MOST SPEED MECHANICS have two burning desires concerning their cars: to boil along at good top speed and to accelerate rapidly. Although engine soup-up will provide additional pep, without proper gearing neither desire can be achieved.

The stock cogs in gearbox and rear end are designed to give a car in stock form acceptable performance. Because the maximum torque developed by the engine is puny compared to the weight of the car, mechanical advantage must be obtained through leverage to move the car and this is accomplished with gears. By multiplying the torque available at the flywheel through speed reducing gears, 300 pounds feet of torque can become as much as 3000 (or more) pounds feet at the driving wheels.
So much confusion exist about gear ratios that clarification of the meaning of terms might prove valuable at this point. Four-to-one is a high ratio, but a low gearing. When used between the engine and rear wheels, it reduces wheels speed to one quarter engine speed and at the same time, (because of leverage advantage), increases torque available at the wheels by four. Two-to-one is a lower ratio but higher gearing: Wheel speed is half engine speed and torque at wheels is twice that available at the engine.
The rear end gear ratio affects the car's top speed and can limit or expand the engine's potential power output. Top speed however, is not necessarily increased when a higher-than-stock gearing is used and many, who substituted a 3.54 rear for a 4.11 for more speed, learn this the hard way. Acceleration through the gears is affected by the overall ratio of transmission and rear end gears.
That it requires more force to overcome the inertia of a body at rest than to keep it moving is a well-known law of physics. The additional force required to move a car from a standstill is made available by transmission gears. Low overall gearing also permits the engine to rev close to the rpm point at which maximum power is developed for good acceleration.
Theoretically a perfect transmission is one in which an infinitely variable gearing is produced, such as in Dynaflow. Unfortunately power losses usually prove to be a disadvantage that is not overcome by theoretical advantages. One of the best gear-box bets is a four speed unit, fitted with close ratio gears, that allows selective gearing of engine speed to wheel speed under a variety of driving conditions. In American gearboxes, the use of three forward gears usually requires a wider spread between ratios and consequently power is lost during shifts and elapsed acceleration time is greater.
As a general rule gearing changes accomplish very little in the stock car. Earlier we mentioned that top speed and that substituting a 3.54 rear, for instance, for a 4.11 for more speed does not result in a gain of 10 mph or so. Usually the stock car's gearing is chosen so that top speed is reached at about the rpm point that the engine develops its maximum hp. Gains in top speed are made with changes in rear end gearing when the engine is reworked to pull more power.
The standard Ford-Mercury boxes have a 28- or 28- tooth main driving gear. The Lincoln Zephyr box carried a 25- or 26-tooth main gear. The lower ratios available in the Zephyr first and second, produce a higher gearing and permit a higher maximum speed in first and second, without changing the speed possible in high (or direct). In addition, Zephyr ratios are closer than in Ford-Merc boxes. As a result of Henry Ford's feeling for standardization, Z gears are interchangeable in most Ford and Merc boxes.
Despite the fact that, among Ford-Merc owners, Lincoln Zephyr gears have been widely used, it would be unwise to order a set for your own rig without a thorough study of your needs and your equipment. Z gears should not be regarded as a cure-all. Unless you've souped or lightened your rig, they will not appreciable improve your car's acceleration, for the higher gearing requires more power from the engine or a lighter wagon to drag. With Z gears it will take a heavy car longer to get going from standstill than when equipped with stock cogs.
In a lightweight chassis, however the Z combination usually spells success and the advantages of Z conversions are most noticeable when used with souped engines or with stock mills capable of handling the load. Combining greatly increased engine output with a light chassis almost always demands the use of Zephyr gears.
It should be obvious that knowledge of your engine's capabilities is necessary to correct analyze the elements that determine proper gearing. A tachometer (discussed in August SM) can be helpful in experimenting with various tire sizes in search of more perfect gearing. If a higher gear will give you more speed, a larger tire size will duplicate this effect. If you obtain the same or a higher reading on the tach, with speed improvement, higher gearing will be of benefit. As it's less expensive to test gearing with borrowed larger tires than to purchase a number of ring and pinion sets, keep experimenting with tire sizes until no further improvement is found.
Determining proper gear ratio can be done, with some rough accuracy due to our omission of wheel slip and wind resistance factors with the formula: Gear ratio = Engine rpm X tire diameter (in Inches) divide by mph X 336 (a constant *). Double check answers thus found against specifications of quick moving cars with similar modifications to those found in your car. The more time you spend checking for the right gear, the less heartache you'll have on road or drag strip.
Zephyr gears can be installed in any (except V-8 60) '32-'48 Ford or in '39-'50 Merc transmission case with little difficulty although some years are a tighter "squeeze" then others in installation. It is impossible to change just one gear in Z conversion; in some models ('40-'48 Fords and '40'50 Mercs) only the cluster gear with rear thrust washers, main drive gear and second gear must be replaced. In other models, it is necessary to make an almost complete parts-changeover.
Boxes built before 1940 were "floor shifters" and later model were "side shifters." Some but not all, speed mechanics think it wise to change to a floor shift box if extensive power modifications are to be made, The floor shift box has a reputation for greater strength that is increasingly appreciated as power is upped.
In the '32-'36 boxes only a few of the original parts are retained in a Z conversion, but in the interest of reliability all parts are usually replaced. The parts changes required on early Fords are: the cluster gear and thrust washer; main drive gear; main shaft; second gear; synchro-mesh assembly (and second gear thrust washers on some models) low and reverse idler. For the proper combination, check all parts needed against a late Ford parts catalog for the current stock numbers.
In '37-'39 Ford and '39 Merc boxes, the above parts (except low reverse idler) are replaced. The '39 second and high shifter fork is used in converting all '37-'39 boxes.
Cost for a Z conversion runs approximately $40 for parts in '40-'50 boxes and somewhat higher for earlier models and labor. If you choose to have the job done, should run at least $30 more. Two series of Zephyr gears are available: the 26-tooth gears ('40-'41 Lincoln) and 25-tooth gears (up to '48 Lincoln).
If you decide to do the job yourself, these are the basic steps to follow:

