I would be more worried about your state then the feds when it comes to road taxes. I looked up the oregon's tax law on dyed fuels and the fine is either $10 per gallon or $1000, whichever happens to be higher. (most states have very similar laws although our fines seems lower than most) We had a friend in eastern Oregon who bought dyed diesel at Pacific Pride which tracks all your untaxed diesel. Well Oregon noticed that his use was way higher than would typically be used in tractors. They dipped his truck's tank and he got a $4000 or so fine. The feds never got involved. It seems likely the feds won't get involved unless the abuse is blatant and in huge excess.

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'84 190d 2.2 - Silver
Converted to 5-Speed
'84 190d 2.2 - Blue
Factory 5-Speed

'76 300D (W115)
Sold, found again in less auspicious circumstances