You May Have To Say Goodbye To Your Toll-Free Highways

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17. IIRC, farmers are able to buy fuel with a different tax structure, or substantial tax refunds.

This is probably different in every state, however.

In Wisconsin, for example:

Who may receive a refund of motor vehicle fuel tax paid?

Off-Road Usage of Motor Vehicle Fuel [sec. 78.75, Wis. Stats.]

Wisconsin law provides that motor vehicle fuel is not subject to the Wisconsin motor vehicle fuel tax when it is used for off-road purposes in mobile machinery and equipment. Exceptions: It is subject to the motor vehicle fuel tax when placed into licensed motor vehicles used for off-road purposes, snowmobiles, recreational motorboats, and all-terrain vehicles (unless the ATV is registered for private use).

Farmers, construction companies, and logging operations often have off-road usage of motor vehicle fuel in non-licensed mobile machinery and equipment. Waste management, ready-mix, liquid waste (e.g., septic service), and utility companies have licensed vehicles with power take-off (PTO) units that share the same fuel supply tank used to power the vehicle. These companies can receive a refund of the motor vehicle fuel tax paid on fuel placed into the vehicles and used by PTO units. The following percentages have been approved by the department when computing the fuel tax refund attributable to PTO units:
waste management - 30%
ready-mix - 35%
liquid waste companies - 15%
boom and derrick/digger trucks - 15% if 20,000 gross vehicle weight or less
30% if greater than 20,000 GVW

Refund claims must be filed within one year of the date fuel is purchased, and must cover at least 100 gallons. To obtain a refund of the fuel tax paid on motor vehicle fuel used for off-road purposes, file Form MF-001. This form may be filed electronically through My Tax Account or by paper.

http://www.revenue.wi.gov/faqs/ise/mofuel.html

And, Federally:

Introduction

You may be eligible to claim a credit on your income tax return for the federal excise tax on certain fuels. You may also be eligible to claim a quarterly refund of the fuel taxes during the year, instead of waiting to claim a credit on your income tax return.

Whether you can claim a credit or refund depends on whether the fuel was taxed and the purpose (nontaxable use) for which you used the fuel. The nontaxable uses of fuel for which a farmer may claim a credit or refund are generally the following.

Use on a farm for farming purposes.

Off-highway business use.

Uses other than as a fuel in a propulsion engine, such as home use.

Table 14-1 presents an overview of credits and refunds that may be claimed for fuels used for the nontaxable uses listed above. See Publication 510, Excise Taxes, for more information.
Topics - This chapter discusses:

Fuels used in farming

Dyed diesel fuel and dyed kerosene

Fuels used in off-highway business use

Fuels used for household purposes

How to claim a credit or refund

Including the credit or refund in income

http://www.irs.gov/publications/p225/ch14.html