gear ratio explanation please...

[sasilverbullet]

My son and I were having a conversation about gear ratios in differentials.

If someone says they have a "3.00" rear end, does that mean that for every three turns of the drive shaft, that the drive shaft makes one revolution? Or is it that the wheels make one revolution? (at the outer edge)

Then, if someone has a "2.93" rear end, and another same car a "3.00" rear end, which is better for towing? The 3.00 right?

Please be very detailed with your answer, my son is a little genius and I have to explain everything to him in great detail!

 

[bullwinkle]

For a 3.00-1 ratio, three turns of the driveshaft=1 turn of the axle(s). The higher the numeric ratio, the higher the towing ability, but higher final drives also mean lower top speed and lower MPGs. For instance, my '89 F-450 originally had a 5.13-1 ratio, which severely limited top speed-at 65 MPH in 5th it would be SCREAMING and drinking fuel (tach doesn't work, but sounded like 3300+ RPM, close to redline). Now, with the 3.73-1 gear that's in it, it cruises fairly well at 65 and can actually make it to 75.

 

[Lead Shoes]

Looking forward to learning something here

 

[NHGUY]

In the 1970s,the ratios went down as low as 2.45:1.That was Chrysler's plan to boost mileage.All it really did was kill acceleration and add top end speed while putting a huge strain on the auto transmissions to push along 4000+lb goliaths with little to no mechanical advantage. In the wartime Jeeps,ratios went sky high to add torque and low gear crawling ability to the weak engines.Top speed was ruined though....45 mph and the valves were floating....

 

[scurvy]

First, the gear ratio is the number of teeth on the ring gear divided by the number of teeth on the pinion gear. Let's say 47 on the ring gear; 12 on the pinion: 47/12 = 3.92 gear ratio.

This means the pinion gear rotates 3.92 times every time the ring gear rotates once (which makes sense as the pinion gear is smaller than the ring gear). But track outward from the differential and you'll see that the engine is connected (eventually, through the transmission, transfer case & driveshaft) to the pinion and the tires are connected to the ring (through the half shafts, hubs & wheels). So 3.92 revolutions of the tires give 1 revolution of the driveshaft.

If you want to be sensible and look at it from the driveshaft point of view, just take the reciprocal of the gear ratio. With the numbers from above, every revolution of the driveshaft gives you 1/3.92 = 0.255 revolutions of the rear wheels. And that is why numerically larger differential gear ratios make for better towing - they are effectively a lower gear ratio when referenced to the driveshaft/transmission/transfer case/engine.

 

[Errtt]

^
Originally Posted By: scurvy
So 3.92 revolutions of the tires give 1 revolution of the driveshaft.


Wouldn't it be the other way around?

 

[HTSS_TR]

Originally Posted By: Errtt
^
Originally Posted By: scurvy
So 3.92 revolutions of the tires give 1 revolution of the driveshaft.


Wouldn't it be the other way around?

It should be "3.92 revolutions of the driveshaft give 1 revolution of the tires"

 

[Errtt]

Just think of it as ratio for transmission gears, bicycle (10-speed) or something.
I once had a Blazer and it had something like 5:25, not very much top end speed, but got me around in the boonies fine. Putting the case in low and in 1st was like useless unless I need to crawl through some scrappy spots. I couldn't hold up well to others on the freeway. Had a lot of birds flipped at me because I wasn't going to let the engine scream and hold it there.

 

[scurvy]

Whoops, fat fingered it. Had it right in one part of my writeup but not in another place.

Originally Posted By: Errtt
Wouldn't it be the other way around?

Originally Posted By: HTSS_TR
It should be "3.92 revolutions of the driveshaft give 1 revolution of the tires"


Quite right. One internet for both of you!

 

[Colt45ws]

In reality you would never notice the difference between the 2.93 and 3.00. They only make certain ratios selections for axles. Some common ones are 2.73, 3.08, 3.27, 3.31, 3.55, 3.73, 3.92, 4.1, 4.56, 5.13..
The actual selection varies dependent on the application. Truck axles will have the deeper (numerically higher) end, and the cars will have the lower end. I think someone told me the IRS in a Dodge Charger is 2.42 or something like that. But, the trans has a really deep 1st so the gear is in there.

 

[55Test]

Originally Posted By: NHGUY
In the 1970s,the ratios went down as low as 2.45:1.That was Chrysler's plan to boost mileage.All it really did was kill acceleration and add top end speed while putting a huge strain on the auto transmissions to push along 4000+lb goliaths with little to no mechanical advantage. In the wartime Jeeps,ratios went sky high to add torque and low gear crawling ability to the weak engines.Top speed was ruined though....45 mph and the valves were floating....


Dodge used a 2.24 RAR even on mid 80's Diplomats with police packages!

 

[Colt45ws]

Originally Posted By: 55
Originally Posted By: NHGUY
In the 1970s,the ratios went down as low as 2.45:1.That was Chrysler's plan to boost mileage.All it really did was kill acceleration and add top end speed while putting a huge strain on the auto transmissions to push along 4000+lb goliaths with little to no mechanical advantage. In the wartime Jeeps,ratios went sky high to add torque and low gear crawling ability to the weak engines.Top speed was ruined though....45 mph and the valves were floating....


Dodge used a 2.24 RAR even on mid 80's Diplomats with police packages!

