Yeah 20 over 17 is faster not slower. I had a yz 250 2 stoke that was insanely slow so I flipped the gears like u are saying it was too fast, ended up going like 17 overc16 or 18 over 16 or 17 whatever worked for chain tension dont remember but yeah 20 over 17 is really fast! Best way is to go by track roll out, search for posts by mtn muncher, he was the original owner / mastermind behind timbersled he did a post on track roll out. Anyway it almost always seem that tge jacksgaft gear comes into play I know a lot of people ended up going 16 on the jack