[Ecky]

So, effective gearing will also depend on tire size and engine torque/HP.

Consider this:

Honda K20 engine = 200HP, 2 liters, ~142ft-lbs of torque
Honda K24 engine = 200HP, 2.4 liters, ~170ft-lbs of torque

The K24 and K20 produce the same peak horsepower. The K24 does it at 20% lower RPM by having 20% more torque from its 20% large displacement. Therefore, to have the same power in every gear, it could have a 20% taller final drive. It would feel the same as the K20.

Or, in other words, the K24 at 2500rpm "feels" the same as the smaller K20 engine if that's spinning at 3000rpm, gearing adjusted by 20%. Both engines would have almost identical driving characteristics.

~

The engine I'm putting in my Insight is from a 2008 TSX. The TSX transmission has a very short 4.764 final drive in part because the K24 is a very high revving engine and Honda wanted it to feel "sporty", but also because the TSX has relatively large 215/50r17 tires, which have an 80" circumference, and a very tall 0.659 6th gear. This results in ~2700rpm @ 65mph in 6th gear, and a very short 15.5:1 ratio in 1st gear.

Moving this engine to my Insight, which has smaller 70.5" circumference wheels, results in ~12% shorter effective gearing due to the change in tire size alone. Highway RPM goes up to 3075rpm because the tires are smaller, and each turn of the engine takes the car less distance.

To compensate for this, I'm replacing the 4.764 final drive with a custom-made 3.4 final drive - absurdly tall for a 4 cylinder. I'm also replacing the 0.659 6th gear with an even taller 0.647.

4.764 x 0.659 = 3.14
3.400 x 0.647 = 2.20

Transmission gearing will be 42% taller, or from a different perspective, RPM will be 30% lower in every gear. However, the tires are 12.2% smaller which partially negates this. So, effectively I'm getting 24.7% taller gearing if I include the the difference in tire size, or 21% lower RPM in every gear.

With the smaller tires, 3.4 final drive and 0.647 6th gear, I can expect 2140rpm @ 65mph. If I change to 185/65r14 tires, which are ~5% larger, I can expect ~2040rpm at 65mph.