Gear Ratio Terminology


Gear Ratio Terminology

The terms higher and lower can get a little confusing when reffering to rearend ratios. If someone changes to a higher gear ratio they are actually changing to a numerically lower ratio. If you were to swap a 2.93:1 gearset with a 3.42:1 gearset you would be switching to a lower gear even though the ratio of driveshaft turns to turns of the wheel is higher. If you swap a 3.42:1 with a 2.93:1 you are switching to a higher ratio. I don't how high ended up meaning low numerically or low ended up meaning high numerically, but they do.

A higher gear ratio would improve mileage and lessen wear on the engine somewhat but at the cost of less acceleration. A car is also capable of a higher top speed with a higher gear. Cars that are meant to do a lot of interstate cruising have higher gears, generally in the 2.48 - 3.08:1 range. Cars meant for acceleration have lower gears, usually something numerically above 3.08:1.

A numerically higher ratio will cause the same tire travel but at higher RPM. For example, a 3.42:1 ratio will cause the same tire travel but at higher RPM than a 2.93.

A lower gear (3.42:1) will require a lesser amount of torque than a higher gear (2.93:1) to accelerate at a given rate. A higher gear will require more torque to accelerate at a given rate.

An engine that puts out a small amount of torque will generally be better off with a lower gear ratio. An engine that puts out a lot of torque will accelerate quite well with a higher gear ratio. That's why the 4500lb 1970 Delta 88 with the police package could run 0-60 in something like 6.9 seconds despite having a 2.93:1 rearend ratio. It had the 455 with 500 ft/lb of torque and didn't need a very low gear to get up to speed quickly. (it also had an estimated top speed of 140). Those of us who just want insane acceleration put a low gear behind a big 'ol 455 in a light car and then spend the rest of their days trying to figure out how to eliminate tire spin!

[ Thanks to Greg Pruett for this information ]

Determining Gear Ratio

You will have to put the rear up on jackstands, put the transmission in neutral. Make sure that the car is quite stable, get under the car, and TURN THE DRIVESHAFT. Count the number of turns of the driveshaft it takes to turn one wheel one time. If you turn a wheel, instead of turning the driveshaft, you will only turn the other wheel (not the driveshaft).

It also works better (more accurate) if you turn the driveshaft enough times to turn one wheel ten times, and count the number of times you turn the driveshaft. This is because it may be difficult to tell if the driveshaft goes around, say, 2.56 or 2.78 times for one turn of a tire. But if you turn the driveshaft enough times to turn a wheel ten times, you will have 25.6, or 27.8, or 41.1 (or whatever) turns of the driveshaft. Then just divide by ten to get the ratio of the rear.

Note also that there are two different ring gear carriers, one for 3-series ratio gears and one for 4-series gears. The difference is the dimension from the ring gear mounting flange to the pinion centerline, due to the larger diameter pinion required for the lower numerical gear ratios. A set of 4-series gears can be installed in the 3-series carrier with a spacer and longer bolts (not desireable due to reduced strength), but the reverse is not true.


These formulas are for giving you a good idea of what to expect not 100% accurate, but pretty close. Some things you need to remember is that if your tire is 25" standing still it will be taller at speed. How much depends on tire construction, rubber compound, inflaltion. It will be shorter on acceleration depending on engine power, wheel spin. Then you have tranny slippage and probably other small variables.

To figure rpm: 168 * gear ratio * mph / tire radius
To figure mph: tire radius / 168 * rpm / gear ratio
To figure gear ratio: tire radius * rpm / 168 / mph
To figure tire radius: 168 * mph * gear ratio / rpm
[ Thanks to Daren for this information ]

N-bolt Terminology

The tern "n-bolt", as pertaining to the rear end of the car, refers to the number of bolts retaining the ring gear to the carrier inside the differential. Fortunately (or unfortunately), this number usually corresponds to the number of bolts holding the differential cover, so you can usually tell whether you have a "10-bolt" or "12-bolt" rear from the number of bolts on the outside of the casing. This is independent of the rear end gear ratio.

The Type O (Oldsmobile) 12-bolt rear ends use a smaller ring gear than the Type C (Chevy) 12-bolt, resulting in lower strength. The good news it that the Type O units retain the axle shafts at their outboard ends, while the Type C units use the infamous c-clips at the imboard end. This is why you see C-Clip Eliminator kits available for the Chevy axles. Also the Chevy rear has a "scalloped" cover where the Olds is smooth.

N-bolt Differences

An Olds 12 bolt rear end is not a "true" 12-bolt. It may have 12 bolts on the outside cover, but it is a 10 bolt gear on the inside. The more common type "C" is very much different than the type "O".

