo Brakes

                                  Automatic Gearbox