IMO (not worth much if you ask Dave...
), the real issue is GRIP.
15" rally tyres generally have a bigger contact patch (widths approx. from 175 to 225) than 14" tyres (widths approx. from 165 to 185) and 15" tyres come in a far bigger range of compounds. I used to run 14" wheels on my 1600 rally car and paid the same price as I do now for 15" tyres on my 180B rally car, the main difference is I can get 4 or 5 different compounds in the 15" tyres (as opposed to 1 or 2 compounds in 14" tyres).
More Tyre GRIP = better acceleration, higher corner speeds and better braking*.
IMO the added grip (especially from better compounds) far exceeds the extra weight of the slightly larger wheel and tyre. Just look at how much tyre compound technology has affect Formula 1!!!
* Assuming all other things are equal.
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