"Give me a lever long enough and a fulcrum on which to place it, and I shall move the world." — Archimedes (287- 212 B.C.)

On the street, everything is a compromise. Power is now easier to make than ever and with that comes an evolution in the approach to gear ratios. Old-school thought was to plug in a deep gear and spin the wee out of the engine. But with today's greater power levels, it takes less leverage than before and there is a gold mine of subtleties that are worth digging into. We'll investigate the advantages and disadvantages of plugging a gear into a drag car and those same ratio options for the street. With a wealth of overdrive transmission conversion options, it's never been easier to build a quick drag or track car that can also be fun to drive on the street. It all depends on how far—and how fast—you want to go.

Let's start with a look at the simple side of gear ratios. Archimedes would recognize that gears are a form of leverage. Input an rpm into a pinion gear that spins a larger diameter ring gear and the resultant motion reduces output speed but multiplies torque. This is necessary because vehicles are heavy and require leverage help to accelerate briskly.

See all 14 photos The only way to accurately determine the gear ratio in the car is to remove the cover or pull the gears and count the teeth on both gears. That is not easy, but at least with GM gears, the numbers are stamped on the side of the ring gear.

If we have a 10-tooth pinion gear and a 41-tooth ring gear and do the math (41 10 = 4.10) that means for every revolution of the ring gear, the pinion will spin 4.1 times. This also multiplies the input torque by 4.1, which is the reason the car jumps when you hit the throttle. A gear with a lower numerical ratio, like 3.08:1, does not multiply torque as much, but with an equal rpm input will produce a higher axle or tire speed, all else being equal. This means that rear gear ratios are a compromise.

But that's just the rear axle ratio. We also have multiplication of engine torque by the transmission ratio. The early days of GM automatics produced the Powerglide with a First gear ratio of a pathetic 1.76:1. Soon after came three-speeds like the TH400 with a First gear ratio of 2.48:1. To determine the vehicle's overall First gear ratio we simply multiply the First gear ratio by the rear axle ratio (e.g., 2.48 x 4.10 = 10.17:1). We'll ignore the multiplication factor of a typical torque converter to keep things simple, so that means if we multiply 400 lb-ft of engine torque by an overall First gear ratio of 10.17, we're applying 4,068 lb-ft of torque to the two rear axles, or over 2,000 lb-ft of torque per axle. That's why the car launches so hard and why parts like axles and driveshafts sometimes twist or break.

See all 14 photos As the gear ratio increases numerically, from 3.08:1 to 4.10:1 for example, the pinion gear tooth count is reduced while the number of ring gear teeth increases. This makes the pinion gear smaller, reducing the contact area between the pinion gear and the ring. Consequently, "taller," or lower numerical, gears are inherently stronger.

Let's burrow down a little deeper into our drag car example with this ratio package. Let's assume that we're running a 26-inch tall rear tire and a 550hp small-block in a 3,600-pound Chevelle and it runs low 11s at 120 mph. It's possible the car might improve traction with a 28-inch tire, but we're unsure how much that taller tire will affect the gear ratio. We'll call this overall drive ratio (ODR). The formula looks like this:

ODR = (Original Tire Dia. New Tire Dia.) x Gear Ratio

ODR = (26 28) x 4.10

ODR = 0.928 x 4.10 = 3.80:1

So, the taller tire will effectively reduce the gear ratio from a 4.10:1 to a 3.80:1, or a change of 7 percent.

The Quarter Pro dragstrip simulation program is a great way to evaluate changes. The nearest 12-bolt gear ratio to 3.80:1 is 3.73:1 and some may think this would be a huge loss of e.t., but the simulation reveals something different. The 3.73:1 gear (leaving the tire size the same) slowed the e.t. barely, by 0.08 second, but the trap speed did drop 2 mph with no change in the 60-foot.

See all 14 photos Be aware that pro series gears are much softer than street gears and, as such, should not be used on the street as they will generate excessive heat and wear out much more quickly. The softer heat treat is designed to absorb the impact load of hard launches.

