Diff Ring Gear Question

[goodnigh]

The tooth count in the picture is obviously different.

What does the 2.75 vs. 3.50 designation mean?

Just another rhetorical question.

mike

 

[CZ-75]

With 2.75, the drive shaft makes 2.75 rotations for every single rotation of the wheel. With 3.5, the drive shaft maked 3.5 rotations for every single rotation of the wheel.

Therefore, a 3.5 rear end will allow for a quicker "off the line" lunge as compared to the 2.75... But the 2.75 rear end will allow lower RPMs at the same speed, which is good for highway speeds.

 

[72HCODE]

like CZ-75 said.

A tall ratio is a lower number and short ratio is a larger number. a 2.75:1 is a tall ratio. and a 4.11:1 is a short ratio.

what it means is the pinon gear is bigger on a tall ratio 1:1 being the tallest ratio. the closer you get to 1:1 the faster the top speed, and the slower the acceleration.

the closer you get to 6:1 the lower the top speed and the faster the acceleration.

basically you are dividing the ring gear teeth by the pinion gear teeth to get the ratio so if you had 80 teeth on the ring gear and 25 teeth on the pinon you would have a 3.2:1 ratio. if you took the same ring gear and placed a smaller 20 tooth gear pinion with it you would have a 4:1 ratio. the 3.2:1 ratio would yield slower acceleration and a higher possible top speed for the same engine rpms. the 4:1 ratio would produce faster acceleration, but limit your top end speed.

this is why people like the 3.25-3.5:1 range of gears. it is a popular ratio so you get some burning rubber and at 65mph you are not running the engine at 5000 RPMS at cruise you are around 3000-3500 rpms

if you used a 2.75:1 at 65mph you would be in the 2200 rpm range. 3:1 2500rpm range and so on. a Very short(deep) gear ratio like 5.25:1 would throw you into the 5000rpm range and 65-75 mph might be your top speed.

real world calculations if you have a ford 351C with C6 trans and you use a 4.11:1 ratio in the rear. top speed in 3rd would be 94mph you would be at redline at that point.

change the rear to 2.75:1 and you would be at 140MPH at redline

3.5:1 110MPH at redline.

there are some other factors not taken into account... different wheel sizes, wind resistance, transmission power loss. etc.

So when somebody says a 2.75:1 is great for the highway. it is because at 55mph the engine is pulling maybe 2000rpms. at steady cruise, you will use much less gas.

change to a 4.25:1 and at 55 the engine is pulling 3500-4000rpms and it will use much more fuel for the same trip. but the car will pop a wheelie and smoke the tires constantly.

but that gives you an idea of how and why things work.

 

[bkdunha]

Great gearing write up. This should be bookmarked for future references. Thank you

 

[goodnigh]

Great info. Thanks.

mike