TDIMAXIMA said:
wish i understood gear ratios.....
GEAR RATIO PRIMER!
Gears work on the old Archimedes principal of mechanical advantage. Remeber "Give me a place to stand, and I will move the world?" Archimedes was speaking of mechanical advantage using torque and the distances from a lever arm. Imagine a see-saw with a pivot point that is adjustable. Move it toward the power source and a very little amount of movement is transferred to a large amount of movement at the other end, however it takes more weight (force) to move the see-saw. Now move the pivot point farther away from the source and you require less force, but more distance traveled.
Gear operate on the same principle. They consist of typically a pair of gears and their ratio is the ratio circumference of the gears with the gear transmitting the power on the bottom of that ratio.. IE a first gear of 3.50 can have a driven gear of 35 and a drive gear of 10 35/10 = 3.5. For every 3.5 revolutions of input you get 1 revolution of output, but torque is multiplied by a factor of 3.5!
Now take a 6th gear. You will have a driving gear of 40 and a driven gear of 24. Which makes the gear ratio .60. So for every .60 turns of the engine, the output turns 1 time however torque is multiplied by .6. A ratio numerically lower that 1:1 is often referred to as an "overdrive".
A numerically high gear ratio (called "Low" gears) will amplify the torque the engine is making at the expense of engine RPM, a numerically lower gear ratio (called higher gears) will reduce the torque with a trade off for decreased speed.
TDIMAXIMA said:
so lowest revs on highway = best mileage.
Now we have to take into account the torque the engine delivers. Diesels typically make their torque way down low and for the lowest fuel consumption you want to run your engine at peak torque, and for diesels that can be as low as 1,800 rpm.
Now you can go to low and loose a great deal of fuel as the engine is no operating off it's peak torque and can't generate enough power to accelerate a mass (your car) in the present gear, which is why we downshift.
