To ensure that prefixes retain their identity when pronounced, the first syllable of each prefix is accented. In particular, kilometre should be pronounced with the accent on the first syllable, as is our custom with units such as kilolitre, kilogram, and kilowatt.
The accent on the second syllable generally applies to measuring devices such as thermometer, speedometer, barometer, and micrometer.
This guide was provided by John Bailes, president of the Canadian Metric Association, and is sourced from the Metric Style Guide. 1975. Council of Ministers of Education, Canada, p. 123.