Gear ratio Basics
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Senior User
Joined: Mar 2004
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From: Houston, Texas
Gear ratio Basics
i have a 94 f150 automatic transmission.
could anyone give me a quick run down on gear ratios?
basically what it is? and why its important?
what dot he different ratios do? whats good for towing? accelerations? top speed? fuel efficiency?
could anyone give me a quick run down on gear ratios?
basically what it is? and why its important?
what dot he different ratios do? whats good for towing? accelerations? top speed? fuel efficiency?
Laughing Gas
Joined: Jul 1999
Posts: 1,047
Likes: 22
From: Lacey, WA
GasMonkey,
I'll take a stab at your question.
Gear ratio is a number that tells you how many turns of an input shaft get you a single turn of the OUTPUT shaft. An example is a rear differential that has 4.10 gears, but really it should be written as 4.10:1 as it's always expressed to a "1". This means that the input shaft to the rear end (the drive shaft) will need to turn 4.10 times to get your rear axle (the tires) to turn 1 time. Make sence?
Now an Automatic tranny will have 3 or 4 gears with 4th being an over drive. The ratios will look something like this; 1st 2.78:1, 2nd 1.68:1, 3rd 1.0:1, and 4th .7:1 These numbers are not exact as I am tring to pull them from memory. What these numbers tell you is the number of crankshaft rotations of the engine to get 1 rotation of the drive shaft. Notice that 3rd gear is a 1:1 ratio and that in 4th the engine turns over .7 times to get 1 complete driveshaft rotation. That's the meaning of "overdrive".
To find out your trucks "final" ratio you multiply the tranny x differential. Example 4th gear's .7:1 x the diff's 4.10:1 gets you 2.87:1 (.7 x 4.10 = 2.87) This shows that the engine has to rotate 2.87 times for your tires to rotate once.
Now on to yer question. If you want to change your ratio the place to do it is at the rear differential as it would be to expensive to change trannys.
Differentials with low numbers (like 2.73:1) are considered "highway" gears because the engine does not have to rotate as much(less gas). Using the example above will get you .7 x 2.73 = 1.911 You will use less gas compared to the 4.10:1 and will have a higher max speed, all things considered. Only problem is, this example would not be a very good tow vehicle or accelerate quickly, again, all things considered. It would be hard on the engine while towing in hills. Towing trucks use something in the 4.10:1 range. You can also just compromise and get something in the 3.5:1 range which is what most 1/2 ton trucks come with stock.
Hope this rambling makes sence.
Lee
I'll take a stab at your question.
Gear ratio is a number that tells you how many turns of an input shaft get you a single turn of the OUTPUT shaft. An example is a rear differential that has 4.10 gears, but really it should be written as 4.10:1 as it's always expressed to a "1". This means that the input shaft to the rear end (the drive shaft) will need to turn 4.10 times to get your rear axle (the tires) to turn 1 time. Make sence?
Now an Automatic tranny will have 3 or 4 gears with 4th being an over drive. The ratios will look something like this; 1st 2.78:1, 2nd 1.68:1, 3rd 1.0:1, and 4th .7:1 These numbers are not exact as I am tring to pull them from memory. What these numbers tell you is the number of crankshaft rotations of the engine to get 1 rotation of the drive shaft. Notice that 3rd gear is a 1:1 ratio and that in 4th the engine turns over .7 times to get 1 complete driveshaft rotation. That's the meaning of "overdrive".
To find out your trucks "final" ratio you multiply the tranny x differential. Example 4th gear's .7:1 x the diff's 4.10:1 gets you 2.87:1 (.7 x 4.10 = 2.87) This shows that the engine has to rotate 2.87 times for your tires to rotate once.
Now on to yer question. If you want to change your ratio the place to do it is at the rear differential as it would be to expensive to change trannys.
Differentials with low numbers (like 2.73:1) are considered "highway" gears because the engine does not have to rotate as much(less gas). Using the example above will get you .7 x 2.73 = 1.911 You will use less gas compared to the 4.10:1 and will have a higher max speed, all things considered. Only problem is, this example would not be a very good tow vehicle or accelerate quickly, again, all things considered. It would be hard on the engine while towing in hills. Towing trucks use something in the 4.10:1 range. You can also just compromise and get something in the 3.5:1 range which is what most 1/2 ton trucks come with stock.