Realistically, if we add the taller tire, it would offer better grip with a taller footprint and perhaps the 60-foot might have been quicker. Part of the reason this car did not suffer a serious loss of e.t. is mainly because the engine makes fairly good torque, especially in the midrange. This is where every car is different. If the car is running a smaller-displacement engine like a 5.3L that we're spinning it to 7,000 rpm, then likely it will be more drastically affected by a reduction in gear ratio because small-displacement engines typically makes less torque.

Conversely, a 540ci big-block making 700 hp and 680 lb-ft of torque will not need as much gear because the engine produces more power to accelerate the car. We simulated that tire change situation of a 26- versus a 28-inch tall tire using a taller 3.55:1 (versus a 4.10:1 gear) and the car lost barely 0.07-second running 10.30s at 130 mph but cleared the lights with the taller tire at 5,800 rpm instead of 6,700. Of course, if every last hundredth is important, then the deeper gear is worth the change, but for a street compromise, the taller gear would work just fine.

See all 14 photos Our LS-swapped Chevelle used in the simulations pushes through a TH400. If we wanted an overdrive, the Gear Vendors unit is a great way to have both the durability of the TH400 and overdrive, with the opportunity to split gears if we want.

Now let's take our theoretical 6.0L Chevelle with its aforementioned 4.10:1 gears and put it on the street with a set of 28-inch tall street tires, but we want to know our highway cruise rpm. To keep this discussion about gear ratios and not about torque converters, we'll ignore slippage. The simple formula for figuring out our cruise rpm at 70 mph looks like this:

RPM = (MPH x Gear Ratio x 336) Tire Diameter

RPM = (70 x 4.10 x 336) 28

RPM = 3,444

This rpm is a little steep for highway cruising. One way to improve this without having to completely change over to an overdrive automatic might be a Gear Vendors overdrive bolted to the back of our TH400. What's that worth? One way to do that is simply multiply the Gear Vendors' overdrive ratio of 0.78:1 by the gear ratio to create our effective ratio: 4.10 x 0.78 = 3.198, we'll round that off to 3.20. If we plug in 3.20:1 to the formula, it works out to 2,688 rpm as our new cruise rpm. A reduction of over 750 rpm. Plus, the Gear Vendors unit can be used to split the TH400 gears, which will generally improve e.t. The most common approach is to overdrive Second so that the shift sequence is First (2.48:1), Second (1.48:1), Second Overdrive (1.15:1), then Third (1:1).

See all 14 photos This is the Strange S-60 in our Chevelle with the 4.10:1 gear and limited-slip. While many consider the Dana 60 way too heavy, the Strange version is only 20 pounds heavier than a 12-bolt, even with its 35-spline axles and 9.5-inch ring gear.

Of course, if the goal was to emphasize the highway cruising side of this equation, we could soften the rear gear a 3.55:1. This would slow the car down on the dragstrip by perhaps 0.10-second, but lower the cruise rpm. That change would net a 70-mph cruise speed of 2,326 rpm. Keep in mind that the highway cruise rpm needs to remain at or above the converter stall speed to prevent building excessive heat in the fluid. Of course, the Gear Vendors can also be used behind a manual transmission.

We haven't forgotten the autocross and track day heroes. Here, the situation is a bit more fluid because drag racers have a set eighth- or quarter-mile distance to manage while track day fans and autocrossers face layouts of considerably wider variation.

For road course guys, the first place to consider is the length of the longest straight. For those of us in Southern California we have three tracks within a few hours. Willow Springs is the longest at 2.5 total miles with a front straight that is roughly 2,000 feet in length, over 3/8ths of a mile. This is much longer than the other courses and is a big factor in the final drive ratio based on rev limit. The quickest way to figure it out if you plan to run a given track is to find a similar performing car and ask them what their maximum speed at the end of the straight is (or peak rpm at the end), along with their gear ratio and tire size. This way you can do the math yourself as verification.

