help with calculating milage with bigger tires

[Jim F]

I was going to suggest a GPS like Robert said. Amazon.com has a basic GPS Garmin xTrex (bright yellow one) for $99 dollars and it will tell you your exact speed. Once you know how far off the speedo is, it just a matter of doing the math to determine the distance you would add or subtract from your trip odometer. I wouldn't suggest using the GPS as a trip odometer. This would mean having to link up to a satelite everytime you went somewhere.