i remember questions like this in my 11plus at school!

if a Virgin train leaves london kings cross for manchester at 8am, travels at an average speed of 100mph, what time will it be cancelled :L

 

:S I don't understand :$

 

Not the best example to use as the RS MTX75 gearbox and the 2.0 MTX75 used in the Focus have almost identical ratios but we can use it to show what you need to know.
Ok, if we also assume that both cars are also running the same size wheels and tyres inflated to exactly the same pressures and also make lots of other assumptions about forces/ friction within components, weather etc and say that the cars are identical apart from the gearbox ratios then we can use a generic speed/ RPM calculator from the web, I have left the tyre size and redline figures as the defaults already in the calculator for ease, I know they are not correct for the Focus, the gearbox ratios are however correct:

First the RS:
Tyre Size: 205/55/16

Final Drive Ratio (Axle Ratio of driven wheels): 3.82

Gear Ratios
1: 3.23
2: 2.13
3: 1.48
4: 1.11
5: 0.85

RPM Redline: 6250

RPM Increment: 500

1 2 3 4 5
0 0.0 0.0 0.0 0.0 0.0
500 2.9 4.4 6.3 8.4 11.0
1000 5.8 8.8 12.6 16.8 22.0
1500 8.7 13.2 18.9 25.3 33.0
2000 11.6 17.6 25.3 33.7 44.0
2500 14.5 21.9 31.6 42.1 55.0
3000 17.4 26.3 37.9 50.5 66.0
3500 20.3 30.7 44.2 59.0 77.0
4000 23.2 35.1 50.5 67.4 88.0
4500 26.0 39.5 56.8 75.8 99.0
5000 28.9 43.9 63.2 84.2 110.0
5500 31.8 48.3 69.5 92.6 121.0
6000 34.7 52.7 75.8 101.1 132.0
6250
redline 36.2 54.9 79.0 105.3 137.5

Now the 2.0 Zetec

Tire Size: 205/55/16

Final Drive Ratio: 3.82

Gear Ratios
1: 3.42
2: 2.14
3: 1.48
4: 1.11
5: 0.85

RPM Redline: 6250

RPM Increment: 500

1 2 3 4 5
0 0.0 0.0 0.0 0.0 0.0
500 2.7 4.4 6.3 8.4 11.0
1000 5.5 8.7 12.6 16.8 22.0
1500 8.2 13.1 18.9 25.3 33.0
2000 10.9 17.5 25.3 33.7 44.0
2500 13.7 21.8 31.6 42.1 55.0
3000 16.4 26.2 37.9 50.5 66.0
3500 19.1 30.6 44.2 59.0 77.0
4000 21.9 34.9 50.5 67.4 88.0
4500 24.6 39.3 56.8 75.8 99.0
5000 27.3 43.7 63.2 84.2 110.0
5500 30.1 48.1 69.5 92.6 121.0
6000 32.8 52.4 75.8 101.1 132.0
6250
redline 34.2 54.6 79.0 105.3 137.5

So (as you would expect) the lower ratios of the RS box in 1st and 2nd gears allow it to reach a higher speed before a gear change is needed so in theory it is quicker.
As said its purely theoretical because of all the varialbes involved but the above gives you some idea.

Steve

 

perfect explanation thanks a lot !!!

 

but the RS mk1 does around 145 kmph at 6250 third gear, my mondeo will only do 125 at same gear and same RPM, is that only due to tire size ???

 

but the RS mk1 does around 145 kmph at 6250 third gear, my mondeo will only do 125 at same gear and same RPM, is that only due to tire size ???

The main reason is weight, the Focus RS weighs 1175Kg whilst a Mondeo hatch weighs 1285Kg.

Steve

 

weight would never affect the actual speed at a given RPM, unless u mean the manufacturer did the shortening of the gear ratios or reducing the diameter of the tires in order to allow the mondeo accelerate better

 

also according to carfolio.com, the focus RS 2002 weights 1278 kg

 

weight would never affect the actual speed at a given RPM, unless u mean the manufacturer did the shortening of the gear ratios or reducing the diameter of the tires in order to allow the mondeo accelerate better

OK so you are assuming both cars have already reached the top speed? if we are talking about a purely theoretical case here of two identical cars apart from the weight, the lighter car will get to the terminal speed quicker than the heavier one and both will in theory have the same terminal speed, however this is not true because the heavier car will put more weight and hence more friction on its drivetrain components so would be slightly slower.

So the short answer is if both cars have identical power then the different terminal speeds are due to gearbox/ final drive ratios, tyre size and type (material and tread configuration), aerodynamics of the car (main factor), drivetrain configuration, friction losses and frontal area of the car.

Steve