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gearing/ratios?

Discussion in 'Chevy Trucks' started by 85w/350, Dec 28, 2000.

  1. 85w/350

    85w/350 Senior Member
    Messages: 190

    Hey guys! Here is a question to throw out there. Anyone running a 350 with a TH400 Tranny and 3.08 gears in the back?!?! I am hearing from select people that have run this setup in past years that the gas mileage is around 17-city and 24-interstate...If its anywhere near that I am very interested...Even a comparison with others may be helpful in my case...Here is my setup and gas mileage. My guzzler is a 2wd 350 paired with a TH350Tranny and 2.73 gears in the back and I get 10.5MPG--city and 12 flat on the highway.(with a light foot) This sounds like 4x4 gas mileage to me and this is running stock street tires p235/r15/75 and 87octane gas(my baby gets nice treatment--besides that I find it runs best on 87) My carb is a Q-jet and from other post I read I see that the Q-jet is the best carb as far as gas mileage. Also I have a Flowmaster 2 chamber straight pipes no cat. which actually bumped my mileage up from what it used to be..also I hear that you can by a push button bolt up overdrive for the TH 350 transmission anyone heard of this?
     
  2. 85w/350

    85w/350 Senior Member
    Messages: 190

    typo

    sorry about the typo...its not 2.73 its 3.73
     
  3. mike reeh

    mike reeh Senior Member
    Messages: 114

    changing gear ratio from 3.73 to 3.08 is not going to double your gas mileage, thats a fact. It might help, I wouldnt say that it would help significantly. Gear Vendors makes an overdrive unit that mounts after the trans/xfer-case.. Its expensive though. I Think their website is gearvendors.com. if not, do a google.com search, there is a lot of info on them out there.

    Unless I could get a smokin deal on the gear vendors setup, Id probably invest in a 700R4 (for your application) instead.

    Is the Q on your truck tuned well? Even a Q will give bad mileage if its not tuned right.

    ALSO Id like to clarify... Qs arent known for GREAT mileage.. The simple fact is that they have smaller primaries, which means if you can keep your foot out of it, it will behave fairly nice, in respect to gas guzzling. a properly tuned holley 650 could be made comparable.. Either way its a CARB. a very crude metering device, by today's standards.. I love em, and so do lots of people, but you cant expect too much from em. Carb technology hasnt changed much in the last 20-30 years. Thats a LONG time, and its pretty impressive, in my opinion..

    woah, off topic time to end post :) anyone agree/disagree?

    mike reeh
     
  4. 85w/350

    85w/350 Senior Member
    Messages: 190

    Thanks for the reply...yeah its either the aftermarket setup or invest a nice $800 for a rebuilt th400...atleast thats what they run around here...I'm still shopping around for options...I'll be sure to have someone look at the tuning sometime soon...that is someone that knows how to mess with it because I sure dont and I know its almost an art or else your truck will run like ****...it may also be time for a rebuild on the carb...thanks again
     
  5. mike reeh

    mike reeh Senior Member
    Messages: 114

    well keep in mind the th400 does not have overdrive. And when I was talking about the gear vendors setup being expensive, Im talking thousands :) Much more than a 700R4 (overdrive automatic) would cost.. Ive heard good things about the gear vendors setup though, they are supposed to be real nice. just pricey :( hope this helps

    mike reeh
     
  6. TLS

    TLS PlowSite.com Addict
    Messages: 1,425

    Hey '85,

    Your milage seems a little low for a 2wd 350. I would look into a good tune-up etc. Might try a switch to bigger rear tires to lower the rpms. I had a '76 350 (worked) 4x4 4sp manual with 3.08:1's and regularly got 19-20 on the highway that was at 75mph! I had 33" tires. If your going to go the overdrive route, I would suggest the TH700-R4, as it has a real low first and a good OD ratio. These are a dime a dozen for a 2wd, and are a weekend project for you and a buddy.

    Good luck.
     
  7. 85w/350

    85w/350 Senior Member
    Messages: 190

    Sounds like a good suggesion with the tires I am in need of some newer ones and I have been trying to justify larger tires being that I do use it for more than just street driving...I will be hooking up a Tach on my truck soon so I should be able to check out the RPM's on my interstate driving soon...I have also heard good things about the external overdrive systems especially being that they are push button activated or atleast the ones I have heard of...in response to them costing thousands I was not aware of that but I have connections through family to that sort of thing....as for the overdrive tranny I'll definately look into the Th700-R4...