A "true 12-bolt" has come to mean a Chevy c-clip 12-bolt rear. Oldsmobile 12 bolt rear ends differ from Chevys internally. In other words, the guts of a Chevy rear end will not fit in an Olds rear end housing, and vice versa. The phrase, "more like a 10-bolt", is usually talking about the size of the ring gear and/or strength of the carrier. The guts of a 10 bolt would not work or fit in a 12 bolt, and vice versa. There are differences between the 10 bolt and the 12 bolt both internally and externally.

Note also that there are two different ring gear carriers, one for 3-series ratio gears and one for 4-series gears. 3 series and 4 series carriers are for the 12 bolt Chevys. The difference is the dimension from the ring gear mounting flange to the pinion centerline, due to the larger diameter pinion required for the lower numerical gear ratios. A set of 4-series gears can be installed in the 3-series carrier with a spacer and longer bolts (not desireable due to reduced strength), but the reverse is not true.

The ring gear size is the size of the largest gear, parallel to the wheels. A 12 bolt Olds has a 10 bolt carrier that measures 8.3", whereas a 12 bolt Chevy has a 12 bolt carrier that measures 8.8". A 10 bolt Olds out of a 1971 has a 10 bolt that measures 8.5". 1968 and 1969 10 bolts might have an 8.2" ring gear. Starting in 1970, 10 bolts might be 8.5". Newer 10 bolts found under the last RWD Cutlass and Chevy are little 7.5".

Oldsmobile bolted the axles from the inside of the housing, whereas Chevy used c-clips. As for Buick and Pontiac, they used different types of posi actuating items called "cones" and "spools", whereas 12 bolt Chevys and Olds used what are called "clutch packs".

Type "O" Oldsmobile Rear Ends

The biggest weak spots in Olds rears are the wheel bearings and the availability of parts. Positive axle retention is good, but put in the new "updated" style wheel bearings for more strength.

Gears for the Olds 12 bolt rear are hard to find. The "O" was only manufactured for 2-3 years. So there isn't enough call for someone to start casting and machining new cases. While posi cases are not available (as far as I know) rebuild kits are. Unless the case you have is outright cracked or otherwise junk you should be able to rebuild it.

Type "C" Chevy Rear Ends

This rear end retains the axle shafts with "C" clips. These do break from time to time, and the result is the axle shaft works its way out of the axle tube. A hint that a clip might be broken is a faint moan from the rear axle when backing up (mine made no noise going forward). Moral of the story: stick with your Olds rear.

The "C" rear can be modified with "a C-clip eliminator kit". That solves that problem of relatively fragile broken "C" clips.

If you want/need a stronger more available rear, the "C" 12 bolt, will bolt in with no changes to your car. It came in the C*evelle after all. The rear U-joint will need to be changed to a hybrid one. NAPA and others carry such a U-joint.

Given the "C" 12-bolt is almost a 9" ring gear, parts are as common as sand on the beach, and almost as cheap. It has some redeeming values once modified for performance. NHRA won't allow the faster cars to run one without the eliminator.

Type "B" Buick Rear Ends

Type "P" Pontiac Rear Ends

Ford 9" Rear End

The 9 inch Ford is the setup these days for many of the same reasons as the Chevy; plus you can change gears by yanking the axels and swapping in the "pumpkin". But getting one setup to bolt into the "A" or "G" bodies is much more expensive at the outset.

The best will have a large "N" cast into the front of the case, between the webs. This indicates it is made of nodular iron, as opposed to slightly weaker regular iron.

Get a 9" out of the boneyard for the axles and the center section, and buy a custom housing from Moser Engineering or Currie Enterprises with your exact suspension system mounts. You could also use the boneyard housing and narrow it, but, the two places I mentioned do it all for you.

Pay particular attention to the brakes and wheel bolt pattern on the 9" you pull out. You could need a redrilled bolt pattern to match your wheels. You can weld brackets onto the narrowed rear yourself, but I think this is something worth farming out, unless you are/know a good welder.

[ Thanks to Cliff Feiler, Brian Kennedy for this information ]

Differential Usage

Actually, 1968 to 1970 Olds 442's (and I believe all Olds for that matter) built in the US plants all had the Type O rear axle. All of the 442's built in the Oshawa, Ontario plant had the Type C (Chevy) axle. In 1971 they went to a GM corporate 10 bolt.

The ring gear size in the 1985 to 1987 442's is the 8.5". Very desireable. The same is used in the Grand National. The Monte SS has the 7.5/7.626" rear. Not nearly as hot as the 8.5" rear.

There were a number of differing 10 bolt differentials made during the 1970's by GM. And all the internals are different - nothing swaps.

Posi or Open Carrier

With both rear wheels off the ground (or front wheels for you Toro-heads!), turn one wheel. If both wheels turn in the same direction, then it is a posi. If they turn in opposite directions, then it is an open carrier or blown posi.

