Another option for you might be to keep a good log book of on/off road miles and then claim a rebate for the fuel tax on the fuel burned off-road with you settle up with the IRS on April 15. Many farmers I know do this. Keeps the dye out of your tank (a little bit goes a LONG way, which is why it is hard to rinse/flush out of your fuel system) and gets you out of paying the road tax on the off-road miles. Talk to your accountant to make sure you keep the right records.