Hope this rambling makes sence.
Lee
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Joined: Dec 2002
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From: USA
Lee did an awesome job explaining that!
I'll add a little by saying that the right gear ratio for your truck depends on a few factors. The biggest factor is tire diameter. The tire diameter is part of the truck's gearing. For trucks with stock size tires, the stock gear ratios are usually pretty good for most uses. But for bigger tires, a lower ratioi (higher numerically) is needed to overcome the torque loss due to the taller tires. It takes more torque to turn taller tires and the axle gearing is a torque multiplier. Lower gears multiply more torque. Also, a heavy truck with a small engine needs lower ratio gears than a light truck with a big engine.
My F350, which weighs 6500 lbs unloaded, was a real dog with a 351 engine, 3.55 gears and 33" tires. I changed to 4.56 and now have alot more power, at the expense of higher engine rpms at freeway speeds. An F150 weighing 4500 lbs with a 351, 3.55 gears and 33" tires would have an easier time pulling hills than my truck did because of the lower weight.
As to the transmission, it could be a C6 3-speed auto without overdrive, or an AOD 4-speed auto with overdrive, or an E40D 4-speed auto with overdrive. You can identify them by counting the number of bolts around the oil pans. C6 has 17 bolts, the AOD has 14 and the E4OD has 20.
Here are the gear ratios of all 3 trans:
C6- 2.46, 1.46, 1.00, rev- 2.18.
AOD- 2.40, 1.47, 1.00, 0.67, rev- ?.
AODE- 2.84, 1.55, 1.00, 0.67, rev- ?.
E4OD- 2.71, 1.54, 1.00, 0.71, rev- 2.18.
The AOD is a manually operated overdrive automatic. The AODE is electronic and will have an overdrive lockout button either on the end of the shift lever or on the dash, as will the E4OD.
I'll add a little by saying that the right gear ratio for your truck depends on a few factors. The biggest factor is tire diameter. The tire diameter is part of the truck's gearing. For trucks with stock size tires, the stock gear ratios are usually pretty good for most uses. But for bigger tires, a lower ratioi (higher numerically) is needed to overcome the torque loss due to the taller tires. It takes more torque to turn taller tires and the axle gearing is a torque multiplier. Lower gears multiply more torque. Also, a heavy truck with a small engine needs lower ratio gears than a light truck with a big engine.
My F350, which weighs 6500 lbs unloaded, was a real dog with a 351 engine, 3.55 gears and 33" tires. I changed to 4.56 and now have alot more power, at the expense of higher engine rpms at freeway speeds. An F150 weighing 4500 lbs with a 351, 3.55 gears and 33" tires would have an easier time pulling hills than my truck did because of the lower weight.
As to the transmission, it could be a C6 3-speed auto without overdrive, or an AOD 4-speed auto with overdrive, or an E40D 4-speed auto with overdrive. You can identify them by counting the number of bolts around the oil pans. C6 has 17 bolts, the AOD has 14 and the E4OD has 20.
Here are the gear ratios of all 3 trans:
C6- 2.46, 1.46, 1.00, rev- 2.18.
AOD- 2.40, 1.47, 1.00, 0.67, rev- ?.
AODE- 2.84, 1.55, 1.00, 0.67, rev- ?.
E4OD- 2.71, 1.54, 1.00, 0.71, rev- 2.18.
The AOD is a manually operated overdrive automatic. The AODE is electronic and will have an overdrive lockout button either on the end of the shift lever or on the dash, as will the E4OD.
Thread Starter
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Senior User
Joined: Mar 2004
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From: Houston, Texas
thanks guys, this really clears things up. id like the lower ratio for better gas milage since i dot he majority of my driving on the highway, but want to still be able to tow and accelerate well. so i guess ill stick with stock. thanks for allt he help, this was actually a decided factor as to when to get my tranny flushed. wouldnt want to flush then get the gears changed and have to refill the tranny.
thanx yall
thanx yall
