GTA I
The fire in GTAI was quite simple. If a vehicle exploded a fire burned in its remains, and the fire of a flamethrower was made up of little individual fires in a line. You and other people were able to catch on fire. The trees in GTAI were simple as well. They changed appearance with each city in the game, such as green blobs in Liberty City, then palm trees in Vice City.
GTA II
The fire in GTAII looked much more natural and fluid-like. Cars would burn if they got too damaged and stayed on fire for a little after exploding. The fire from the flamethrower looked better and was a stream of fire instead of individual fires.
The trees in GTAII were a slight improvement over GTAI's.
GTA III
The fire from the flamethrower in III was similar to II, but in a different perspective or angle. Vehicles caught on fire and would burn before exploding. Many of the trees in III were not solid, and were basically two pictures of trees put together. The solid trees were more complex.
GTA: Vice City
Fire in VC behaved basically the same as fire in III, but it looked slightly different.
The trees in VC were slightly better looking than those in III. There were more solid trees and more variety.
GTA: San Andreas
GTA:SA had much better looking trees than the previous installments. There were many types, from the Redwood in the countryside to the palm trees in Los Santos. The fire also improved. It was able to spread on the ground and wouldn't go away every time unless it was extinguished. Fire also appeared on the ground after explosions.
GTA IV
The fire in IV was much more realistic looking. It spread more slowly than in SA and was able to go out without you extinguishing it manually. When you caught on fire, you stopped, dropped, and rolled to try and put it out. Vehicles wheels were able to burn off and leave the rims, and it stayed burning in cars for a bit after the car exploded. Fire spread through cars, too. There was no flamethrower this time around. You did not always instantly catch on fire when you went through small ground fires.
There was less variety in tree type in IV and there was improvement in their appearance.
GTA V
The trees in V look to be the best in the series, so far.
There will be much more variety in tree type.
This time around, you are supposedly able to make a trail of oil with an oil can and then set it on fire.
Specifics on how fire will behave have not been revealed, but it will probably be similar to IV.
It will spread through cars like in IV.
