Tell us about your weekend ride

There's one major difference for both examples you use, racing.

An 84" gear is a bit low for racing really, but for general on road fixed riding it's excessively high, and the damage it could possibly do to your knees is only something you'll find out about after it's too late.

There's a reason why a gear in the mid 60s to low 70s has become the norm for road riding over the years, because it allows you to maintain a decent average speed whilst allowing you to tackle most hills you'll encounter in the UK. And it won't trash your knees.

But if you're riding shorter distances for speed on the flat with no traffic lights...