How To Improve MPG & Performance With An Overdrive Transmission - CHP How It Works

Going Broke Filling Up Your Hot Rod? Here’s How to Improve MPG and Performance With an Overdrive Trans

Stephen Kim Jun 15, 2011 0 Comment(s)
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GM automatic transmissions quite literally rule the world. The venerable Powerglide has been the gold standard of durability and efficiency in drag racing since the ’50s, and the workhorse TH400 has been factory installed in everything from big-block Chevelles to Ferraris to military-spec Hummers. In the hot rodding world, GM automatics are routinely swapped into Fords and Mopars, but you’ll hardly ever find a C6 or a TorqueFlite in a Chevy. The reason for this is two-fold. First off, GM simply builds kickass transmissions. Secondly, what GM does better than any other manufacturer on earth is stick with a small handful of trans models, keep them in production for decades, and continually enhance their durability along the way. This yields an economy of scales that drives down costs, and ensures parts compatibility for both factory and aftermarket components for decades. The only downside to this is that there are so many great transmissions to choose from that several GM transmissions will work great for any given application.

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Fortunately, that’s a great problem to have, and the solution is usually quite simple. In the wake of gas prices that have once again eclipsed the obscene mark, our focus this month is on GM overdrive automatics. To help distinguish between the pros and cons of each of GM’s overdrives—in terms of cost, durability, weight, size, and efficiency—we sought the expertise of the best trans builders in the business. They include Stan Poff of TCI Automotive, Achilles Thomas of Monster Performance, and Zack Farah of Gearstar Performance Transmissions. We also consulted with Rick Johnson of Gear Vendors to explore the benefits of a more nonconventional approach to overdrive performance.

Overdrive Advantage

Zack Farah: You can usually expect a 30 percent savings in fuel consumption with an overdrive transmission due to three primary factors. First off, most GM overdrives have a Fourth gear ratio of 0.70:1. Let’s say you have a car with a 3.73:1 ring-and-pinion ratio and 27-inch diameter rear tires. With a TH350 or a TH400, your car would be turning about 3,000 rpm at 70 mph. On the other hand, that 3,000 rpm multiplied by an overdrive ratio of 0.70:1 yields a cruise rpm of 2,100. That’s a reduction of 900 rpm. Another factor to consider is that popular overdrives like the 200-4R and 700-R4 are low-inertia transmissions. They require less torque and horsepower to spin them up, compared to a TH350 or a TH400. Less parasitic drag equals more power to the rear wheels. Lastly, the 200-4R and 700-R4 have lower First gear ratios than the TH350 and TH400. The First gear ratios on the 200-4R and 700-R4 are 2.74:1 and 3.06:1, respectively. In comparison, a TH400 has a 2.48:1 First gear ratio and the 350 has a 2.52:1 ratio. Therefore, the overdrive transmissions offer a lower overall First gear ratio, which translates to better off-the-line performance and better fuel economy since they don’t load the motor as much when getting the car moving from a stop.

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Stan Poff: Overdrive transmissions substantially lower cruise rpm. With gas as expensive as it is these days, that translates to increased fuel mileage and savings at the pump. With an overdrive unit, cruise rpm on the freeway is typically reduced between 25-33 percent. This can easily equate to a 3- to 5-mpg improvement and make for a much more pleasant cruise as well. Most hot rods have free-flowing exhausts, so a vehicle will be much quieter when turning 1,500-1,800 rpm on the freeway with an overdrive versus 3,000-3,500 rpm without one.




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