Differential Gear Ratios

2021, Gear Ratio, kenworth -

Differential Gear Ratios

Differential gear ratios; there is a lot of confusion in the trucking industry about what is the best gear ratio for pulling power, fuel mileage, and a good cruising speed.  This article will pertain to the over the road semi truck, NOT heavy haul, or vocational local trucking. To pick the right rear gears you must know the peak torque RPM of your engine, the ideal cruising RPM, and the speed you mostly drive on the interstates.  Today almost all transmissions are double overdrive which is a .73 final drive gear.  The standard 13 and 18 speed transmissions are all double overdrive, which means in the 13 speed transmission, 11th gear is direct, 12th gear is .85, and 13th gear is .73.  With the 18 speed transmission 16th gear is direct, 17th gear is .85 and 18th gear .73.  Back around 1990, Eaton did make a single overdrive 18 speed in which 18th gear is .85, however, this is a very rare transmission. I happen to have one in my 1989 T-600 Kenworth.  This article is pertaining to most 1995 and newer 13 and 18 speed transmissions.  If you have an Eaton 10 speed autoshift or a 10 speed manual transmission and have 3.36, 3.55, 3.73 rear gears you still have a double overdrive transmission, however, your 9th gear is direct and 10th is .73. Unfortunately this transmission has a big gap between 9th and 10th gear.  I do not care for 10th gear in the 10 speed.  The 10 speed it’s still considered a double overdrive because of the .73 overdrive gear and because you cannot split the gears with the large gap between 9th and 10th gear. 

What is the most efficient gear in the transmission for pulling and cruising?  It’s direct gear.  Why?  Because the power from the engine goes straight through the transmission to the differentials.  When pulling in overdrive the power coming into the transmission is diverted in the overdrive section of the transmission and consequently power is lost, heat is generated, and the transmission produces more noise.  1 to 1 or direct gear, is always the best gear for fuel mileage, quietness, cooler temperatures in the transmission, and more pulling power. However, to run in direct gear, the rear gears must be either 2.64, 2.47, 2.21 and I think there is even a higher gear than the 2.21.  

The average speed today is 70 miles per hour. Engines want to be near the peak torque so it’s up to YOU to call the manufacturer of your engine and ask them at what RPM is the peak torque, and what RPM do they want the engine to cruise at.  Now you can determine what fingal gear ratio you need for your average driving and speed.  

Next is a list of speeds vs RPM running low pro 22.5 tires.

2.64 gears; 62 mph=1390 rpm, 65 mph=1450 rpm, 70 mph=1625 rpm

2.47 gears; 55 mph=1200 rpm,  60 mph=1280 rpm, 65 mph=1375 rpm

3.08 gears, which is perfect for an X-15 Cummins running in double overdrive, 13th or 18 gear; 65 mph=1265, 70 mph=1340, 75 mph=1430 rpm. 

The reason I’m telling you about the 3.08 is because Kenworth and Peterbilt do NOT understand running in direct gear, and will usually not build the truck with a 2.21 or 2.47 rear gear. So you might be stuck running in overdrive and in that case the 3.08 is a better gear than the 3.36 or 3.25 that most of the Paccar trucks on the lot are equipped with. Again this is all predicated on low pro 22.5 tires.  

If you have an older truck, 1995 up to about 2014, and you are thinking about changing the rear gears and like the rpm’s the engine is currently turning at your cruising speed, than take your current gear ratio and subtract 91 from it and that will put you very close to the correct gear to run in direct gear.  For example, 3.55-91 =2.64,  3.36-91=2.45 so you would install the 2.47 gear, and the 3.08 -91=2.17 or the 2.21 gear ratio.  The gears mentioned in this article are Eaton specs, Meritor has the equivalent gear just a few numbers difference. I do not have the speed vs the rpm of the 2.21 gear ratio, you will have to go on Eaton’s web site and look at the gear ratios available and determine the speed vs rpm. 

My good friend Mike Lane put it this way for people that do not understand that pulling in direct has more power than double overdrive. Think of a breaker bar, if it’s 36” long you will be able to torque more than if the breaker bar is 18” long. The 36” long bar is direct gear, the 18” bar is double overdrive.   

Pittsburgh Power is NOT a dealer for gears or transmissions, however we do change rear gears, transmissions, remove auto shift transmissions and install 13 or 18 speed manual transmissions.  Usually we purchase the rebuilt differentials and transmissions from various rebuilders, and we have a local transmission rebuilder near us if you want to stay with your transmission and just need it rebuilt. If you want to play around with different gear ratios and tire sizes, please download the Gear Calculator on our website. You’ll need Microsoft Excel on your computer to make it work, but it’s a very useful tool. 

This is Don Griesmann’s 2018 Kenworth W900L. It’s been a great truck for Don but he came across the opportunity to buy a beautiful ‘99 Pete that just had a body-off restoration so he’ll be looking to sell this W900L. It’s a 283 inch wheelbase truck. It has a Cummins X-15 605 HP engine that has been tuned to 730 HP right here at Pittsburgh Power and verified on our dyno. Power is delivered through an 18 speed trans with a 3.25 rear end. The truck features full lockers, all disk brakes, a tool box with steps, and 22.5 low pro tires with low miles. Upgrades include JW speaker headlights, a PDI intake manifold, Espar bunk heater, and Holland aluminum 5th wheel.  Recent maintenance includes recent DPF cleaning, new clutch, Fleet Guard filters, and new batteries. It only has 271,000 miles so it’s barely broken in! Don also has all the maintenance records, receipts, and oil samples. He’s asking $130k and his number is 610-390-7497. If you’re interested please give him a call. 


Written by;

Bruce Mallinson

Andrew Wilson