Final Drive Ratio

A

ll Kas destined for the United Kingdom market with power steering leave the factory with a final drive ratio of 3·588. Those early Kas without power steering were given a taller final drive, with a quoted model number of 4·059, and as such, these cars benefit from taller overall gearing and lower fuel consumption.

Given that I’m tight /cough/ I’ve been considering replacing Kermit’s final drive in order to lengthen his gearing, and save some fuel (around six percent, if Ford’s figures are to be believed). Another advantage of doing this is that the engine will be a little quieter at a motorway cruise, and the intermediate gears will allow higher speeds to be reached.

Of course, with these benefits, you also get some negative points. The first is that the Ka is likely to lose some of its immediacy around town. Although outright performance will be unchanged, in-gear acceleration will be reduced - it will probably add a second or so to the 30 to 50 time in fourth gear, for example. It will also reduce bottom end flexibility - basically, when driving about town, the Ka will need more gear changes.

It is possible that lengthening the gearing will actually reduce the quarter mile time of Kermit, since it will mean more time in first and second, and less time in third. In any event, any difference will be slight.

As to the technicalities, the simplest way to adjust the final drive is to simply replace the gearbox. Second hand Ka gearboxes are cheap and reasonably plentiful, and if there’s nothing wrong with the one we have, it may be possible to simply swap the part. Then all we need to afford is the labour...

More Notes: July 2004

There are two final drive ratios used with the Ka, but the IB5 has the advantage that it’s used in several cars, ranging from the Ka, through the Fiesta, Puma and Focus.

This means that if one doesn’t want to use the non-PAS final drive, or the shorter ratio of the PAS Ka, one can use any other final drive.

Fortunately, a number of Fiestas use a final drive somewhere between the two Ka extremes - the diesel and XR2i use a 3·88 final drive, which is approximately half way between the two extremes. And this in turn makes it handy!

The advantages of having a final drive mid-way between the two extremes is that Kermit should get most of the benefits of taller gearing with few of the disadvantages.

Advantages

• Lower engine noise from a moderate cruising speed upwards.

• Lower fuel consumption.

Disadvantages

• Slower in-gear acceleration.

• Less flexible engine.

• Less responsive engine.

Kermit’s usual commute involves a thirty mile drive across mixed roads, varying from urban streets to a very brief stretch of dual carriageway.

Gear Ratios – miles per hour per thousand engine revs

4·058 Final Drive

1: 4·97

2: 8·14

3: 12·24

4: 16·48

5: 20·74

3·88 Final Drive

1: 5·28

2: 8·65

3: 13·00

4: 17·51

5: 22·03

3·588 Final Drive

1: 5·62

2: 9·21

3: 13·84

4: 18·64

5: 23·45