Gas Saving Modifications - Fight Back Oil Prices

Performance Modifications That Also Conserve Fuel

Bob Mehlhoff Dec 21, 2005 0 Comment(s)

We all understand the gas that used to cost 20 bucks now costs 40, but before you swap your very fast '69 Camaro for a kerosene-powered Kia, please read on. By making some easy choices, doing some part-swapping, and putting in a little effort, you can raise your musclecar's performance and improve its mileage as well.

Yes, you read that right. We've outlined several straightforward ways to do this, while making your cool ride a breeze to drive. We're talking about modifications that lower your engine rpm at freeway speed, provide more low-speed power, allow your engine to breathe better, and in the end, make your car more versatile to drive. Think of it as a menu to help you save money, gain performance, and minimize wear-and-tear on your powertrain.

The Spin Factor
One of the best ways to gain mileage while maintaining or increasing performance is to install an overdrive transmission that offers low-rpm cruising at freeway speeds and improved acceleration from a lower First gear ratio. To demonstrate this, we'll assume a typical '72 Malibu fitted with a small-block 350. In general terms, this engine requires about 50 lb-ft of torque to spin the crankshaft, and at 5,250 rpm this equals about 50 hp. We're also going to assume a 3.42:1 axle ratio and 26-inch-tall rear tires. The first transmission for our case will be a TH400 (three-speed automatic) that utilizes a stock 1:1 High gear and a stock torque converter.

Our first step is to determine engine rpm at a given speed (we'll use 60 mph). To do this we'll use the mathematical formula: mph x final drive ratio x (336 / tire diameter) = rpm. First we divide 336 by 26 (tire diameter in inches) to get 12.92. Next we plug our data into our formula: 60 (mph) x 3.42 (final drive ratio) x 12.92 = 2651.18 rpm. We'll round that to 2,650 rpm. Next, we know most stock non-lockup converters will add about 250 rpm to the total engine speed at 60 mph due to slippage. So our final reading at 60 mph will roughly be 2,900 rpm.

If we install an overdrive automatic with a lockup converter (either a TH200-4R or a TH700-R4), we're going to have an overdrive ratio of about 0.70:1 (0.67 for the TH200-4R). Multiplying the 0.70:1- overdrive ratio by the previously established engine rpm at 60 mph (without slippage) equals (2,650 rpm x 0.70) 1,855 rpm, which is a reduction of about 1,000 rpm. But what's also important here is that we've not only reduced the engine rpm at a given speed with an overdrive gear and a lockup converter, we've also reduced the frictional drag soaking up power at the 2,900-rpm level.

Overdrive automatics and most overdrive manual transmissions also offer deeper First gear ratios compared to the older traditional 1:1 High gear units found in the TH350, TH400, and Muncie four-speeds. As an example, inside a TH200-4R you'll find a steeper 2.74 First gear compared to the 2.48 of a TH400; a TH700-R4 uses a 3.06 First gear ratio. Manual overdrive transmissions' First gear ratios range from 2.65 to 3.27, which are far deeper than that in any Muncie four-speed. A close-ratio Muncie (M21 or M22) has a 2.20 First gear and a wide ratio (M20) has a 2.52 First gear ratio. The First gear ratios found in most overdrive transmissions (manual or automatic) allow you to retain your stock 3.08, 3.31, or 3.42 rearend gears and still have excellent off-the-line acceleration.

Since many modern overdrives have been in production for 10 or 20 years, finding a viable candidate in a wrecking yard is generally easy to do. Better yet, if you have the budget, the aftermarket offers some great new overdrive choices from manual gear-boxes to add-on overdrive units. Keisler Engineering provides many five- and six-speed manual transmission kits for all the popular GM platforms, and they include a bellhousing, driveshaft, shifter, cross-member, clutch pedals, floor hump (in some cases), as well as the transmission.

If you'd like to retain your existing automatic transmission (or some custom manual-transmission installations), Gear Vendors offers a selection of overdrive units that mount to the rear of the case. Another benefit of the Gear Vendors unit is that the system doubles the amount of gears you currently have, so if you were currently running a TH350 trans, you'd have six forward speeds.Another option is to find a factory overdrive manual transmission from a local swap meet or on eBay. When we checked eBay we found many from $1,000 to $2,000. We even found an all-new Borg-Warner T56 six-speed trans for $2,049 with a crossmember to fit a Third-Gen Camaro. The ratios on this one were 2.97, 2.07, 1.43, 1.00, 0.80, and 0.62:1. Shipping in the lower 48 states added $100. This type of transmission would work fine behind a moderately built small-block since the 2.97-ratio T56 has a 400 lb-ft torque rating. One of the most common donor cars is the '93-97 Camaro.

The Torque Monster
In general terms, big-duration cams can make big power upstairs, but if your new goal is to save a little fuel and increase lower engine-speed performance, consider installing a cam with 225 degrees duration at 0.050-inch lift (or less). The lesser duration will provide more manifold vacuum and allow your car to accelerate quicker without having to dip as far into the throttle. Most camshaft companies offer a list of hydraulic bumpsticks to fill this bill. Consult the manufacturer's listing for the one that matches your intention.

Fuel Systems
Keeping your engine running properly is foremost in achieving the best mileage and power. If your engine is carbureted, make certain the jetting is correct. The best place to start testing is with the stock jetting, which is often the optimal point. Don't forget to check the air/fuel adjustments (with the engine warmed up), air filter, and make certain there are no vacuum leaks. A carburetor jetted too lean will typically develop a surge during part-throttle acceleration. Remember to also check or replace the fuel filter if needed, and inspect all the fuel lines.