See all 14 photos This is the 6.0L iron block LS engine in our Chevelle test mule that makes 550 hp with a pair of TFS cathedral port heads, FAST LSX-R intake, and Hooker LS-swap headers. The trans is a TCI TH400 with a 10-inch converter. We'd love to swap in a GM 10-speed but, as yet, there are no stand-alone controllers.

What is at least as important as the final drive may be the gear splits between First and high gear. This will focus our attention on manual transmissions. The advantage of the manuals is that even with a four-speed, you have ratio splits closer together than a three-speed automatic. The exception to this is the new line of 6-, 8-, and, now, 10-speed automatics. But these transmissions have not found their way into the mainstream as yet, so we'll focus on the manuals.

For a road race/autocross situation, we'd like to have a multi-geared manual transmission to allow us to keep the engine in its power range at all times. While four-speeds get the job done, a five-speed with a close spread is even better. We've listed the gear ratios for several transmissions in the Late Manual Trans chart. If we divide a higher gear (like Third) by the lower gear (Second), we can see the change in rpm expressed as a percentage.

On a TKO-600 five-speed for example, Second gear is 1.89:1 and Third is 1.28:1. The math works out as 1.28 1.89 = 0.677, a 32.3 percent rpm drop. This pulls 6,500 rpm in Second down to 4,400 in Third. A close-ratio 2.43 First gear Super T-10, shifting from Second to Third drops the rpm by much less at 24 percent and a T-56 six-speed is in between at 27 percent. But now consider the Richmond five-speed. The split between Third (1.57:1) and Fourth (1.23:1) is only 22 percent—closer by a significant margin. At 6,500 rpm the rpm drops to a higher 5,070 rpm. Of course, one downside to the five-speed is an additional 35 pounds.

See all 14 photos This is the Richmond five-speed whose origins can be traced back to Doug Nash. This trans is considered a close-ratio with 18- to 19-percent rpm drops between 3-4 and 4-5. This keeps the engine in its power range.

With the coming of the new 8- and now 10-speed GM automatic transmissions, they take the close-ratio approach to the next level. The 10-speed, for example, offers gear splits of 17 to 18 percent compared to a TH400 of 33 percent between Second and Third gears. This may not sound like much of a difference, so just for kicks we compared our 550 hp LS-powered Chevelle with its TH400 package to a swap of a brand-new GM 10L80 that enjoys a 4.70:1 First gear ratio.

The overall First gear ratio with the TH400 and 4.10:1 rear gear would be 2.48 x 4.10 = 10.168:1. But, 10.168 4.70 (to determine the equivalent rear gear ratio) equals 2.16:1, which is not realistic. So, we plugged in a 2.73:1 rear gear. We also added 90 pounds to simulate the added weight of 10-speed. Not surprisingly, the 10-speed was quicker at 11.06 at 123.3 mph compared to the TH400/4.10 gear package at 11.25 at 121.8 mph. The 10-speed was quicker by nearly two tenths of a second and 1.5 mph faster. This is because the 10-speed maintains engine rpm right in the sweet power spot with minimal rpm drops between gears.

See all 14 photos Here's an option to consider. The Richmond five-speed offers a deep 3.27:1 First gear. With a 3.08:1 rear gear, the effective First gear ratio is equal to a 2.20:1 First gear Muncie four-speed with 4.56:1 rear gear. In the quarter-mile, Fourth gear is equivalent to running a 3.79:1 rear gear. But shift into 1:1 Fifth gear and enjoy the highway cruising rpm of a 3.08:1 rear gear.

The 60-foot times were exactly the same for both transmissions and no other changes were made. The 10-speed simulation experienced tire spin (which slowed the 60-foot) because it had much more overall First gear. With traction, the 10L80 would have been even quicker. Another variable would be the power absorbed by the 10-speed's heavier rotating mass. These would slow our simulation slightly, but this does reveal the advantage to a closer gear spread. Remember when a 10-speed bicycle was considered exotic?

All of this is an attempt to approach what a continuously variable transmission, where ratios are always changing, does during acceleration. We're not there yet, but 10-speeds are closer than 4's. We've come a long way from the days of the lowly Powerglide.

