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Old 22nd February 2021, 12:30
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There are likely to be issues with fouling and/or usability if you go to big or too wide, while there's a period/visual fit issue with anything very low profile.

On a small-chassis Triumph based car the maximum tyre diameter you can get away with without moving the front bulkheads back is approximately 25 inches (636mm), and even then you might have to 'realign' the bulkheads with a lump hammer or cut and reweld them to stop the tyres fouling them.

Additionally, wheels wider than 5.5" can foul the suspension so you may need to add a 'stop' to restrict the steering travel, while tyres wider than 175mm can foul the inside of the wheel arches, dependant on the profile, and definitely make the steering too heavy at low speeds.

If you want the tyres to look period, you also need to go with a higher profile aspect ratio. Big wheels wrapped in tiny rubber bands weren't available back in the day and the look of the wheels and tyres makes a big difference to the whole look of the car.

As an example, the 195/45/15 tyres mentioned above have sidewalls a full 2" (50mm) lower than the 175/80/14's I had fitted to my Spyder, so would look very different -

Various different options were tried on Sammio Spyders and I'm guessing that the space available in your arches is similar to those, and the most used sizes were as follows:

165/80/14 - no fouling issues
175/80/14 - bulkhead may need 'adjustment'
175/70/14 - no fouling issues but sidewall can look a little small
155/80/15 - bulkhead may need 'adjustment'
175/70/15 - no issues but limited availability
175/65/15 - no fouling issues

This calculator is quite useful when checking just how big a tyre will actually be -

Anyway, good luck choosing a wheel and tyre combination that works for you.
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