Calculate diff ratio without counting teeth?
Turbo Mini Forums

Hi guys,
One of the largest oversights I made when building my engine from a donor car was not looking to see what the FD ratio was on the diff before I finalised the build and dropped the engine into the car.

The engine works a treat but the gearing is incredibly short. I can't get past about 15 mph in 1st without the drop gears screaming and can't stay on boost for more than a second or so before having to change.

The car doesn't see motorway miles so a short ratio is 'OK' but whatever it is on at the moment is way too short.

Without taking the engine out and looking at the diff, is there a way to calculate the ratio to actually understand what I am running with currently?

That would then help me to understand what might be a good solution.

Yeh. Get it on level ground. Plugs out. Mark the crank pulley and a tyre, put it in 4th gear. Pull it along counting how many times the crank turns while a mate counts how many times the wheels turn. Then devide the two.

If you've an accurate speedo or gps, you could work it out with the revs? A bit rough but it could work

Use guessworks gearbox ratio calc, plug in a few things and see what comes up the closest in each of the gears with regards to speed/rpm relations