What does ratio mean for a semi truck?

Answer 1

There are actually two possibilities to what they're referring to. The one which wasn't mentioned previously was the transmission drive ratio, which factors heavily into how you spec out a truck.. there's a world of difference between running, say, 3.08 rears with a direct drive transmission (1 to 1 ratio), a low hole overdrive transmission (.85 - .90 to 1 ratio), and a large hole overdrive transmission (typically .73 or .74 to 1 ratio). What this refers to is the number of turns of the transmission input shaft to the number of turns of the transmission tail shaft. If that ratio is 1 to 1 at the Top Gear, that's called direct drive. If the tail shaft turns faster than the input shaft, that's called overdrive. The one mentioned already refers to rear end gear ratio, and it's the number of turns of the pinion gear to the number of turns of the ring gear - or, to simply it, the number of full rotations the driveshaft makes for each full rotation of the axle. So, if you see a truck with 3.36 rears, it means that driveshaft makes 3.36 rotations for each rotation of the axle.

Answer 2

When referring to ration in semi trucks what is meant is rear end gear ratios. If you drive mostly mountains and/or city you will want something with a higher number. If most of your routes are flat country then you will go with a lower ration number. The higher the ration the lower the gearing. If at 65 mph your truck runs at 1400 rpm that is a lower ratio/tall gearing. If at 65 your truck is running at 1800 rpm then its a higher ration/short gearing. Higher gearing will give you better power for city stop and go plus will pull the hills better but fuel mileage will likely suffer. The opposite is true for lower gearing. You will have lower rpm and likely better fuel mileage but not as quick off the stop and you will lug up the hills slower.