You can check the ratio by marking the wheel or brake drum, and the propshaft UJ. With the gearbox in neutral, and the car jacked up safely, note where the propshaft UJ starts - easier from the 6 o'clock position. Then slowly rotate the wheels. Make sure that both wheels turn at exactly the same rate - you may need a helper. 3.07 is only a shade over 3 revs. 3.70 is closer to 4 revs, so 1 revolution of the roadwheel will produce either 3 revs of the prop, or nearly 4.
Not all Roadsters have had the same axle ratio. It depends on the gearbox, and whether top gear is direct drive 1:1 or overdrive (0.85ish).
From memory......the S1 had a direct drive top gear with a 3.07 diff.
The S2 and S3 had an overdrive top gear with a 3.7 diff.
I don't know what the ratio is of 6th gear on the current box, but 3.7 could well be the right ratio.
