Tech Question - Gear Ratios

.Longshanks.

Is it possible to work out what rev's a car will be doing at, say 120KMPH in 6th gear, if you know that the gear ratios are?

Gear ratios: 1st – 3.82:1, 2nd – 2.16:1, 3rd – 1.48:1, 4th – 1.07:1,
5th – 0.88:1, 6th – 0.74:1, Final drive – 4.18:1

Never understood how those figures worked. Whats the difference between 6th and final drive?

Tea drinker

Final drive doesn't equal top gear.
final drive is a sort of multiple for all the gears....I think it it's the gearing at the differential - but not sure

Alun

Tea drinker wrote: »

Final drive doesn't equal top gear.
final drive is a sort of multiple for all the gears....I think it it's the gearing at the differential - but not sure

That's right.

Also, in order to work out road speed, you'll have to know the rolling radius of the tyres.

crosstownk

No matter what gear is selected, the final drive reduces it further -4.18:1 in your example. Essentially, the final drive is always in the equation.

Alun wrote: »

That's right.

Also, in order to work out road speed, you'll have to know the rolling radius of the tyres.

I'm being pedantic, but surely you mean 'rolling circumfrence'?

Pete67

Circumference can be calculated from the rolling radius, which is easy to measure - measure vertically from the ground to the centre of the driveshaft where it passes through the front wheelhub (FWD).

Circumference is 2 * PI *r where r is the radius as above. PI is a constant with approximate value 3.142

Worked Example: (guessing rolling radius = 285mm, substitute your own measurement)

Rolling radius: 285 mm
Rolling circumference = 2*3.142*285 = 1790.7 mm = 1.7907 m
Road speed = 120 kph = (120/3.6) = 33.33 m/s
Wheel angular velocity = (33.33/1.7907) revs/second = 18.615 rev/sec
Wheel rpm = rev/sec * 60 = 18.615*60 = 1116.9 rpm

Final Drive ratio = 4.18:1 so if wheel is rotating at 1116.9 rpm them gearbox output shaft is rotating at (1116.9 * 4.18) = 4668.5 rpm

6th gear ratio is 0.74:1, so if the gearbox output shaft is rotating at 4668.5 rpm then the input shaft is rotating at (4668.5 * 0.74) = 3454.7 rpm. And of course the input shaft is rotating at the same speed as the engine, so this is the engine speed.

Changing down to 4th at the same road speed would give an engine speed of (4668.5*1.07) = 4995.3 rpm.

Changing to second at 120 kph, if you were stupid enough to do it would give an engine speed of (4668.5 * 2.16) = 10,084 rpm and a large repair bill!

.Longshanks.

Pete67 wrote: »

Circumference can be calculated from the rolling radius, which is easy to measure - measure vertically from the ground to the centre of the driveshaft where it passes through the front wheelhub (FWD).

Circumference is 2 * PI *r where r is the radius as above. PI is a constant with approximate value 3.142

So on a car with 17" wheels and 215/45 tyres what would the RPM be at 120KMPH in 6th gear? Or is it more complicated than that?

Pete67

See above example..........