*Vomit* Ive heard of early 80's CVs being sold with 255s and 2.73s. Simply criminal.

 

[TFB1]

Originally Posted By: Colt45ws

*Vomit* Ive heard of early 80's CVs being sold with 255s and 2.73s. Simply criminal.


I dunno about that but since Ford's first overdrive auto trans(AOD) became available in 1980, I kinda doubt it...

In the late '80s to early '90s, the civvy CV had 3.08 and later 2.73, while the tow package and ahem "performance models" got 3.27... The same vintage CVPI mostly got 3.27 but did have 3.55 a few years...

 

[supton]

Here's some fun stuff for you: RAV4's get 3.08 finals, while CRV's get 4.44:1 finals. The I4 RAV doesn't bother with overdrive in the 4spd auto, while the V6 does get a mild OD. Yet the CRV gets not one but two overdrive gears! Similar overall effect though, as the CRV simply uses a less deep first gear.

Even more fun: Neither uses the same FD in the front and rear differential! Both use the transfer case to gear down the drive to the rear end so as to make up for a steeper rear end. I'm sure the overall effect is numerically the same; and if not, then the viscious coupling used makes up the difference (plus, it seems on these setups it is generally 100% FWD until slippage is detected--I don't know if Subaru uses different front/rear FD's but the automatics have run 80/20% power split for some time now).

RAV4: Transfer case: http://www.vibratesoftware.com/html_help/2011/Toyota/Toyota_Transfer_Units.htm#GF1A
RAV4: Gear Ratio: http://www.vibratesoftware.com/html_help/2011/Toyota/2009/2009_Toyota_RAV4.htm

[I've misplaced the links for the CRV and Ridgeline, but I'm 99% sure they are the same. It may well be that most setups that use a viscious center coupling (aka transfer case) uses dissimilar front/rear end ratios, in some attempt to reduce fuel consumption--spin the rear driveshaft slower, and save something.]

The point is, rear end ratio is but one part of the puzzle. The transmission first gear ratio, and final drive, and the jump between gears, all comes into play. Also, the torque convertor of conventional automatics can make a big difference: due to the torque multiplication under slippage, it was common (for years) for automatics to get by using one less gear than the similarly-equipped manual transmission car. A manual transmission must slip the clutch--no torque multiplication there--and therefore has to use a deeper first gear. Also, it was in vogue (still is) to use the locking torque convertor as a "half gear" or something: many vehicles will unlock the convertor when a hill is encountered, so as to get a slight boost in engine rpm's.

What Packard was it that got by on a single speed transmission? Huge V8, used a locking torque convertor and one speed! Early 50's IIRC.

 

[mechtech2]

Originally Posted By: Errtt
^
Originally Posted By: scurvy
So 3.92 revolutions of the tires give 1 revolution of the driveshaft.


Wouldn't it be the other way around?

Note scurvy's long and pretentious explanation to arrive at the wrong conclusion!


sasilverbullet -
The higher the rear end or final drive #, the more the engine revs for the same vehicle speed. You generally get better acceleration and pulling power. 2.93 to 3.0 would hardly be felt. It takes a change of 2.75 to 3, or 3 to 3.25 [at least!] to make a difference really noticeable.
You are right about the driveshaft turning about three times as fast as the axles.
Most cars have transaxles now, with no drive shaft. They still have a final drive, with an input and output ratio.
A high numerical ratio is called a 'LOW' ratio rear end, and a low numerical ratio is called a 'HIGH' ratio rear end [final drive].

 

[scurvy]

Originally Posted By: mechtech2
Note scurvy's long and pretentious explanation to arrive at the wrong conclusion!


The OP asked for a detailed explanation. I tried to give a similar explanation that I would do for my kids - detailed enough to understand the principles without going into minutiae.

I freely admit I swapped the drivetrain bits and the end of my writeup and missed the edit timer, even gave credit to the two after me that politely pointed it out.

I was just answering the OP.

My apologies if I offended you with a simple error, but it looks like you were well offended even before that.

Have a nice day.

 

[sasilverbullet]

Thanks everyone for the explanation! I ran this by my uber smart son and now he's happy!

 

[Gotch]

Originally Posted By: 55
Originally Posted By: NHGUY
In the 1970s,the ratios went down as low as 2.45:1.That was Chrysler's plan to boost mileage.All it really did was kill acceleration and add top end speed while putting a huge strain on the auto transmissions to push along 4000+lb goliaths with little to no mechanical advantage. In the wartime Jeeps,ratios went sky high to add torque and low gear crawling ability to the weak engines.Top speed was ruined though....45 mph and the valves were floating....


Dodge used a 2.24 RAR even on mid 80's Diplomats with police packages!


I had a 1979 Pontiac Grand Prix (my first car). Axle ratio was 2.14:1, the highest gear ratio ever installed by the big 3. When I swapped the original 301 2bbl for a SB 350 Chev with a 5700rpm redline, it would go 70mph in first gear. Total slug out of the hole. Swapped the 2.14 for a complete 3.73 axle out of a Monte SS and what a difference! Knocked 3sec. off of quarter mile (17s to low 14s). Strangely enoungh, fuel ecconomy improved due to not lugging the engine so much...

 

[SteveSRT8]

Same story in my past with a 78 Firechicken. Had a 2.56 and swapped it out for a 3.73 one day. It was amazing.