Diff gears explained please?

[undersuspicion]

Just wondering if someone can explain to me the difference in diff gears and exactly what they do? i.e. what do 4.11s have that 3.9s dont or etc.
Thanks

[wraith]

There's countless information if you searched google. But any way. The larger the number the smaller the ratio meaning more acceleration but less top end and vice versa.

For example.
4.11 means for every 4.11 turns there's 1 turn to the wheels.

[old mate]

the diff gears act as a torque multiplier.

in a diff with 3.08 gears, it takes 3.08 turns of the tailshaft to make 1 revolution of the wheels. so when you change the diff gears to 3.9's, it takes 3.9 turns of the tailshaft for 1 wheel revolution. that means that the torque will be multiplied more than with the taller (lower number) gears. when a car is geared shorter (bigger number), it will accelerate faster but wont reach the same top speed. just like when you put a taller diff ratio in, the car will lose some acceleration but gain more top speed.

the reason people change their diff gears is usually to gain more acceleration. in commodores the standard ratio is around 3.08 (depending on the model of course). so they will go to 3.45 - 3.5 - 3.55 - 3.9 - 4.11 to gain some more acceleration without any engine mods. the car will then rev higher at the same speed on the road than before (example my vs ute revs at 2055rpm with 3.08 gears at 100km/h, but if i changed to 3.55 gears it would do 2369rpm at 100km/h). this will possibly hurt fuel economy a little but but the car will accelerate faster than previously and 'feel' more responsive on the throttle.

commodores are geared a little tall from the factory to get a good fuel economy, rather than performance. they are usually geared that they will never reach the rev limiter in top gear so its a bit of a error if you are going for the best performance. using my ute as an example again, it will be able to do a 'theoretical' top speed of around 270km/h in top gear on the rev limiter. we all know that there is no way a standard v6 will ever get it moving that fast, so when you change the diff ratio to a shorter one the maximum top speed drops, but it will be more achievable. if it had 3.55 diff gears instead of the standard gears, it would have a theoretical top speed of around 235km/h, which would be more realistic.

so in reality, going for that change in ratios wouldnt affect the top speed of the car as it never could reach the maximum speed of the gearing anyway, but it increases the acceleration. but the downside is higher rpm when driving at a given speed so fuel economy may worsen.

hope this helps.

[Not_An_Abba_Fan]

The two numbers are the ratio between the crown wheel and pinion. So, 4.11:1 gears, the crown wheel will turn 1 revolution to the pinion gear's 4.11. You can also calculate it by spinning a wheel and counting the revolutions of the tail shaft. 4.11:1 will be 1 complete revolution of the road wheel to 4.11 revolutions of the tail shaft.

The higher the first number the less effort the engine needs to get the car moving, but the higher it will rev at speed. This ratio is used for quick launches IE: drag cars, lower ratios like 3.23:1, or 3.08:1 are more for cruising where the engine is revving relatively slowly at speed.

(Beat me to it)

But I will say that gears do affect top speed. The higher the ratio the slower the top speed will be, the lower the ratio or as some people say, the taller the gearing, the higher the top speed will be.

[undersuspicion]

No worries thanks for the assistance guys. So if I was wanting a bit more performance but not losing too much top end speed my best bet would be around 3.9s?

[old mate]

what sort of car? what engine/trans/wheel size/tyre size?

all plays a part of choosing diff gears

[undersuspicion]

would be looking at putting in either a vx ss/clubbie or a vy ss/clubbie... 5.7litre. 17 inch rims/225/45 probably tyre size. Most probably auto tranny

[DrZ]

with a set up like that the only next question isss.... what do u want out of ur car? saying its an auto i would say 3.7 is better as its more streetable.. ie u wont b chewin ur poket outta dough for fuel on highway like 4.11 would.. but if u dont do much highway.. go 3.9..

I prob wouldnt go near 4.11s if ur in an auto.. (unless ur makin a dragster) holden autos have a short gear ratio as it is, so shortening it even further with 4.11s cld end bad.. if u do go manual though.. as it sounds like u dont have the car yet.. instead of 3.7-3.9 for an A4 change the range to 3.9-4.11 (for M6) as that final gear is rather helpful..

DrZ

[ari666]

The two numbers are the ratio between the crown wheel and pinion. So, 4.11:1 gears, the crown wheel will turn 1 revolution to the pinion gear's 4.11.

this answer is correct. all the rest are wrong.