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Automatic Transmission

  1. Feb 17, 2006 #1


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    I've just bought an automatic transmission car (7 speeds), and I am used to drive manual cars. My question is the next:

    -do you have any advice for ensure a long duration of the transmission?. I mean, sometimes I don't know if it is correct to drive with the overdrive speed at low velocity, such as when I stop at traffic lights. Do you usually put the Neutral when waiting for a green light?. What is the difference between drive and overdrive gears?, should I drive with the drive gear unless I am in the freeway?

    And yeah, it's a Pontiac Firebird. :biggrin:
  2. jcsd
  3. Feb 17, 2006 #2
    Overdrive gears are simply a lower (numerically) ratio so that the wheels will turn more frequently than the engine does. Therefore, you are likely to experience increased gas mileage. There is no reason to put the transmission in neutral. In fact, I would think doing so would cause greater wear on the internals by having them engaging and disengaging. It is common on vehicles that are used to tow something to not engage overdrive. Usually there is a button allowing the driver to select "Tow/Haul" mode. The reason that the overdrive gear is not desirable is that most engines do not have sufficient power to remain in that gear while driving down the highway. Thus, the transmission continually shifts in and out of overdrive. This continuous work (shifting) plus the additional strain from the load being towed cause the transmission to overheat and wear out.

    As an example, 1997-2005 Corvettes with an Automatic transmission had the following ratios:

    Code (Text):

    Gear   Transmission           Rear Axle       Final Ratio
    1st     3.06                       2.73             8.3538
    2nd     1.63                      2.73              4.4499
    3rd      1.00                      2.73              2.73
    4th     .7                          2.73              1.911
    What year Pontiac Firebird did you purchase with a 7 speed automatic? The newer corvettes are available with a 6 speed automatic.
  4. Feb 17, 2006 #3


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    Ok, thanx for your reply. I will try to underline the essential points:

    -It's a 1991 Firebird, with speeds: PNRDO12 (maybe you don't count R as a speed)

    -You told me overdrive would cause more gas mileage than driving with drive. But I don't understand it. I mean, it would happen for all regimes (freeway and city)? Also you said:

    "The reason that the overdrive gear is not desirable is that most engines do not have sufficient power to remain in that gear while driving down the highway".

    I thought it was the contrary. Actually I thought that Overdrive was a gear with small torque and high RPM, for running in freeways. So I would think that the transmission and mileage is suffering at low RPM, similiarly to what happens with a manual car if you drive in city with the 5th gear.

    Therefore the fundamental question is: is the Overdrive gear a whole of gears with smaller torque capacity and for higher rpm's compared with those of Drive?. If so I would expect to have more gas mileage driving with Overdrive in city.
  5. Feb 17, 2006 #4


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    Ok, its a 4 speed automatic. Its model number TH700R4 and its the transmission used in the Firebird/Camaro, Corvette (until 2006), S10 and 1/2 ton and 3/4 ton pickups up to 7500lb GVWR. It is the same basic tranny updated in 1993 to the 4L60 model number and later the 4L60E in 1994 with an electronically controlled valve body.

    Put it in overdrive and leave it there. It will automatically select the appropriate gear based on load and will lockup the converter as often as it can. A 91 Firebird is a 3200lb car, note the tranny was used in applications to twice that weight. Yes, if the torque converter is unlocked it will create more heat, but the 5.0L and 5.7L engines have plenty of torque to avoid this on all but the steepest hills and the 3.1L engine will have more load and cause the transmission to select a lower gear (plus have less power to create heat in the torque converter).

    Heat kills tranmissions, if you are pulling a trailer or hauling a full load of dirt in the back of a 4 wheel drive pickup with this same transmission, you'll want an external transmission oil cooler. Otherwise, like my 4L60E with 130k on it and shifting strong, you can expect a long life from it with a regular filter and fluid change at maybe every 50-60k.

    If you live in a very hot place like Arizona and sit in stop-n-go traffic or have performance modifications on the motor, an external transmission cooler might be worthwhile and would be cheaper than any other method.
  6. Feb 18, 2006 #5


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    Hey Clausius, I take it this is your first car? Congrats and I hope it works out for you for a long time.

    Looks like you have a lot of good advice already. I'd say Cliff's advice about changing the oil regularly and installing an oil cooler if driving in severe conditions covers the most important issues. Don't forget to change the filter when you change the oil.

    I'd also agree with Texas that slipping it into neutral while driving isn't going to help anything and may hurt. Unless you're into street racing, just putting it in Drive and leaving it there is the best option.

    Regarding overdrive, the transmission has a "torque converter" in it that acts a bit like a clutch in a standard transmission, except it normally allows a small amount of slippage between the input shaft from the engine and the output shaft to the transmission. When on the highway, there's a feature which locks these two shafts together so there's no slippage and no loss of energy associated with that. This is what overdrive does, it simply negates the torque converter and energy losses associated with it as long as the car is cruising and not in need of changing gears.

    Along with the transmission, you also want to check other things. If you got an owner's manual with the car that should tell you what to check. If not, you can pick up a book at your local auto parts store - they're always a good investment. You'll want to check and change your motor oil, radiator fluid, tire pressure, etc... on a regular basis. Those kinds of things go a long way in increasing reliability of your vehicle.

