About that fire....how many tires were used total?....can a few tires burn at 1800 degrees (number from the forensic dentist) without something pumping air into it? Blacksmiths need an air source, a "blower" or bellows to get coal up to 1400-1500 degrees for iron and steel work. Without that air source, the coal will normally not get anywhere near hot enough unless the fire was massive.
Also, just as a side note, the hotter the fire, the more air it needs and the faster the fuel source (like tires) get used up. Tires normally ignite at around 750 according to cursory research.
I found a guy who says he has burned tires and the hottest part of the burn lasted only for 30-45 minutes with one tire. And the whole body needs to be wreathed in cremation temperature flames...that sounds like a fairly large number of tires to me.
Edited to add the info below:
A traditional Hindu funeral pyre takes six hours and burns 500–600 kilogrammes (1,102–1,323 pounds) of wood to burn a body completely
From wikipedia:
https://en.wikipedia.org/wiki/Pyre