Gearing By Request
Several emails recently asked me to explain gearing again. When a truck runs on the Dyno, the transmission is always in direct gear, or 1 to 1. Direct gear is the BEST pulling gear; the engine and transmission run cooler and quieter. This pertains mainly to 2018 and older trucks. In direct gear, 1 to 1, the additional horsepower gained is about 80, and the torque is about 240 pound-feet. A loaded semi-truck running in double overdrive approaches a hill, and the driver has to drop 2 1/2 gears, such as a 13-speed dropping to 11 gear and 18-speed dropping to 16th gear, which is 1 full gear drop. Now, the engine comes to life, and you go over the hill or long grade. Just think, if you were already in direct gear, 1 to 1, you would not have to touch the gearshift because you are already in the best pulling gear. Many people think you will lose power on the hills, but the opposite is true, you will gain power because the downshifting is eliminated; remember, the increase in torque in 1 to 1 or direct gear is 240-pound feet, so the truck will go over the hill easily. Now, a truck that runs in double overdrive, which is almost all OD transmissions since the mid-1990s, and you drop 1 full gear, and now in direct, yes, the gain in torque is 240-pound feet. However, the speed is 5 to 6 mph. Running in direct gear will increase the fuel mileage by 1/2 mpg at the same speed.
So, how do you know what gear to install in the differentials? If you like the speed and RPM, the current gear ratio is producing for you, subtract 90 from that number. Let’s say the current gear ratio is 3:55, minus 90 is 2:65; the closest gear ratio to that number is 2:64, which is pretty simple. Now, a 3:36 gear minus 90 is 2:46; no such gear ratio; however, there is a 2:47, close enough. There can be a problem going to 2:47, as the ring gear diameter may not fit in the existing housing. A sharp person at a gear house will be able to tell you if it’s possible once you give him the model number of your differentials. Older Walmart trucks ran a 2:47 rear gear with a 10-speed direct. So the housings are available, and Rob from LKQ in Easton, Md., will be able to help you. Sometimes, he can sell you a cutoff with disc brakes and the correct gear ratio for a little more than the price of the two drop-ins or pumpkins.
If you are running an older 60 series Detroit, the gear ratio may be 3:70 or 3:73. If you install the 2:64, your RPM will be about 60 to 80 RPM less; however, your engine will pull harder. A 2:64 gear, low pro 22.5 tire, will run 1460 RPM AT 65 mph. At 70 mph, the RPM will be 1560 RPM, which is okay for engines 2012 and older.
There is a 2:53 gear and the speed vs RPM is 65 mph is 1425 RPM, and 70 mph is 1540 RPM.
The 2:47 gear at 65 mph is 1375 RPM, and I do not have the RPM at 70 mph. This gear ratio is a little high for an older 60 series, but it is great for a 3406 E through a C-15 Caterpillar.
New low-RPM engines, like the Detroit, X-15 Cummins, Volvo, and Mack, use a 2:28 gear to keep the RPM lower where the engine is built to operate. Older engines DO NOT OPERATE at the lower RPM like new ones
Running in direct 1 to 1 is great for over-the-road trucks. It’s not meant for farms, logging, or any application starting out in soft dirt or thick gravel. This is not for heavy haul trucks either.
Tire size: A 22.5-inch rear tire will increase the speed by 2 to 3 mph, and a 24.5-inch tire will be 5 to 6 mph faster. When we are talking gearing, it’s always with low-pro 22.5 tires.
Annual Snow Mobile Conference Update
We changed the location. We are now at the Comfort Inn at Ogden Utah, I-15 and 21st Street. $99.00 plus tax for two queen beds or one king bed. They are pet-friendly. Meet on Wednesday afternoon, February 19, and ride on the 20, 21, and 22. Depart Sunday the 23rd. Wednesday at 4 pm, we visit Karl Malone/ ADS Power Sports, one of the largest in the USA. On Thursday evening, we will watch Smokey and the Bandit. We go to a local country western bar with a large dance floor and dance on Friday. Several restaurants in the area, one right in the parking lot.
The riding man, Cisto, is a 35-minute drive into the mountains. It’s a beautiful drive, and the snowmobile rentals are right there at the trailhead. The trail riding and off-trail riding are phenomenal, with Mike Lane as the tour guide.
Written by: Bruce Mallinson, Pittsburgh Power inc., 3600 S. Noah Drive, Saxonburg, PA, 16056. Phone: (724) 360-4080 Email: [email protected] Website: www.PittsburghPower.com