Can't find a calculator to do what you requested either.However try
http://oee.nrcan.gc.ca/autosmart/calculator/index.cfm They don't have results for the Vibe/Matrix yet but you can go to: Find and compare vehicles. Celica's should be close. Choose your cars Select one of your cars to "insert into calculator" switch between l/100km and MPG on the drop down box to convert back and forth.This service is using imperial gallons. To convert to smaller U.S. mpg divide by 1.2
Frosty 5 speed Vibe Power & Safety Packages. Naturally Aspirated my gallery