Different numbers don't change what you've done at all. However, if your car were that far off, I think you'd have k nown it by now. It's a lot easier for a bike 'puter to be off.

Another option - but also not totally reliable - go to "google pedometer" and zoom in on your route. (Since all my speedometers are broken, it's what I ahve to do whenever I ride a dfferent route to get my mileage.)