On models up to '48, remove the rear axle-floor boards if necessary. Remove the clutch equalizer shaft, detach the shaft rods, place a support under the engine unbolt the transmission. Disassemble transmission. Remove U-Joint (one bolt holds it in place). Remove clutch throw out bearing and unbolt front and rear bearing retainers, and transmission cover. Push countershaft out trough rear, allowing cluster gear to lie in case. Pull main drive gear through front of case, push mainshaft to the rear and pull rear bearing. Lift mainshaft out through cover opening, keeping synchromesh and low and reverse in position. Do not move synchro assembly; it is spring-loaded and parts can be lost easily if disturbed. Lift cluster gear from case and drive out reverse idler gear shaft and lift of gear.
Slide reverse and low gear off the shaft. Remove lock ring from front of shaft but keep synchro gear centered on the assembly. Wash parts to be reused and inspect carefully. Worn parts should be replaced.
The Zephyr cluster is first replaced on reassembly, with other parts to be replaced in reverse order to that used in disassembly.
The development of the Ford Thunder-Bird almost from the beginning meant much more to the professional speed mechanic than merely the appearance of a new American sports car. It didn't take these professionals long to recognize that the T-Bird synchro-mesh box provides still another excellent source of good transmission gear ratios.
Some such as Milby Jones of Exton Pennsylvania have already made use of the T-Bird's 24-tooth clusters in reworking late Ford and other gearboxes. Jones, one of the East's veteran "Sultans of Swap" says he'll be glad to answer any specific questions concerning installation of T gears.
To take advantage of the new ratios available, it is necessary to procure the following T-Bird parts:
B5S-7017A-----Input shaft and gear
B5A-7102C-----Second Speed gear
B5S-7113A-----Cluster Gear.
Other modifications depend upon what engine and transmission case is used, but one thing that must be done in whatever transmission case is used to line-bore the cluster shaft hole from .750 (or ¾") to .875 (or 7/8") because the shaft that accompanies the T cluster is larger and uses 50 instead of 44 needle bearings. As the T input shaft is two inches longer than previous shafts, it is ideal for coupling to '51-'53 Chrysler engines with a deep bell. Usually the shaft must be cut shorter and resplined for general conversion use.
To those speed mechanics, seeking the ultimate in a both performance and reliability in a box that must handle the output of a big and hairy power plant, a full-scale shift to pre-war Cadillac, La Salle and Packard brute gear boxes is called for. Attention auto wrecking yards: Brace yourselves for a boom in good boxes!

Transmission Gears and Ratios
26H Lincoln 25-tooth Low Second High 2.12 1.44 1.00
06H-56H Lincoln 26-Tooth Low Second High 2.33 1.58 1.00
Ford-Merc Lincoln 28-tooth Low Second High 2.82 1.60 1.00
Ford-Merc Lincoln 29-Tooth Low Second High 3.11 1.77 1.00
Ford Thunderbird 24-tooth Low Second High 2.32 1.48 1.00

* The Constant is used to equate inches and miles and minutes and hours, and minutes and hours, and diameters and circumferences.