I have an 18" rear wheel on my Tenere (really? what else have you done to it Jenny?) and you'll certainly need to change the gearing significantly to get it back to a similar ratio. You'll also need a longer drive chain than stock.

For info, I'm running 14/50 now (with a 140/80x18 Michelin Desert) - you might get away with 15/50 - but running a 48T rear is not enough to lower the ratio back to what the stock 17" rim had...

Of course the increase in wheels size/gearing also compounds the speedo over-reading, it's like 15% out now...

However, the benefit in running an 18" rear does outweigh the disadvantages - tyre choice like you say, plus the steering is slightly sharper (unless you put different forks on too of course x)... I use a GPS nearly all the time anyway, so that is my accurate speedo these days...

J xx

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