RATIO

Their is no best raito. Truck are geared different for many reasons. Different engines make power at different rpms. Some trannys are direct drive some are over drive. Different size tires need different gear ratios to keep a engine in its peek opperating zone. Some trucks are geared high for better fuel econemy. Some lower for better pulling power, both getting the load moving and pulling hills. OTR trucks at 80,000lbs or less ar eusually geared kinda high to keep rpms down and get better fuel milage. Heavy haulers, construction trucks, farm trucks, and some local inner city truck are geared lower. This helps get a very heavy load moving. A truck on soft ground like a dump truck or concrete mixer need real low gears to get moving. Continus stop and go with an inner city truck is hell on the drive line if its geared to high. These are just a few reasons trucks have different gear ratios. You need to figure out what you will do with a truck, what weight you will haul. What rpm you engine makes power at and need to cruse at. What tire size you want to run. What tranny you want and how fast you want to run before you can chose a ratio. A 550 cat with an over drive tranny, 11r24.5 tires would run about a 3.36 rear gear where a cummins needs a 3.55 and a detroit needs a 3.70 because thay all make peak power at different rpms. Adump truck would run 3.90, 4.10, or 4.30s to help off road. Just remember the smaller the number ( 3.36 ) the higher the gear. The bigger the number ( 4.30 ) the lower the gear. Hope this helps.

 

Just as melpromud said, a bunch of ratios for a reason. 4.30 in one truck and near perfect for what i do(some off road, and always heavy). 3.70/5.04 2 speed in other truck and makes perfect setup, but... heavy and expensive. The low gears are alittle tough on the MPG's, but it is nice to know when i let clutch out, the driveshaft, etc is going to take it.
This has been brought up before and most will argue with me but i will burn alittle extra fuel and have the truck geared for the job i am doing.
Just my opinion and preference....

 

Is that Trans ratio or differential ratio thats is being talked about? Because in a car the higher the ratio 4.11 is quicker than lower ratio 3.11. And doesnt the lower ratio allow for higher top speed at lower RPM? Educate me

 

there are lots of variables, tire size from 22.5 to 24.5, two different ratios like 3.42 with 22.5 and 3.55 with 24.5 could result in about the same engine rpm at say 65 mph. Transmissions can be spec'd with different overdrive (or even direct, and underdrive) ratios, so what they are combined with for rear's ratio will effect engine speed at a specific road speed.

But in general, a lower number will give you less rpms at speed, and hopefully better fuel mileage. Higher ratios will enable the truck to pull heavier loads and improve the trucks starting ability on steeper grades of hills.

 

car with 4.11 should be quicker through the 1/4mi than a car with 3.11 but the car with 3.11 gears will be capable of a higher top speed given both have the same trans.

 

The gear ratio is the amount the engine turns compared to the rear wheels. Say your trans high gear is 1 to 1. That means in high gear the engine and the output shaft of the trans turn the same speed. If the rear end ratio is 4.10 the engine and drive shaft turn 4 and 10 onehundreths, or 4 .1 times every time the rear wheels turn once. The lower the number the higher the gear. At 2000 Rpms a truck with 3.08 rears will be moving much faster than a truck with 4.30s, but the 4.30s will get a load moving a lot easier.
Try to think of gearing like a pry bar. If a 3 foot bar isn't enough leverage to move an object than a 4 foot or 5 foot bar will be needed. With vehicle gearing if a 3.08 wont move the load a 4.10 or 5.38 gear might be needed.
The more leverage (gearing) the higher the RPM and at road speeds the lower the fuel econemy. Most trucks and cars for that matter are geared somewhere in the middle. Low enough to get moving but high enough to keep the RPMs low when crusing.

 

Just remember the ease of turning the wheels sacrifices are made in mpg. The easier it is to turn the wheels with that 4:10 rear also means a higher consumption of fuel than a truck with 3:70 rears

A truck that is working short hauls and mostly in steep terrain where speed is not necessary a higher ratio would probably be preferred. Also if you plan to move things like buildings and what not.

A truck that is going to be operating on the wide open spaces like hte plains of the midwest will do well to have some thing like 3:55 rears. It is your choice. Yet make the right choice, as your choice will define wether you make a profit. And this industry requires profit.

 

Their is no one right gear for running OTR 48 states. What matters is what engine, what HP and torque that engine produces, what trans you have, what tire size you have, how heavy you are, how fast you plan to drive, and what part of the country you plan on running the most. Change any one of these veriables and the answer to the right gear changes. Their are to many veriables to say one gear raito is the best. A Cummins usually needs one gear lower than a cat and a Detroit one lower than a Cummins but the new Detroits I see seem to be geared the same as the cummins. The reason different engines need different gearing isnt so much because one has more power than the other but because the make peak opperating power at different RPMs.