All About Gearing

A few years ago I found a 5 speed and matching R200 rear from an '81 280ZX for sale on the internet.  This is widely considered to be the "hot" setup for an S30 with a few more horses than stock, and I snapped it up and added it to my collection of parts. When I started planning my L28 build, I got around to thinking about gearing and looked up the ZX ratios and did some figuring, and was a little surprised - the ZX gears seemed like a poor fit for what I was after. It was time to do some math...


Lets start with the basics.

If there is a little gear driving a big gear, the big gear will turn slower than the little one. Imagine the little gear has 10 teeth and the big gear has 40 teeth.  If you watch right at the point where the gears mesh, one turn of the little gear means 10 teeth of the little gear will go by. Since the teeth on the two gears interlock, 10 teeth of the big gear have to go by as well. That means the big gear has only turned 10/40 (or 1/4) of a whole rotation.  The little gear would have to make 4 turns for the big gear to go around once.  We call this a 4:1 gear reduction because the output speed is reduced by 4x.

If you connect the output of one gear reduction to the input of another, the end-to-end gear reduction is found by multiplying the two reductions.  A 3:1 reduction connected to a 4:1 reduction gives a 12:1 end-to-end reduction.

Simple physics tells us that a gear reduction reduces the speed and increases torque at the output shaft - but it can't create energy or power - the same amount of horsepower (or kilowatts if you're metriclly inclined) comes out as goes in. That may not be obvious, but remember power is torque x RPM; a 4:1 gear reduction increases the output torque by a factor of 4, and reduces the output speed by a factor of 4, and the two changes cancel each other out as far as horespower. In the real world, friction between the gears actually consumes a bit of the power transmitted through the gears, changing it into heat. The RPM relationship of input and output is fixed, but the torque (and horsepower) transmitted through the gearbox drops a bit from the ideal. For a typical manual transmission and rear-end the rule of thumb is that about 85% of the engine's power makes it to the wheels.

The speed of an engine's crankshaft is way too fast to drive the wheels directly, so there has to be a gear reduction between the two.  The gears can't make power, but the power generated by the engine goes up the faster the engine turns so having a bigger gear reduction allows more power to be sent to the wheels at any vehicle speed, allowing for more acceleration - but the top speed (at redline) in that gear will be lower.

There is no one perfect gear ratio: a ratio that maximizes acceleration will limit top speed, while a gear that is perfect for highway cruising wouldn't allow the car to start moving from a stop. In your typical Z-car with a manual transmission, there are 4 or 5 gear ratios to choose from to let the driver match the speed and power requirements of the car to the output of the engine.

The ratios available in the transmission combine with the final gear-ratio in the rear-end to determine the overall or end-to-end ratio. For example, 1st gear in the ZX transmission I bought has a ratio of 3.08:1, and the ZX R200 rear end has a ratio of 3.9:1, yielding an overall end-to-end reduction ratio of:

3.08:1 x 3.9:1 = 11.94:1.

That means it takes nearly 12 rotations of the engine for the rear axle to rotate just once.  2nd gear in the same transmission has a ratio of 1.858:1, giving an overall ratio of 7.24:1. The transmission and final drive ratio can have a big effect on the performance and drivability of the car.  Let's dig into the details.

First gear is a bit of a special case as it needs to deal with getting the car moving from a stop.  If we pull away from a stop sign in the usual way - rev the engine to around 1500 RPM and ease out the clutch - the 12:1 end-to-end gear ratio means the car wants to be going about 5mph. The speed of the car can't change instantly to 5mph; the clutch has to slip a bit until the speed of the car catches up to the speed of the engine. Giving 1st gear a big reduction makes the speed the car wants to go low so the clutch doesn't have to slip very much.

With that big reduction, an aggressive (high RPM) launch in 1st gear can easily send more power to the wheels than the tires can put to the pavement - at 5mph it only takes about 20hp to break the tires loose. In everyday driving, you typically shift out of 1st as soon as the speed of the car is enough to use 2nd gear (about 20mph).  After an upshift to a higher gear (a numerically smaller gear reduction) the engine has to slow down to match the speed of the car at the new ratio, which usually means the engine is producing less power in the higher gear.