A posi differential will have a metal tag attached to a lower right hand cover bolt warning about the type of diff. oil to use. Open diffs usually have no tags, except Chevy diffs usually have a code and ratio tag attached to a lower left cover bolt.

Posi (short for posi-traction) is an option that is not specific to amount of bolts (ie. 10 bolt or 12 bolt), but was a factory option. You can change a single track car to posi by putting in a positraction differential. In other words you can put a posi unit in a 10 or 12 bolt, as long as you have the right posi unit. There are posi units for 10 bolts, and posi units for 12 bolts (not interchangeable).

Clutch and Cone Anti-Spin

The cone units have advantages and disadvantages versus the clutch type units. The most immediate advantage is that they seem to work fine with most any gear lube. Friction discs and clutch plates are not used. Resistance is achieved by the steel cone (which has a series of raised spiral groves) rubbing on the smooth steel case. Under acceleration the cone is forced into the case which results in limited slip. I have never had to use lubes specifically formulated for posi units in my cone posi's. Even though the car came with a limited slip lube tag on the housing. Mobil synthetic works fine, as is. But doesn't work fine in the C rear without additive.

When a cone posi wears out it is because the cone has bottomed out in the case. While you can't make it good as new, you can restore it to like new or better limited slip action by grinding the lip off the bottom of the cones and deepening the recess in the case. Olds/Pontiac cone posi's use a split case. Open it up by removing the eight (I think) grade 8 bolts which hold the two sections together. Grinding a ¼ total separation between the case and cone is more than enough. I then use side bearing shims to take up the slack in the gears. These are the same shims you need to use to set backlash. You put them between the splined cone and splined driven sections on each side. Installation is a lot easier if you use an axel shaft to keep the splines aligned. If you over pack the shims, the cone unit will actually act like a locker style rear. Been there, done that and changed it to a more reasonable preload.

You can also shim, modify or replace the triple coil springs which serve as preload. I've used an old exhaust valve to increase the preload once. Increased it a LOT. The case halves should go together with some preload, just don't over do it.

I don't know how long one restored this way will last, I'm still using one after several years, lots of street use and some drag racing. It still works great.

All the ruined cone type posi's I have seen were severly abused. They should last quite a while if not abused. I still have the _original_ cone posi that shipped with my '67 in the car. It has never worn out. The restored unit cost me $10 bucks from a Pontiac guy in Hemmings.

If I had access to either cone or friction plate posi's I would go with the cone, first because the original poster of this question stated he was told they are cheaper. Next, in my experience, the cone works as well or better than the more common design. And, given the smaller ring gear, they are stonger than the other designs (four pinion gears is much more than double the strength of two). The case is also almost full circle, there is no need for a large opening through which the plates and frictions are installed. Only enough space to ensure full lubrication is needed.

FWIW, this is the type of posi Olds used for the 66-67 W-30's. As noted above, I've never broken it.

In the final analysis, both designs work and will last a long time if not severely abused.

There are also other designs such as locking, Torsen (sp?), air lockers and others I am sure. I have no experience with these units but they receive rave reviews in the 4X4 mags. Just remember that magazines have two main functions. To show you good stuff so you'll buy the mag again and to sell the goods their advertises pay them to push. The latter usually prevails.

The 3.55 posi is almost certainly a two pinion posi, while good, it is not extremely strong. It is also a cone type posi unit. If that is shot it cannot be rebuilt in the same fashion as the plate/friction type. They can however be made functional with a little work. The 4-pinion posi case is pretty good, and a lot stronger. You can not, without swapping parts around, install anything lower than 3.55 gears. The case changes for the 3.90 and lower gears (higher numerically).

[ Thanks to Bob Handren for this information ]

Locker Differentials

So you want a locker. Don't do it, you don't need it. Just buy the Ford Motorsport 4 pinion traction-lok diff. Its about $100 cheaper, and is much better suited for a mostly street and some strip car. A "Detroit Locker" is streetable but only for true weekend warrior street/strip cars. The are noisy, and tend to cause the car to plow in corners (cause the axles are "locked" most of the time) unless you are taking a real sharp turn.


The 1965 thru early 1967 axels are weaker than later axles.

In the late '70s and early '80s, GM converted axle shaft retention from bolted in, to being held in with c-clips in the differential housing. There was a problem with some axles. The groove in the axle shaft that the c-clip held on to was machined too wide. This allowed excess endplay in the axle shaft. Over time, the clip and the groove would wear due to the excessive side movement and the clip would fall out. Then the axle shaft comes right out of the axle carrier.

[ Thanks to Bob Handren, Steve Ochs for this information ]