Gear "ratios" are determined by "both" the drive and driven gear...
So in your example if you were running a 72 tooth #35 and lets say a 24 tooth drive (for ease of math)...
72/24=3:1 ratio
So to get the same from the #40/41 size with a 45 tooth rear you would need a 15 tooth drive to get same gearing...
45/15= 3:1 ratio
The 45 tooth rear is going to be "about" 7" in diameter...
You could get the same "ratio" in #40/41 chain with a 48 tooth rear and 16 tooth driver...
48/16= 3:1 ratio... at 8" diameter..
Then if you want to change "ratios" and pulling and to speed characteristics, you start interchanging drive and driven gears...
48 tooth rear with 15 drive or 45 tooth rear with 16 drive..
48/16= 3.2:1 ratio and 45/16=2.8:1 ratio....
Then depending on engine torque, chassis traction and track length...you can adjust from there....
The 3.2:1 will "pull hard" and wind out quick than either of the 3:1s will...and the 2.8:1 will have higher top speed at loss of pull from corner...
Got it..?