Which brings us to second gear.  2nd gear is the one chance to closely match the power output of the engine to the power the tires can handle. This is a little surprising; it works this way because engine power depends on engine RPM and the power the tires can handle depends on the speed of the car, and with the right overall gear ratio the two increase in sync. Getting this ratio just right is a big part of a good 0-60mph sprint. The only problem with 2nd gear is that the engine will usually hit its maximum RPM at around 60mph, and we often want to drive faster than that...

Moving on to third gear.  Shifting into 3rd drops engine RPM; that means the engine can't make  as much power as when it was in 2nd gear.  For typical 5-speed gearing, a 2-3 shift at 7000 RPM will drop the engine RPM to (aprox) 4500RPM.

The horsepower required to accelerate the car doesn't change, but having less power available means acceleration drops significantly. If the car was geared for aprox 0.5G of acceleration in 2nd, 3rd gear acceleration will be about 0.3G (which is still more more acceleration than most drivers ever use in street driving).

Something to think about: 3rd gear acceleration is more important than you might think. If 2nd gear is chosen to give maximum acceleration up to 60mph (at roughly max engine RPM), you can't use 2nd for highway speed passing - which can be pretty important! 3rd gear will usually be good to about 90mph.

If  your engine has horsepower to spare - its a turbo or V8 swap - then you might be able to pull 3rd gear as hard as 2nd and go 0-100mph in under 10 seconds. This takes in the ballpark of 400 crank horsepower. Doing this with a streetable NA L6 is pretty unlikely.  More important is that a 5-3 downshift at 70mph puts the engine at the RPM where the cam is working - around 4500RPM.

Lets skip ahead to fifth gear.  This gear is meant for cruising; it is designed so the engine delivers just enough power to maintain the car at a steady 70-80mph at as low an RPM as possible.  5th gear in a Datsun transmission is typically not a reduction at all; it has a ratio of about 0.75:1  (the transmission output shaft turns faster than the crankshaft) that combined with the final-gear ratio gives an overall gear reduction around 3:1.

In a 5 speed transmission set up for the street, fourth gear is basically just a gear between 3rd and 5th; it avoids a big drop in available acceleration when shifting up from 3rd gear, and it allows a downshift from 5th for climbing a hill or modest acceleration at cruising speed for leisurely highway passing.  It also comes in handy for cruising along on surface streets at less than freeway speeds. In a 4 speed transmission, fourth takes the place of 5th as a cruising gear. For reasons that boil down to tradition, in both 4 and 5 speed transmissions fourth gear usually has a 1:1 ratio.  On the street you can't really use much of 4th gear before you're going ridiculously fast - even with a low final drive ratio you'll get to 100mph at less than 5000RPM.

There are a couple take-aways here:

• You can't look at a single rear gear or transmission ratio and think "that's too high" or "that's to low" - it is the combination of transmission and final gear ratios and tire size (more about tire size in a minute) and the RPM range of the engine that determine what makes sense.
• 1st gear isn't all that important: any 1st gear ratio is probably fine.  This isn't completely true if you're running slicks or wide high-performance tires that can handle a hard launch, but anything close to 205mm street tires aren't up to full throttle in 1st gear with any of the common gear ratios you'll run into.
• On the other hand 2nd gear is really important; this is the one place we can really take advantage of high RPM engine power (that we've gone to great lengths to achieve).  If the end-to-end 2nd gear reduction is too small - numerically low - we'll give up acceleration and run out of speed limit before taking advantage of the engine's peak horsepower.
• Keeping the ratio change from 2nd to 3rd as small as possible makes 3rd more useful from a performance standpoint as that will minimize the inevitable power and acceleration drop on a 2-3 up-shift.

