With gasoline costs uncomfortably high, many enthusiasts have started thinking twice before trekking to a car show an extended distance from their home. A set of low rear gears coupled with a 1:1 direct drive top gear ratio in a classic Mopar’s manual or automatic transmission usually equals poor fuel economy. Maybe it is time to consider an overdrive transmission to replace the traditional non-overdrive Mopar transmission, but which one is the best fit?
Above: The Silver Sport Transmissions’ Speed Analyzer requires minimal input from the operator and provides valuable data about the vehicle’s speed and engine rpm. In addition, there is a pull-down box filled with all the Silver Sport Transmissions and the most popular factory transmission offerings. Also, at the bottom of the page is an aspect ratio calculator to determine the diameter of any tire/wheel combination.
There are several parameters that must be considered when selecting an overdrive transmission. First, what transmission is desired, manual or automatic? Second, what overdrive gear ratios of a 5- or 6-speed better complement the vehicle’s engine and differential combination? Third, will the floor and driveshaft tunnel require modification to fit the transmission? Lastly, how do you figure in the tire size?
Silver Sport Transmissions has developed the Speed Analyzer to help resolve the concerns of a transmission selection. The Speed Analyzer provides an understanding of how each transmission changes the vehicle’s performance.
Above Left: A hypothetical small-block, factory 4-speed, A-body equipped with 4.10:1 rear gears spinning 255/60R15 drag radials revs the engine to 2800 rpm at 55 mph. Above Right: Raising the speed to 70 mph pushes the engine’s rpm to 3600. At that rpm, the engine is buzzing, there will be plenty of vibration, and the fuel economy will be poor.
To illustrate the power of the Speed Analyzer, we will use a typical Mopar A-833 4-speed connected to a small-block wedge in an A-body. The 8 ¾-inch rear end has 4.10:1 gears that spin a pair of 255/60R15 drag radials. With the vehicle traveling 55 miles per hour (mph), the engine is spinning at 2800 revolutions per minute (rpm). If the speed is raised to 70 mph, the engine revs at 3600 rpm. At that rpm, the engine will roar, the car will vibrate, and the occupant’s cabin temperature could rise considerably. Also, the engine will wear at an accelerated rate, and the gas mileage will suffer.
Above Left: Removing the 4-speed and installing a Silver Sport Transmissions’ TREMEC TKX close-ratio 5-speed manual with a 0.68 overdrive will significantly drop the engine speed. For example, at 70 mph, the engine rpm would be reduced from 3600 rpm to 2430 rpm. Above Right: If an automatic is more your style, an A41 with a 0.70 overdrive will result in an engine rpm of 2500.
To reduce the engine rpm at cruising speed, installing a Silver Sport Transmissions’ TREMEC TKX close-ratio 5-speed manual with a 0.68 overdrive would drop the engine speed from 3600 rpm to 2430 rpm while maintaining 70 mph. If rowing through the gears is not for you, a Silver Sport Transmissions’ A41 Automatic, 4-speed with a 0.70 overdrive is a great option. At 70 mph, the engine would rev to 2500 rpm. Both options significantly reduce the frequency of fuel stops and the ride’s noise, vibration, and harshness (NVH).
Decreasing the engine rpm is good, but there can be too much of a good thing, and the Speed Analyzer can reveal that too. For example, rather than a 4.10:1 geared differential, if 2.93:1 gears were coupled with the TKX and the 0.68 overdrive, the engine rpm would drop appreciably. The engine would only need to spin at 940 rpm at 55 mph and 1185 rpm to maintain 70 mph. Depending on the engine requirements, the lower rpm may cause the engine to lug.
Above Left: Dropping the rear gears to 2.93:1 will result in only 940 rpm at 55 mph. The rpm may be too low for smooth engine operation. Above Right: With the same gears at 70 mph, the engine rpm will be 1185 rpm. Again, this rpm could be too low for cruising, leading to engine lugging. Silver Sport Transmissions has other transmission overdrive ratios that will slightly raise the engine rpm to a more efficient engine speed at 55 or 70 mph.
Instead of the 0.68 overdrive, a TKX with a 0.81 overdrive may be the better choice because it still provides the benefits of an overdrive without significantly affecting the vehicle’s drivability while maintaining a more efficient engine rpm at cruising speed.
If you are considering plus-sizing your tires from a 14-inch to a 15-inch wheel, the Speed Analyzer also has an aspect ratio calculator to determine the new tire’s diameter. Although the example is a 14- to 15-inch wheel, the aspect ratio calculator is a robust tool that can calculate wheel diameters from a tiny wheel to wheel diameters beyond the scope of reality for an automobile.
Above: Although we used a Mopar A-body for our transmission example, Silver Sport Transmissions offers products for ’64-’76 A-bodies, ’62-’74 B-bodies, and ’70-’74 E-bodies. Silver Sport Transmissions has a transmission kit to meet anyone’s needs, whether a TREMEC TKX 5-speed, a Magnum 6-speed, or an A41 automatic.
A quick call to Silver Sport Transmissions will start the order process after completing the calculations. A representative will ask you a few questions about your vehicle and construct a custom parts list with every part necessary to exchange the current transmission for the transmission of your choice. Additionally, the rep will discuss if any modifications to the floor will be required. If mods are necessary, the representative can provide alternative transmission options that may better fit your application.
Above: In addition to the transmission kits, Silver Sport Transmissions has plenty of old-school and modern-look components to upgrade the transmission installation.
Silver Sport Transmissions is a one-stop shop that can help with decision-making by weighing the pros and cons of each transmission based on your individual needs/desires. As proof, Silver Sport Transmissions offers reproduction shifter handles that maintain the classic Mopar look or provide a more modern appearance with aluminum shifter sticks or carbon fiber-styled pistol grip handles.
Silver Sport Transmissions has every TREMEC manual transmission from its catalog in stock, ready for immediate delivery, and the A41 requires only two weeks to build to the purchaser’s specifications. For more information, check out the Speed Analyzer and call 888-609-0054 for quality advice. Installing a transmission from Silver Sport Transmissions will have you and your Mopar comfortably cruising in no time.