This article is very misleading.
Speedy Racer: Fastest diesel-powered street-legal car in the world : Big Country : Abilene Reporter-News
He is NOT running 1200 hp and getting 35 mpg. On the highway the nitrous system is not running. The diesel computer is dialing back the wastegate on the turbo to an economy mode. When he goes racing, he UPs the boost big time and turns on the nitrous big time.
On the highway I would be surprised if he is producing 250 hp.
What the article doesn't say, says a lot more than what is says.