In a perfect world, we could hand pick each transmission ratio to match modern tires and engine mods, and while we were at it add an extra ratio or two to help us keep the engine RPMs up at all vehicle speeds - but when it comes to Z-cars practically all we can do is pick one of a few commonly available transmissions and match it with a commonly available rear gear ratio to get the best compromise we can come up with. Lets take a look at some of the more common options.

Common Options

The transmissions and rear ends used in the 240Z, 260Z, 280Z and 280ZX are largely interchangeable, and the 5 speed transmission used in the 240SX can be used with relatively straightforward modifications (with enough time, money and machine tools its possible to swap nearly anything into anything, but we're talking largely bolt-in mods).  Here is a listing of transmission gear ratios:

• 260Z 4 speed: 3.592, 2.246, 1.415, 1.0
• 280Z 4 speed: 3.32, 2.077, 1.308, 1.0
• 280Z (early) 5 speed: 3.32, 2.077, 1.308, 1.0, 0.864
• 280ZX (late) 5 speed: 3.062, 1.858, 1.308, 1.0, 0.745
• 280ZX turbo (T5) 5 speed: 3.5, 2.144, 1.375, 1.0, 0.78
• 240SX 5 speed: 3.32, 1.902, 1.308, 1.0, 0.759

And here is a list of widely available rear gear ratios.  I've included a few Subaru R180 rear-ends that can be fit into the early Z-car with minimal mods:

• Nissan R180 : 3.364, 3.545, 3.90, 4.11, 4.375
• Nissan R200 : 3.545, 3.70, 3.90, 4.11, 4.375
• Subaru R180 : 3.90, 4.44:1

Most cars with small displacement engines and manual transmissions are geared so that the engine hits peak horsepower at 60mph in 2nd gear. This isn't a hard and fast rule; if  the engine has horsepower to spare (e.g. a stout L31 stroker) you could gear the car to reach a higher speed (like 65 or 70mph) at the top of 2nd gear (in my How Fast is Fast article I gave guidelines for figuring out how much horsepower is needed to reach various speeds).

But to get a specific number we first need to take that diversion to look at tire sizes.  The outside diameter of the driving tires tells us the speed of the car at a specific axle RPM.  A stock Z-car tire is about 2 feet in diameter; multiplying by pi (3.14159...) tells us the distance around the outside of the tire (the circumference) - about 6.28 feet. Every revolution of the rear axle moves the car forward 6.28 feet.  Lets say the axle is turning at 1000RPM; after 1 minute the axle has turned 1000 times and the car has moved forward 1000 x 6.28 = 6280 feet - a little over 1 mile in a minute (it works out to about 72mph). Combining all of the unit conversions lets us write:

tire-diameter-in-inches x axle-RPM / 336 = speed-in-mph

We  know that the axle-RPM is just the engine-RPM divided by the total gear reduction.

axle-RPM = engine-RPM / end-to-end-gear-ratio

So we can do some simple algebra and find:

end-to-end-ratio = (engine-RPM  x tire-diameter-in-inches) / (speed-in-mph x 336)

Now we can plug in some numbers: lets say we've got 25 inch diameter tires and we've built our engine for peak horsepower at 6500RPM, and we want to hit that RPM at 60mph:

end-to-end-ratio = (6500 x 25) / (60 x 336) = 8.1

One thing this tells us is that a smaller diameter tire requires a numerically smaller gear ratio to get the same speed-to-RPM relationship; mounting a smaller tire is like installing a lower rear gear - without having to tear the rear suspension apart.

Is there a way to find the tire diameter without visiting a tire store with a tape-measure?  The diameter is actually encoded in those mysterious numbers in the tire-size.  For example, a stock S30 tire is a 195/70-14.  The "14" is the diameter of the wheel in inches, "195" is the width of the tire in mm, and "70" tells us the side-wall height as a percentage of the tire width.  So the outside diameter in inches is:

((195 mm / 25.4 mm/inch)  x .70 x 2) + 14 inches = 24.75 inches

Note that going up a size in tire-width - for example swapping from a 195/70-14 to a 205/70-14 will increase the outside diameter; in this case about 14mm or .5 inches. If you want to run a wider tire and keep (approximately) the same diameter, you also need to switch to a lower profile tire; in this example, a 205/65-14  (which is an odd size you're unlikely to actually find).

