What is the best size tyre to fit to my MGB?
Most MGB’s with wire wheels have 4.5 inch width X 14 inch diameter wheels.
Early roadster steel wheels and early MGBGT were 4 inch wide X 14 inch diameter. However some time after the introduction of the GT their steel wheels became 5 inches wide.
The roadster and the GT steel wheels became 5 inch wide with the introduction of “Rostyle” steel wheels although some sources suggest that even early “Rostyle” steel wheels were 4.5 inches wide.
It can become confusing but all wheels will have the width stamped into the rim and for example my “Rostyle” wheels have 5JFHx14.
The original tyre size fitted to MGB’s was a 5.60 x 14 x 80 aspect ratio and had, when new, a diameter of 24 inches.
Therefore to avoid speedometer error it is best to seek a tyre as close as possible to have the same diameter of 24 inches.
The table above has highlighted the best fit tyres at the respective tyre width to minimise speedometer error.
It is also important to note the recommended wheel width for each tyre width and to avoid too wide a tyre for the wheel width.
Fitting a wide tyre to a too narrow wheel will allow the tyre to potentially roll too much relative to the wheel when negotiating corners at speed. It is possible to pull the tyre bead away from the wheel rim allowing air to escape with disastrous consequence.
When selecting tyres first check what wheel width is fitted to your car.
Some MGB’s, if the wider tyres are fitted, have tyre rubbing occurring on the rear wheels . This could be either the tyres are too wide or is caused by the axle and rear spring assembly not central to the car centreline. This usually occurs when only one side tyre contacts the mudguard.
Graham Haywood
October 2021