Terminoligy

hey. i was watching some rolling cb interviews and when the drivers gave the specs for thier trucks one of them said he had 3.55 rears. what does that mean. 3 and a half feet tall rear tires??? also alot of guys had custom 1999 379's. i guess peterbilt did a damn fine job that year is that why everybody has one?

hey. i was watching some rolling cb interviews and when the drivers gave the specs for thier trucks one of them said he had 3.55 rears. what does that mean. 3 and a half feet tall rear tires??? also alot of guys had custom 1999 379's. i guess peterbilt did a damn fine job that year is that why everybody has one?

Drive axle gear ratio, aka: "rears". The higher the ratio, greater power, reduced fuel economy, and slower top speed. Opposite is true for a higher gear ratio number.

Drive ratio is usually carefully matched with the transmission and engine to provide an optimal amount of horsepower, torque, fuel economy and top speed for moving specific types of loads over different types of terrain and roads.

A dump truck that moves heavy loads over rugged terrain will have a lower rear ratio. An OTR tractor that will spend 85% of the time cruising at highway speeds will have a higher tear gear ratio.

hey. i was watching some rolling cb interviews and when the drivers gave the specs for thier trucks one of them said he had 3.55 rears. what does that mean. 3 and a half feet tall rear tires??? also alot of guys had custom 1999 379's. i guess peterbilt did a damn fine job that year is that why everybody has one?

Drive axle gear ratio, aka: "rears". The higher the ratio, greater power, reduced fuel economy, and slower top speed. Opposite is true for a higher gear ratio number.

Drive ratio is usually carefully matched with the transmission and engine to provide an optimal amount of horsepower, torque, fuel economy and top speed for moving specific types of loads over different types of terrain and roads.

A dump truck that moves heavy loads over rugged terrain will have a lower rear ratio. An OTR tractor that will spend 85% of the time cruising at highway speeds will have a higher tear gear ratio.

Meant rear in last sentence.

hey. i was watching some rolling cb interviews and when the drivers gave the specs for thier trucks one of them said he had 3.55 rears. what does that mean. 3 and a half feet tall rear tires??? also alot of guys had custom 1999 379's. i guess peterbilt did a damn fine job that year is that why everybody has one?

Drive axle gear ratio, aka: "rears". The higher the ratio, greater power, reduced fuel economy, and slower top speed. Opposite is true for a higher gear ratio number.

Drive ratio is usually carefully matched with the transmission and engine to provide an optimal amount of horsepower, torque, fuel economy and top speed for moving specific types of loads over different types of terrain and roads.

A dump truck that moves heavy loads over rugged terrain will have a lower rear ratio. An OTR tractor that will spend 85% of the time cruising at highway speeds will have a higher tear gear ratio.

Meant rear in last sentence.

Sorry, really messed this up, second sentence in the first paragraph should have said the lower the ratio. I should know better to do this when I'm tired.

That is the gear ratio for the rearends. In this case the driveline will complete 3.55 revolutions for 1 revolution of the drive tire. The higher the number the lower the gear ratio. So if someone has 4.10 gears they are going to have an easier time at the starting line but will sacrifice fuel mileage on the highway. This is why you will see highway specd trucks running 2.30 to 3.33 rears while something like a dump truck will run 4.10 gears because it is easier to get that truck moving and they do very little highway driving.

Then heavy haul trucks can be specd with dual range rears like a couple of ours do. In low range they are running 5.38 or something like that and in high range they are running 4.10 gears.

The closer to "1" you get with the gears, the harder it is to start off the line.

That is the gear ratio for the rearends. In this case the driveline will complete 3.55 revolutions for 1 revolution of the drive tire. The higher the number the lower the gear ratio. So if someone has 4.10 gears they are going to have an easier time at the starting line but will sacrifice fuel mileage on the highway. This is why you will see highway specd trucks running 2.30 to 3.33 rears while something like a dump truck will run 4.10 gears because it is easier to get that truck moving and they do very little highway driving.

Then heavy haul trucks can be specd with dual range rears like a couple of ours do. In low range they are running 5.38 or something like that and in high range they are running 4.10 gears.

The closer to "1" you get with the gears, the harder it is to start off the line.

Pat gave the best answer, I reversed high vs low ratios and their effect on power, speed and fuel economy.

This is all like reading greek to me haha... I'm sure I'll learn some of this someday but right now it's a whole other language.

I'm sure I'll learn more of that one day but thanks. All your answered are great! You guys are really helpful on here!

I'm sure I'll learn more of that one day but thanks. All your answered are great! You guys are really helpful on here!

Michael, not to worry, unless you are specing your own truck(s) it's something that you will never need to worry about. And again I apologize because I reversed high vs low (tired and trying to fat-finger my phone) so please disregard my dislexic rambling and focus on Pats response. He is a heavy haul driver so his knowledge of drive gear ratios is at an expert level.