Q:
I have a classic 1971 Chevrolet Impala that I love to drive but after driving it this summer, I wish it had better fuel economy. It has a 350 cubic inch engine, 4 barrel Rochester carburetor and three speed automatic transmission but I am looking at installing an overdrive automatic transmission. One of the problems is the new transmissions are electronically controlled. Also, how do I connect the speedometer cable on an electronic shift transmission? Do you have any suggestions? Harry
A:
First, I would ensure that your engine is operating with the best fuel economy. There are a couple things you should change or check to maximize fuel economy. First, change the float in the carburetor. The Rochester Quadrajet carburetor is a good carburetor for fuel economy but the floats do soak up fuel over time and this raises the fuel level in the carburetor. This results in a rich fuel air mixture and poor fuel economy. You also want to replace the spring for the power valve piston (the mechanism that holds the metering rods). If this spring is weak, it lets too much fuel into the engine during cruise.
Next, I would check the advance mechanism in the distributor and make sure it is free and lubricated. Over the years, this often overlooked part gets gummed up and sticky. This results in incorrect ignition timing and poor fuel economy.
As for an overdrive transmission, you have a couple choices. The 200-R4 overdrive transmission was introduced in the early 80’s and still uses a cable drive for the speedometer. This transmission uses an electrically controlled lock up torque converter for even better fuel economy. The converter operation can be controlled by a simple vacuum switch on the engine intake manifold so this is a fairly easy conversion. You will need to have a custom driveshaft fabricated but there are lots of driveline shops that can do this for you.
Another option if you want a stronger transmission is the 4L60 (700-R4) transmission used in most GM pickup trucks and SUV’s from the 80’s to about 1994. Later models of this transmission were electronically controlled by a computer and were called the 4L60E (E for Electronic). Earlier models used a speedometer cable while the later ones had electric speed sensors but these can be switched to the cable type drive for the speedometer with minor transmission disassembly.
If you want to keep your original speedometer, these transmissions are the best bet. If you go to a modern speedometer, there are units that now get their speed signal from a GPS sensor that comes with the speedometer kit. Because they get their signal from satellites, they can determine the exact location of your vehicle and how fast it is moving. They are very accurate but only work if you are in satellite range. In parking garages and sometimes in mountain driving, the signal disappears for a short while.
Q:
With colder weather coming, I would like to know when I should switch to winter tires. Also, what is the best pattern to rotate the tires to maximize wear? Dorothy
A:
Tire manufacturers recommend switching to winter tires when outside temperature averages drop below 7 degrees C. When temperatures get lower than this, pavement has less traction and the tread on all-season (3-season) tires gets harder and has less grip. Don’t wait for snow to fall and as a bonus, you beat the last minute panic when everyone wants to get winter tires on.
Tire rotation patterns depend on several factors. If you have directional tires on your vehicle designed to roll in one direction only, you must rotate front to rear, assuming your vehicle has the same size tires on both the front and rear. If they are a different size, then you can’t rotate them.
If the tires are not directional, an X pattern is best for maximizing tire wear. For example, move the right front tire to the left rear position and the left rear to the right front. Do the same for the other two tires.
Rotation patterns used to incorporate the spare tire as well but in modern vehicles, very few spare tires are designed for long term use so this rotation pattern is not used anymore. Be sure to check tire pressures at least once a month and add air if necessary to maximize your tire’s tread life.
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