Early Manual Transmission Gear Ratios 1st 2nd 3rd 4th M20 close 2.20 1.64 1.28 1.00 M21 wide 2.52 1.88 1.47 1.00 S-T10 2.43 1.61 1.23 1.00 ST10 2.64 1.75 1.34 1.00 ST10 2.88 1.91 1.33 1.00 Show All

Late Manual Transmission Gear Ratios 1st 2nd 3rd 4th 5th 6th 7th GM V-8 5-spd. 2.75 1.94 1.34 1.00 0.74 --- --- McLeod 5-spd. 2.95 1.99 1.34 1.00 0.63 --- --- TKO-500 3.27 1.98 1.34 1.00 0.68 --- --- TKO-600 2.87 1.89 1.28 1.00 0.64 --- --- Richmond 5-spd. 3.27 2.13 1.57 1.23 1.00 --- --- T-56 2.66 1.78 1.30 1.00 0.74 0.50 --- TR-6060 2.66 1.78 1.30 1.00 0.80 0.63 --- TR-6070* 2.66 1.78 1.30 1.00 0.74 0.50 0.42 TR-9070 DCT 3.14 2.05 1.43 1.10 0.86 0.68 0.56 TR-9070-DCT 3.24 2.02 1.45 1.08 0.81 0.63 0.50 *There are actually three different ratios available in this transmission. The 2.29:1 First gear version is rated at 635 lb-ft of torque. Show All

Early Automatic Transmission Gear Ratios 1st 2nd 3rd 4th PG 1.76 1.00 --- --- TH350 2.52 1.52 1.00 --- TH400 2.48 1.48 1.00 --- 4L60E* 3.06 1.63 1.00 0.70 200-4R 2.74 1.57 1.00 0.67 4L80E 2.48 1.48 1.00 0.75 *Same ratios as 700-R4 Show All

Late Automatic Transmission Gear Ratios Trans 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th TCI 6-speed 2.97 2.23 1.57 1.18 1.00 0.75 --- --- --- --- 6L80E 4.03 2.36 1.53 1.15 0.85 0.67 --- --- --- --- 8L80 4.56 2.97 2.08 1.69 1.27 1.00 0.85 0.65 --- --- 10L80 4.70 2.99 2.15 1.80 1.52 1.28 1.00 0.85 0.69 0.64 Show All

See all 14 photos This is GM's latest automatic: the 10L80. While heavy, it offers a killer 4.70:1 First gear ratio with six close-spread ratios between First and Seventh (1.00:1). Our simulation indicates it could be worth a couple of tenths in the quarter-mile over a typical three-speed automatic.

See all 14 photos This is a TKO five-speed trans going into an early Camaro. The most popular version is the TKO-600, not because of its greater strength but its more tractable First gear ratio and steeper overdrive. With a 0.64 overdrive, this converts a 3.55:1 rear gear to a 2.27:1.

See all 14 photos An overdrive spins the driveshaft at a much faster speed. It's a complex subject, but if using overdrive in a high-speed situation, you must consider something called critical speed. At critical speed, the driveshaft attempts to turn into a pretzel and bad things happen. With a 3-inch diameter, a 54-inch long driveshaft will hit its critical speed at 136 mph using a 3.55:1 gear, a 26-inch tall tire, and a 0.76:1 overdrive ratio. Think about that.

See all 14 photos Gear Vendors offers this bolt-on 22 percent overdrive system for a multitude of different GM transmissions, both manual and automatic. The overdrive can also be used to split gears on wide-ratio split transmissions. The torque capacity of this unit is sufficient to live behind 3,800-pound Drag Week cars running in the 7s.

See all 14 photos Among the more interesting evolutions to the manual transmission is the new TREMEC TR-9070 DCT. This is a seven-speed manual that uses a wet, dual-clutch arrangement in a housing that looks like a torque converter. The clutches are connected to two separate input shafts. Shifts are controlled by a computer—a version of this transmission is in the C8 Corvette as a transaxle. This trans has a torque capacity of 664 lb-ft.