Pronunciation of -ar in Madagascar

Solution 1:

In American English, we hear the -car that ends the name Madagascar pronounced just like the word "car" meaning a motor vehicle. The American pronunciation is clearly said the same as in if an American were to say, "I was mad a gas car wasn't available."

In British dialects, however, the A is de-emphasized and so less tall, being most commonly pronounced like the -car at the end of the name Oscar, which tends to be slightly taller at the back of the throat than words that end in -er but not much taller, certainly not as tall as the word car.

Solution 2:

I found that it's an American convention.

The link to the 1828, American dictionary entry for Templar that @Peter Shor provided lists, in another transcription system, /tɛmplɑɹ/. I was also able to find one online American dictionary, Merriam-Webster, that does list the /mædəɡæskɑɹ/ pronunciation of Madagascar but no English dictionary that does so. The Oxford English Dictionaries and the Cambridge English Dictionary, for example, list only the schwa pronunciations for Madagascar and Templar.

This offers a few possible conclusions. As @choster pointed out, many words that end in -(s)car are pronounced /(s)kɑɹ/, so it could be the natural tendency to pronounce other words this way. @Peter Shor says that this is the standard way to reduce the vowel in unstressed syllables, so it could be reduced to the schwa pronunciation for speakers that normally have /ɑ/, depending on speed. Finally, @Benjamin Harman says it's most common for British speakers to reduce a final vowel of this type to /ɐ-ɜ/ (if I'm reading correctly).