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i own a Isuzu bighorn 3.1dt 1997,its fitted with 245x70x16 tyres , want to keep original rims but have wider tyres, would like 275 or 285 but not sure about speedo reading, whats the widest i could go ?

Assuming you keep the same height profile (70), the wider tyre would produce a slower indicated speed in comparison to true speed.

With the 275 tyre, the indicated speed would read 5.6% slow which means at 100kph indicated, your actual speed would be 105.6kph.

With the 285 tyre, the indicated speed would read 7.5% slow which means at 100kph indicated, your actual speed would be 107.5kph.