2001 Prius Manual says:
"Fuel type: UNLEADED gasoline, Octane Rating 87 (Research Octane Number 91) or higher."
Depending on where you live (say, outside the US), will depend upon what number is listed as the octane rating on the pump.
from a quick search on google:
"Gasoline pumps typically post octane numbers as an average of two different
values. Often you may see the octane rating quoted as (R+M)/2. One value is
the research octane number (RON), which is determined with a test engine
running at a low speed of 600 rpm. The other value is the motor octane
number (MON), which is determined with a test engine running at a higher
speed of 900 rpm. If, for example, a gasoline has an RON of 98 and a MON of
90, then the posted octane number would be the average of the two values or
"At present three systems of octane rating are used in the United States.
Two of these, the research octane and motor octane numbers, are determined
by burning the gasoline in an engine under different, but specified,
conditions. Usually the motor octane number is lower than the research
octane. The third octane rating, which federal regulations require on
commercial gasoline pumps, is an average of research octane and motor
octane. Under this system a regular grade gasoline has an octane number of
about 87 and a premium grade of about 93."
http://www.shell.ca/code/motoring/encyc ... ctane.html
"Technically there are three different "octane numbers" associated with
every gasoline. The Research Octane Number, or RON, is measured under fairly
easy test conditions. The Motor Octane Number, or MON, is a tougher test
measured at higher engine speed and temperature.
The value that relates most closely to actual driving conditions is the
average of these two values: Road Octane Number = (RON + MON)/ 2.
Occasionally, less scrupulous gasoline outlets will use the confusion of
these different octane measurements to exaggerate their octane rating
claims, by advertising their fuel's Research Octane Number - which will be
higher than the Road Octane Number. Motorists should always be sure that the
octane number a vendor advertises is its Road Octane value, not its RON."
[note: those URLs I copied from an email I sent back in March 2002, so the links may not work or the text may be slightly modified now...]