Rainbo Oil is offering 90-octane unleaded gasoline and a money back guarantee for those whose cars experience "knock or run-on" after purchasing at least 10 gallons of the fuel.

The "One Tank Test" guarantee is good until March 31 for Rainbo's Golden Supreme unleaded gasoline, a spokesman said.Rainbo introduced its high octane gasoline to Utah in November to prevent detuning of engines equipped with "knock-sensors," said Rainbo Oil general manager Frank Sida.

"We are so confident in the superior ability of Golden Supreme to prevent knock and run-on that we're offering the `One Tank Test' guarantee to prove our point."

In the event of a refund, a customer must fill out an application available at a Rainbo outlet and mail it to Rainbo's Salt Lake offices.