Dear Ole Mechanic;
I have a 2003 Chevy ¾ ton HD pickup with a 6.0 liter V8 and automatic transmission. I have got to see if I can get better fuel mileage. I have been told that if I change the rear axle ratio from the current 3.73 to a higher speed (lower number) ratio like a 3.08 or even a 2.90, my fuel mileage would increase. I checked with a local salvage yard and they have a 3.08 ratio axle that they say will fit for $250. The mechanical swap of one complete axle housing for another seems simple enough. Is there anything that could keep this from being a simple rear axle swap?
for Better Mileage
If this was a swap in a 1983 truck instead of a 2003, then all I would warn you about is getting the correct speedometer gear at the transmission end of the speedometer cable and to make sure that the brakes were the same size. The warning about the brake size would apply to the 2003 even more because there are some big differences between the regular ¾ ton truck and a ¾ ton HD truck--especially in the brake systems. That “will fit” axle could come out of a regular ¾ ton with smaller brakes or a regular 1 ton with bigger brakes. That “HD” carries a lot of changes over either the regular ¾ ton or a regular 1 ton or a 1 ton HD, but they all “will fit”. The wheels may be weaker or stronger, the housing smaller or heavier, the brakes can have less or more stopping capacity, and there can even be less lug nuts and studs. Some lighter GVW (Gross Vehicle Weight) rating ¾ ton trucks have 6 studs and nuts while most will have 8 studs and nuts. If the “will fit” doesn’t have the 8 studs like your truck has, then carrying the weight of an extra spare helps send the fuel mileage the wrong way as carrying more weight decreases fuel mileage.
The speedometer is another can of worms. Don’t bother looking for a speedometer cable, because there isn’t one. There is an electronic speed sensor where the speedometer gear once was. The same sensor is used for all ½, ¾ and 1 ton trucks. To correct the calibration of the speedometer, the dash speedometer must be sent to a Delphi (Delco) or other authorized repair center along with the trucks VIN (Vehicle Identification Number), the axle ratio, and the tire circumference, so that the speedometer can be reprogrammed. Yep, there is a mini-computer in the speedometer.
The next computer you may run into is in the anti-lock brake system. Different capacity axles can have a different sensor tooth count due to the size of the parts. If the front and rear sensor tooth counts do not agree, then the anti-lock computer will sense that the truck is already skidding and may go into an anti-lock brake application when the brakes are applied. More than likely the anti-lock computer will see the incompatible sensor readings, shut itself down, and turn on the anti-lock warning light on the dash. You would have regular brakes but no anti-lock and no inspection sticker when that came due.
One thing that you could run into is even worse mileage. Your automatic transmission has an overdrive top gear. Couple that with a high-speed axle ratio and tall 16” tires, and the engine will run at too low of an RPM. This is more commonly known as "lugging down" the engine. Since the engine would not be in its most efficient RPM range, the fuel-injection system would dump more fuel into the engine and the fuel mileage would go down. A different manufacturer--not Chevy--ran into this problem about 15 years ago. Their “fuel economy” 6-cylinder engine was over-geared to the point that the 6 cylinder engine got worse mileage than their V8 engines.
If you can find a higher ratio rear out of a nearly identical HD truck, you may be able to improve your mileage some, just don’t expect it to haul the heavy loads that the HD’s were set up to pull. And I don’t know if the fuel savings will be enough to off set the cost of the axle and the reprogramming of the speedometer. Also, if the axle ratio change is not that great, say from your current 3.73 to 3.54 (the next normally available ratio), then don’t expect a big fuel-mileage increase. I helped a friend of mine change out his 4.10 ratio for a 3.73 ratio, and the change in mileage was less than one mile per gallon. It was an older truck, so there was no speedometer or anti-lock system to reprogram. He also got the higher ratio free in a swap, so all it cost him was labor, rear axle grease, and some brake fluid to bleed the rear axle brakes. He still says it was not worth all of the work.
You need to remember that fuel mileage is more a function of speed, aerodynamics and weight. If you are moving less weight, then you will use less fuel. Aerodynamics is just what the manufacturer made it, and grill guards, bug deflectors and light bars sticking up above the cab hurt mileage. Speed is one of the biggest single fuel eaters, because as speed doubles, the air drag goes up 4 times. Jack rabbit starts and racing to the next stop light gobbles fuel too. If ya got to go somewhere in a hurry, then be prepared to pay for it.
Herr Professor Nuzanbolts