Explain gear ratios?

Question

If I want to know how many rpm a car will be doing at say 70mph in top gear and I have the gear ratios, how do I calculate this? Do I even need the gear ratios?

They are:
Gear
1st
2nd
3rd
4th
5th
6th
Gear Ratio
4.46
2.71
1.33
1.09
1.33
1.09
Layshaft
1
1
2
2
1
1
Final Drive Ratio
2.88
2.88
4.25
4.25
2.88
2.88
Overall Ratio
12.7
7.7
5.7
4.6
3.8
3.1

(its a focus st170, if that helps explain the layshaft bit)
Thanks

Answer 1

You need one more piece of information - the effective rolling radius of the tyres.

Then you need to convert 70mph into m/s (31.286 m/s)
Divide this by the affective rolling circumference (calculated from the effective rolling radius) of the tyres. For a 215/45R17 tyre this will be about 1.95m. You may need to check the tyre manufacturer's technical datasheet for the exact effective rolling radius of your tyres.

This allows you to calculate the number of times the wheels turn per second at 70mph: 31.286/1.95 = 16.04

Times 60 for rpm = 962.6

Times the total gear ratio for the engine speed = 962.6 x 3.1 = 2984rpm.

Answer 2

The gear ratio is the relationship between the number of teeth on two gears that are meshed or two sprockets connected with a common roller chain, or the circumferences of two pulleys connected with a drive belt.

This is the first paragraph. For more information, see the link below.