(petitioner utilized specialized trucks and "belly-dump" trailers in its business; use of trucks and trailers occurred on-road and off-road; trucks and trailers were street legal, but had been modified for heavy use; approximately 40 percent of use was off-highway; petitioner claimed I.R.C. Sec. 34(a)(3) and 6427(l)1) credits for off-highway business use; court upholds IRS determination that credits disallowed; tractors and trailers not analyzed together for purpose of interpreting statutory term "vehicle" because tractors and trailers could each perform designed functions with other vehicles; tractors not specifically designed for off-highway use and could operate at normal highway speeds; tractors do not qualify for "off-highway transportation" exception of I.R.C. Sec. 48.4061(a)-1(d)(2)(ii); for post-2006 tax years I.R.C. Sec. 7701(a)(48) defines "highway vehicle" more narrowly).