Changing the fuel system from a carburetor to a fuel injection system may offer more precise fuel management and better drivability if it is installed correctly. Contrary to belief, the wiring of an electronic fuel injection system is generally not the most difficult part of the installation. What does require careful attention is the design of the fuel delivery system from the tank to the engine and back. Although there are plenty of fuel injection systems outfitted to late-model Chevy powerplants, perhaps the most versatile add-on systems to older Chevy V-8s are offered by the aftermarket. If you'd like to really update your vehicle, consider installing a late-model Chevy V-8 with factory fuel injection and computer control. With the help of companies like Year One, installing an LS1 and a 4L60E automatic or T56 manual is relatively easy with its conversion kit for early Camaros.

A Diet for Mileage and Performance
The curb weight of your car may be another area to look to improve performance and mileage. Generally, increments of 100 pounds are the most noticeable. Items like hoods, trunk lids, cylinder heads, exhaust systems, and wheels and tires are where weight can be removed. In the mid-'70s, when GM was first attempting to increase the fuel economy (and regain some perfor-mance) of its products, it re-engineered models to save weight. Instead of looking for one or two areas where they could remove 100 pounds at a time, they looked for 1,600 places to remove one ounce. In other words, weight can be saved almost anywhere.

Stay in Tune
Increasing power and fuel economy is largely dependent on engine tune. Open the hood and check the plug wires and spark plugs. Next, check the engine timing (on pre-computer-controlled engines). If your vehicle has a "check engine" light on, scanners are available at most auto parts stores to help pinpoint the cause of failure. Don't overlook items like O2 sensors, which generally should be replaced every 100,000 miles.

Getting the most power and mileage from your car is dependent on many factors. Like all other facets of hot rodding, the best results come from having a goal, doing some research, and then making the changes. By improving the tuning, gear selection, and more, you can improve mileage, performance, and drivability. In the end, your musclecar will be more enjoyable to drive.


Air-filter elements, such as those offered from K&N, may increase performance and mileage by providing an unrestricted flow of air to the intake system.

Installing a camshaft with shorter duration amounts (below 225 degrees at 0.050) will provide higher engine vacuum and not require much relative throttle opening to move the car.

Tires and wheels significantly larger than the originals may decrease fuel economy because they are heavier, have more rolling resistance, and require more force to accelerate. This condition may be more evident on vehicles driven mostly around town or in hilly areas. In flat areas at sustained highway speeds, a taller tire may increase fuel economy because engine rpm will be decreased at a given vehicle speed.

To keep your engine running its best and to maintain fuel economy, replace your spark plugs sooner rather than later.

Overdrive automatics from the '80s can be retrofitted to almost any body style. This 200-4R was pulled from an '86 Olds Cutlass and features a Chevrolet bolt-pattern. The 0.67:1 overdrive ratio, combined with the electronic lockup converter, will help elevate fuel economy and keep crankshaft speed down, thus increasing engine life...

...The lockup converter found on these transmissions (when electronically signaled) uses an internally locking clutch that completely eliminates slipping, unlike a non-lockup converter.

To install an '80s (or later) overdrive automatic transmission with a lockup converter into a pre-computer-controlled car, you'll need to wire the converter switch so it can be operated by the driver at cruising speed...

...The original external power connector for a computer-controlled TH700-R4 lockup converter is the four-position connector (shown top). A single-blade connector (GM PN 24205373) installed in the same position and wired to a driver-activated switch will allow the converter to be locked up on command...

...To complete this straightforward conversion, contact JET Performance Products and ask for PN JET-700211 (Fourth gear lockup kit).

Add a deep 3.27:1 First gear and a 0.68:1 overdrive to your musclecar with a Keisler Engineering five-speed conversion. Although Fourth gear is 1:1, the Fifth gear overdrive ratio (0.68:1) will transform a 4.10 axle ratio into a fuel-friendly 2.72:1. To learn more, visit

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Gear Accordingly
Here's the math to show how Overdrive transmissions provide great acceleration and economy. A good all-around performance combination that will generally not kill mileage is a combined First gear ratio of 10.0:1 (First gear multiplied by the axle ratio) and the capability to drive at freeway speeds at low rpm with overdrive. In other words, if your current rear axle ratio is 3.31:1 and the overdrive transmission you'll be adding has a 3.06 First gear, your total combined gear ratio in First gear will be 10.13 (3.31 axle ratio x 3.06 First gear = 10.128:1). This combined First gear ratio is equal to having 4.56 axle gears and an M21 or M22 Muncie in First gear (2.20 low). We can illustrate this by dividing the final combined ratio of 10.128 by the Muncie M21 First gear ratio of 2.20:1 (10.128/2.20 = 4.60:1). Additionally, your effective High gear ratio with the 0.70:1 overdrive and 3.31:1 axle ratio will equal 2.32:1 (3.31 x 0.70 = 2.317:1). At 75 mph, this combination will put your freeway engine speed at 2,250 rpm.


TCI Automotive
Ashland, MS 38603
JET Performance Products
Huntington Beach , CA
Year One
Tucker, GA 30085
Gear Vendors, INC.
Keisler Engineering
Rockford, TN 37853
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