    Best of luck with your new car!
    Last edited: Feb 18, 2006
  7. Feb 18, 2006 #6
    By increased gas mileage, I meant better MPG (miles per gallon). Again, the reason is the wheels are turning less often in overdrive as compared to a lower gear (1st, 2nd, or 3rd). Thus, to achieve a given MPH the engine can/will be at a lower speed (RPM). Thus, if the engine is performing fewer revolutions but achieving the same MPH, then less fuel is consumed (i.e. better MPG) because there are fewer engine cycles occurring over a given period of time. Typically, a truck that is used for towing will have a higher numerical rear axle gear ratio (e.g. 4.10 vs 2.73). If the transmission and the tires remain constant, then a car with 2.73 rear axle gears might only require 1200 RPM’s to achieve 60 MPH in overdrive whereas a car with 4.10 rear axle gears will require 1700 RPM’s to achieve 60 MPH. Thus, for every minute spent traveling at 60 MPH the car with 4.10’s will have 1700 revolutions vs 1200 for the car with 2.73’s, or 250 more cycles (500 / 2). Thus, more fuel is consumed.

    It is funny, I am on many different automotive performance enthusiast boards, and there are always misconceptions about fuel economy. Fuel economy is directly related to how much fuel the motor consumes. I know that sounds stupid, but most people don’t think about how simplistic it really is. Modern cars (I should say car manufacturers) have the burden of meeting EPA emission regulations. As such, the desired Air-to-Fuel ratio is 14.7:1. A modern Electronic Fuel Injection engine can run much leaner (less fuel) while cruising down the highway. If one knows how to manipulate the computer controlling the engine, then you can command leaner fuel mixtures. Thus, less fuel is consumed and MPG improves. :) The risk being that leaner fuel mixtures are detrimental to engines when they placed under load. The proper A/F is critical for thermal management. You can easily run a modern EFI engine at 16:1 with no load cruising down the highway (straight road, no hills, no high headwind, etc). If you tried to run the motor at wide open throttle (lots of load) with an A/F of 16:1, you would probably only get to do so once as the motor (particularly the pistons) would melt.

    This leads me to my point about using overdrive when towing. What I meant when stating that overdrive is not desirable because most engines don’t have sufficient power is that most engines do not have enough power to sustain a given MPH when conditions change (a slight grade, headwind increases, or even a turn). Thus, the car’s computer tells the transmission to downshift (i.e. go from overdrive, 4th, down to 3rd gear). In a modern car’s computer, there are a couple of conditions that must be met for the computer to command the transmission to shift to a different gear (either up or down). Typically, these conditions are primarily based on speed (MPH), RPM, and throttle position. So, the computer actually manages the gear shifts for you. You are correct in that you would get worse gas mileage (lower MPG) if you always drove around in overdrive. You would find yourself having to push the throttle down farther to achieve your desired acceleration which consumes more fuel. Just put the car in overdrive and let the computer do the shifting.

    BTW, in many modern cars with manual transmissions there is a solenoid that forces the driver to shift from 1st to 4th under certain circumstances. The end result is better fuel economy.
  8. Feb 18, 2006 #7


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    That is one nice looking car Clausius, congrats.

    To sum it up... put it in O and go. When you are backing up put it in R, when shutting down put it in P. Thats all there is there ain't no more. IF you don't get up to highway speeds there will be no differencr between D and O.

    As for hauling and Towing, unless you mean hauling a$$, .... DON"T!
    Last edited: Feb 18, 2006
  9. Feb 19, 2006 #8


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    And when you want to have some fun, you can leave it in a lower gear (1, 2, or 3), and it won't upshift beyond that. That's useful if you want more throttle response, are climbing hills, or driving on winding roads.
  10. Nov 8, 2006 #9
    This is not correct. Overdrive is simply another gear above D (actually, below is more technically correct, as it is lower than 1:1, as .7 in the example). It allows the engine to drop to a lower RPM at cruising speeds.

    As to the torque converter, it is a feature of the converter ITSELF that once the speed becomes steady, it physically locks itself, reducing RPMS a wee bit more, and reducing heat. It is referred to as a "lock-up" or "locking" TC, sometimes a Stall converter. No automatic comes be default with a locking converter, but it is very common since the 80s.
  11. Nov 9, 2006 #10


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    Can't help intruding with a personal prejudice here, buddy. Turf that bastard and throw in a Rock-Crusher. (I hate GM crap, but that is one excellent transmission.)
  12. Nov 9, 2006 #11


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    LOL I was like "a 7-speed auto tranny in a firebird?! WTF?!"

    It's definitely a 4-speed automatic. You don't get to count reverse, neutral, or park :rofl: You describe any transmission by the number of FORWARD gear ratios it has.

    As for increasing lifespan, using fully synthetic transmission fluid is an excellent option. I use transmission fluid from AMSOIL for it's excellent lubricant characteristics and higher temperature performance. make sure you check your fluids often and change them on regular intervals as suggested by the car's manufacturer and AMSOIL.

    Congratulations on the new car :tongue2:
  13. Nov 9, 2006 #12


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