What Difference Would Changing a 3.92 Gear Ratio to 3.55 Make on a 1999 Dodge Dakota 3.9L?by Robert Bayly
The 1999 Dodge Dakota pickup was available with either a 2.5-liter inline-four, 3.9-liter V-6, or a 5.2-liter V-8 engine. Available in either two- or four-wheel-drive, the Dakota used either the Chrysler 8.25 or 9.25 inch rear axle and a five-speed manual, or four-speed automatic transmission. Available axle ratios were 3.21, 3.55- and 3.92-to-1 for the manual, and 3.55- and 3.92-to-1 for the automatic transmission. Changing the axle ratio can have a striking effect on performance.
Axle ratio is determined by the number of teeth on the ring and pinion gears. For instance, a ring gear with 39 teeth and a pinion gear with 10 teeth, would result in an axle ratio of 3.90-to-1. This is calculated by dividing the ring gear by the pinion gear; 39 divided by 10 equals 3.90. This means that for every 3.9 revolutions of the driveshaft, the rear wheel turns one time.
Higher and Lower
Gear sets are generally referred to as "high" and "low." It sounds counterintuitive, but high gear sets are those with a numerically lower number and low-gear sets are those with a numerically higher number. For instance, 3.92-to-1 gears are lower than 3.55-to-1 gears. High gears -- numerically lower -- tend to give better highway mileage than low gears -- numerically higher -- because at any given engine speed, the engine will be turning fewer rpm. The tradeoff is that high gears do not accelerate as quickly. Low gears offer better acceleration because more engine rpm are being used to turn the wheels at any given speed. The tradeoff is lower gas mileage at highway speeds.
A 3.92-to-1 gear ratio would be considered a low gear set for the Dakota. Off-the-line acceleration, load carrying and towing capacity would be superior to the 3.55-to-1 gear ratio. A little math will illustrate this. Using the 1-to-1 transmission ratio of third gear in the automatic and fourth gear in the manual transmission, you can calculate engine rpm at a given speed. Using the stock tire diameter of 27.5 inches, a speed of 65 mph and the formula (mph x axle ratio x 336) tire diameter, you get 65 x 3.92 x 336 = 85612.8 / 27.5 = 3113.19, which is rounded to 3,113. The automatic transmission has a fourth-gear overdrive ratio of .69-to-1, which effectively changes the 3.92 gears to 2.70-to-1 and the engine rpm to 2,148. The fifth gear on the manual transmission is 0.79-to-1 for an effective ratio of 3.10-to-1, and an rpm of 2,459. The effective, or "final drive" ratio is calculated by multiplying the transmission overdrive ratio by the axle ratio, for instance, 0.069 x 3.92 = 2.70 for the automatic transmission.
These gears would offer better gas mileage at highway speeds because of lower engine speeds, at the cost of acceleration and pulling power. Using the same formula, we get engine rpm of 2,819 in third or fourth gear, 2,148 rpm for the automatic and 2,227 for the manual transmission in overdrive. If you do not want to do the math, there are handy calculators on the Internet.
Different Gear Ratios
Another stock gear ratio available for the Dakota is 3.21-to-1, which would further increase gas mileage at highway speeds. Gear ratios beyond the stock offerings can be found through a variety of aftermarket suppliers, which offer gear sets in ratios from 2.76-to-1 to 4.11-to-1 and beyond.