speedo recalibration?!?!?!

What speedo drive should I be using on 33x12.5x15 tyres ?

I designed a circuit to adjust the number of pulses from the transducer and feed out pulses in a different ratio allowing me to adjust the TD5 speedo to any size but once I'd finished I realised that you can already buy one!! Not only that but when I checked my speedo against a GPS unit it was absolutely spot on