((205 / 25.4) x .65 x 2) + 14 = 24.5

Tire makers are a little loosey-goosey with the actual dimensions of their tires so using the tire size numbers is a bit of an approximation, but it at least gives us a starting point.  If instead we used a low-profile track tire, like a 205/55-14, the diameter is:

 ((205 / 25.4) x .55 x 2) + 14 = 22.9 inches

A lot of Z-cars are moving to 16 inch tires (on aftermarket wheels) just for the bigger tire selection. A 205/55-16 is a pretty good match for stock size and widely available; 205/50-16 and 205/45-16 are also fairly common if you like the modern short-sidewall look.

((205/25.4) x .55 x 2) +16 = 24.9 inches

((205//25.4) x .50 x 2) + 16 = 24.1 inches

((205/25.4) x .45 x 2) + 16 = 23.3 inches 

These may seem like tiny differences, but swapping a 205/55-16 for the 205/50-14 track tires is equivalent to swapping a stock 3.54 rear gear ratio to a 3.9:1 ratio.

Finding 16 inch performance tires wider than 205mm in a size that works for the S30 can be a bit of a challenge as modern high performance cars have moved to 17 or 18 inch (or larger) wheels - which look pretty ridiculous on a Z-car.  For example very few 16 inch tires are available in a 215mm width. 225mm is about the widest tire that will fit on an S30 without flares - and there are lots of 225-50/16s but for some reason 225/45-16s are much harder to find.  Its a good idea to plan your gearing around readily available tire sizes.

OK, back to gear ratios.  If we want an 8.1:1 end-to-end gear ratio in second gear, we can look at the 2nd gear ratios in the available transmissions to figure out the corresponding rear gear ratio (by dividing 8.1 by the 2nd gear ratio):

Here are the 2nd gear ratios in the commonly available transmissions that will bolt up to an L6 and the corresponding final gear ratio to get an (approx) 8.1:1 end-to-end ratio.

• Early 280Z 5 speed - 2nd gear ratio 2.077:1  -> final gear 3.90:1
• Late 280Zx 5 speed - 2nd gear ratio 1.858:1 -> final gear 4.36:1  (nearest avail ratio 4.375:1)
• Turbo 280Zx T5 5 speed - 2nd gear 2.144   -> final gear 3.78:1   (nearest  ratio 3.7:1)
• 240SX 5 speed - 2nd gear ratio 1.902:1 -> final gear ratio 4.26:1 (nearest ratio 4.11:1 or 4.375)
• Late 4 speed - 2nd gear ratio 2.246        -> final gear ratio 3.32:1 (nearest ratio 3.36:1)

If you're familiar with the Datsun gear ratios the one thing that stands out is that these final gear ratios are much lower than stock. The original ratios simply weren't designed for modern tires and engines turning high RPMs (working backwards, Nissan designed around a 5500RPM max performance shift-point). Running ratios this low can get old quick in casual street driving - you'll be shifting a lot and the engine will always seem a little busy; a better pick for a pure street driven car might be one "step" higher (e.g. a 3.7:1 where the numbers alone recommend a 3.9:1)

Now about 3rd gear....  All of the Nissan built 5 speeds have a third gear with a 1.308:1 ratio. It might not be obvious, but that means the transmission that can use the lowest rear-end gear will give the best 3rd gear performance. The T5 transmission that Nissan out-sourced for the turbo 280ZX has a lower 3rd gear - 1.375:1 - but it also has a very low 2nd gear that works best with a 3.70 or 3.90 rear gear. The end-to-end 3rd gear reduction of the T5 with the proposed rear gear ratios are numerically lower than the late 280ZX or 240SX transmissions, so the T5 ratios don't work as well with an NA motor that needs to rev to make power.

