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, as 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. When a loaded semi running in double overdrive approaches a hill, and the driver has to drop 2-1/2 gears, such as a 13-speed dropping to 11th gear or an 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 up the 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 think you will lose power on the hills, but that is not true. You will actually gain power because the downshifting is eliminated. Remember, the increase in torque in 1 to 1 or direct is 240-pound feet of torque, so the truck will go up the hill easily. Operating a truck that runs in double overdrive, which is almost all OD transmissions since the mid-1990s, when you drop 1 full gear, down to direct, the gain in torque is 240-pound feet but the speed is only 5 to 6 mph. Running in direct 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 – there is no such gear ratio, however, there is a 2.47, which is close enough. There can be a problem going to 2.47, as the ring gear diameter may not fit inside 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 might be able to help you, as well. 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, but your engine will pull harder. A 2.64 gear, with low pro 22.5 tires, will run 1,460 RPM at 65 mph. At 70 mph, the RPM will be 1,560, which is okay for engines 2012 and older. There is a 2.53 gear, and the speed vs RPM is 65 mph at 1,425 RPM and 70 mph is 1,540 RPM. The 2.47 gear at 65 mph is 1,375 RPM, but 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 3406E 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, but it’s not meant for farms, logging, or any application that requires the operator to start out in soft dirt or thick gravel. This is not a good setup for heavy haul trucks, either. Final thought: 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 about gearing, it’s always with low-pro 22.5 tires.
To finish off this month’s article, here is an update on our Annual Snow Mobile Conference. We are staying at the Comfort Inn in Ogden UT (I-15 and 21st Street). The price is $99 per night plus tax for two queen beds or one king bed, and the hotel is pet friendly. We will meet on Wednesday afternoon, February 19, ride on the 20th through the 22nd, then depart on Sunday the 23rd. Wednesday at 4:00 PM, we visit Karl Malone/ADS Power Sports, one of the largest in the USA. On Thursday evening, we plan to watch Smokey and the Bandit, and then on Friday night we will be going to a local western bar to listen to some music and do some dancing (if you want). The riding area is Monte Cristo, and it is a 35-minute drive into the mountains. It’s a beautiful drive, and the snowmobile rentals are right there at the trailhead. Both the trail riding and off-trail riding are phenomenal, and Mike Lane will be our tour guide. For more information or to reserve your spot, call us as soon as possible at (724) 360-4080.