Okay, from what I learned off Howstuffworks.com, Gear ratio affects the RPM. Is there more to this? And what is a Final Drive Ratio?
Thanks guys; I couldn't find the general technical forum section.
Drivetrain Gear Ratio/Final Drive Ratio
the tires, rear end, transmission all affect the rpm at a given road speed.
final drive ratio is the ring and pinion [rear end in a RWD car].
to figure the road speed at engine speed, you need the variables.
rpm-3000 rpm, lets say 3rd gear is 1.302:1, final drive [rear end] is 3.916, and you have a rear tire size of 255/40-17
3000 rpm into the transmission- through 1.302 gears equals 2304.15 rpm. [3000/1.302=2304.15]
now the transmission is turning the driveshaft at 2304.15 rpm, which turns the rear end input {[differential] a front wheel drive car doesnt have a rear end diff. in that case the differential and final drive gears are intergral with the transmission. the diff in a FWD car turns the axle shafts}
with 2304.15 rpm input at the differential, the 3.916 ratio final drive turns the wheels at 588.39 rpm [2304.15/3.916=588.39]
for a 255/40-17 tire- 255 mm * .40 [aspect ratio] = 102 mm sidewall.
102/25.4 [mm per inch] = 4.016 inches.
4.016 * 2 [2 sidewalls [top and bottom]] = 8.032 inches. 8.032 inches + 17 inches [wheel diameter] = 25.032 overall diameter.
25.032 * 3.14 = 78.6 inches per tire revolution.
78.6 inches * 588.39 rpm = 46247.45 inches per minute.
46247.45 / 12 [inches per foot] = 3853.95 feet per minute
3853.95 fpm * 60 [minutes per hour] = 231237 feet per hour
231237 fph / 5280 [feet per mile] = 43.79 miles per hour.
therefore, with the above specs, this car would be travelling 43.79 MPH at 3000 rpm in 3rd gear.
final drive ratio is the ring and pinion [rear end in a RWD car].
to figure the road speed at engine speed, you need the variables.
rpm-3000 rpm, lets say 3rd gear is 1.302:1, final drive [rear end] is 3.916, and you have a rear tire size of 255/40-17
3000 rpm into the transmission- through 1.302 gears equals 2304.15 rpm. [3000/1.302=2304.15]
now the transmission is turning the driveshaft at 2304.15 rpm, which turns the rear end input {[differential] a front wheel drive car doesnt have a rear end diff. in that case the differential and final drive gears are intergral with the transmission. the diff in a FWD car turns the axle shafts}
with 2304.15 rpm input at the differential, the 3.916 ratio final drive turns the wheels at 588.39 rpm [2304.15/3.916=588.39]
for a 255/40-17 tire- 255 mm * .40 [aspect ratio] = 102 mm sidewall.
102/25.4 [mm per inch] = 4.016 inches.
4.016 * 2 [2 sidewalls [top and bottom]] = 8.032 inches. 8.032 inches + 17 inches [wheel diameter] = 25.032 overall diameter.
25.032 * 3.14 = 78.6 inches per tire revolution.
78.6 inches * 588.39 rpm = 46247.45 inches per minute.
46247.45 / 12 [inches per foot] = 3853.95 feet per minute
3853.95 fpm * 60 [minutes per hour] = 231237 feet per hour
231237 fph / 5280 [feet per mile] = 43.79 miles per hour.
therefore, with the above specs, this car would be travelling 43.79 MPH at 3000 rpm in 3rd gear.
no problem. i was bored.
automatics usually have a taller [numerically higher] final drive ratio due to the gear spacing and the torque converter.
remember, a torque converter [auto] is much different from a clutch.
when the car is not moving, and you give it gas, the TC actually mulitplies the torque the engine is feeding it until the rotors even out in speed. it acts aot like gears do, but the whole means is different. we wont get into the TC's behavior.
automatics usually have a taller [numerically higher] final drive ratio due to the gear spacing and the torque converter.
remember, a torque converter [auto] is much different from a clutch.
when the car is not moving, and you give it gas, the TC actually mulitplies the torque the engine is feeding it until the rotors even out in speed. it acts aot like gears do, but the whole means is different. we wont get into the TC's behavior.
in an automatic, gear ratios arent exact mathematically due to the fact that the torque convertor never spins the turbine at the exact same speed as the rotor since they are fluid coupled..
there is always a slack zone between the rpm with your foot on and off the gas with the car moving at a specific speed.
there is always a slack zone between the rpm with your foot on and off the gas with the car moving at a specific speed.