A 4.375:1 rear combined with a 1.308:1 3rd gear equates to 4800RPM at 60mph and 6300RPM at 80mph; right in the meat of the power band  for an L28 built around a 7000RPM redline. If we geared up a step, a 4.11 rear gear would lower those revs a bit, to 4500RPM at 60mph and 5900RPM at 80mph - giving up a bit of power (and acceleration) in exchange for slightly lower RPMs.

And now 5th gear. The cost of using a lower than stock rear gear is that the engine will turn at higher RPM during cruise conditions (highway driving).  Datsun set up their 5 speed gear ratios so that the engine was turning about 3000RPM at 75mph (remember when S30s were new, there was a 55mph "National Speed Limit" so high speed cruising was not all that important). Changing to a 4.11 or 4.375 rear end ratio will push that up to about 3200-3400RPM, which sounds like a lot, but is actually about 100RPM less than the 4-speed equipped 260Z and 280Z in 4th gear. Since most Z-cars are not long-distance cruisers, a little extra RPM and noise doesn't seem all that big a price to pay (but maybe avoid the boy-racer muffler if you decide to go this route).

Rear-end Options

Datsun used two different rear-ends in the S30: the R180 and long-nose R200 (there was also a high performance R190 available, but its pretty rare).  The 180 and 200 refer to the outside diameter of the ring-gear in mm. In general, the bigger the ring gear the more torque the gears can handle. The 240Z and 260Z used the R180, Nissan switched to the R200 for the manual transmission 280Zs.  The two rear-ends are close enough to the same size that they can be swapped; the major difference is the "mustache bar" that supports the back of the rear-end; you need the right bar for whichever rear-end you plan to use in the car (if you've got a 1970 or 71 240Z you need a few other suspension parts for the swap).

The R200 is bigger and stronger and heavier and more expensive than the R180, but there is more to this story than meets the eye. A big reason the stock R180 is marginal behind the bigger 280Z motor is that it has a relatively weak two-pinion-gear open differential. The open differential refers to the gears inside the rear-end that allow the rear wheels to turn at slightly different speeds when going around a corner (rear-ends are sometimes called differentials, even though the differential is really just one internal part of the total assembly). An open differential is one where if one wheel looses traction and spins, all of the power is directed to that 1 spinning wheel and none to the other wheel. An open differential makes it easy to do a 1 tire burnout, but it is hard on the internal differential gears. An R180 with a limited-slip differential (LSD), or open differential with heavy-duty gears, will reliably handle at least 250 horsepower (and more if not abused).

While only a few S30s received LSD units from the factory, there are aftermarket LSDs that can be added to a stock R180. The LSD units are expensive ($1000+ for new parts), but part of that cost is offset by using the cheaper R180, and it adds to the performance of the car by preventing tire spin under hard power-on cornering.

There is also another option: Subaru uses essentially the same R180 rear-end in the modern WRX right down to the bolt patterns; these rear-ends are available in a few interesting ratios and with either LSDs or heavy-duty 3-pinion differentials. The Subaru rear ends do require special aftermarket stub-axles to work with the original Datsun half-shafts. Options include a 3.90:1 ratio with Torsen-style LSD and 4.44:1 with a clutch-type LSD.  Just like the original Datsun rear-ends, these are typically used parts, but the Subaru rears typically have quite a bit less wear and tear than the vintage Datsun parts.

Practical Considerations

By now all of the original Nissan transmissions are getting old. The 240SX 5 speed transmissions typically have 10 years less wear and tear and and were built to handle more power than the S30 gear boxes. And they can be modified to bolt up to the L6 engine with some simple machine shop work. Its gear ratios aren't as close as those in the 280ZX transmission, and they are a little lower - but if you plan to keep your Z-car for a long time, you may well end up using the 240SX gearbox - you may want to plan ahead and run a slightly taller (numerically lower) final gear ratio that will work with the SX transmission, even if that means giving up a little 2nd gear performance with your current 280Zx 5 speed transmission.