Speedo reads about 7% fast on the stock bike, but the bigger tyre (which alters the gearing) will reduce that degree of error. But...
The speedo error is deliberate and built into the display. The odo and trip are as accurate as the manufacturer can make them, so if anything they're probably underestimating the distance covered because of your bigger tyre.
That said, I doubt if your tyre has a very big effect, perhaps about 3%, and I did say 'as accurate as the manufacturer can make them'. That isn't as accurate as you might like to think lol.
The average mpg from the trip computer is never accurate. The bike just isn't very good at measuring the amount of fuel used. I suspect that there is also a deliberate error built in. Every vehicle I've ever had has had a trip computer that's read about 10% better on fuel consumption than the real fuel consumption.
As posted, for an accurate mpg figure, use a GPS to record your mileage and be aware that that's probably only accurate to around 2 or 3%, but as it can both under read and over read it tends to balance out over a distance. Gps or trip, record the amount of fuel you actually put in to the tank and calculate your mpg yourself.
Why does the tyre make a difference? Because the sped sensor measures the rpm of the front sprocket. Tfe speedo calculates speed by knowing how far the bike moves for each revolution of the sprocket. If you change the gearing or the tyre size it moves a different distance from that programmed in to the ecu.
One other thing.
Because the speedo reads high and the odo doesn't with the bike in stock trim, if you use a speedo healer to correct the speedo you'll throw the odo and trip out.
Last edited by williamr; 11-19-2012 at 08:22 AM.