[cnold]

toydog,
I had the same question when I noticed that the octane number posted on regular unleaded pumps in Colorado was lower than what Toyota recommends. Looking to the Internet for answers, I found several reports on octane requirement tests. One test concluded that computerized engine management systems that adjust the octane requirement may also reduce the power output on low octane fuel, resulting in increased fuel consumption but no gas mileage test were none. My personal gas mileage tests indicate the opposite to be true but that may be due to the higher ethanol content of premium fuels sold in some areas. All of the tests showed that the octane required went down as the altitude went up. The number fluctuated between a 0.2 and 0.5 reduction per thousand feet gained in elevation. Taking the 0.2 number, your Tundra would do just fine with 86 octane fuel at Denver and would only need 85 octane at 10,000 ft.