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2002 3/4 ton, tires swapped lost gas mileage. HELP PLEASE ANYBODY!!

Discussion in '1992-Present Chevy & GMC models' started by speed_deed, Jan 21, 2002.

  1. speed_deed

    speed_deed 1/2 ton status

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    A buddy of ours got a 2002 3/4 ton chevy pick up and automatically changed the standard tires to bf goodrich trail 285 16's and lost gas mileage. He took it to the dealer and they switched out his speedometer and said thats all they could do. Is there anything else out there he could do? Anything else out there he could add on or buy to correct this. I am sure he will not go back to smaller tires because he does alot of off-road traveling and he likes those big tires. But if there was nothing else he could do maybe he would give me a deal on them and go small. HELP PLEASE!!!

    '85 k-5 gmc jimmy 4x4 sierra classic. '84 k-5 gmc jimmy sierra. '70 cuda 440 6 pack executive edition shaker hood, steering column shift.
     
  2. MudNurI

    MudNurI 1/2 ton status

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    Re: 2002 3/4 ton, tires swapped lost gas mileage

    well, first off is this 3/4 a HD or not? second- what exactly does 28.5 break down to??? or was that suppose to be 285? if it was 285 I DOUBT that it messed up his gas mileage, the truck should have come from factory with 245's or 265's, by going up 2 sizes, the most it would have been is NOT going to mess up his gas mileage enough to be noticable... YOU did not mention what he has for a tranny either- if he has the ali- he wants to be careful with bigger tires- a LOT of dealers WONT fix anything with the speedo- they claim bigger than 265's will mess with the tranny... we had those 305's on our D/A with no problems, but not for much time- we sold them Saturday- so they were on for probably 3-4 weeks tops

    hope this helps

    brandy

    LOVING the new Duramax!!!
     
  3. speed_deed

    speed_deed 1/2 ton status

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    Re: 2002 3/4 ton, tires swapped lost gas mileage

    Yeah I didn't mean to put in the period. It is automatic so it has to be the allison 1000 series 5 speed. or else it would be the zf 6 speed manual, true? It is a HD extended cab. He isn't loosing much but as much weekend hunting as he does it is enough to be a loss of money. It's only like 4 or 5 mpg but he does notice.

    '85 k-5 gmc jimmy 4x4 sierra classic. '84 k-5 gmc jimmy sierra. '70 cuda 440 6 pack executive edition shaker hood, steering column shift.
     
  4. MudNurI

    MudNurI 1/2 ton status

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    Re: 2002 3/4 ton, tires swapped lost gas mileage

    to be honest-- thats about what 50 miles to the tank he is losing- I woudn't think its the tires... I just cant see 4-5 miles on tires that are only 1 size bigger-I am kind of lost here, maybe we didn't notice becuase with have a duramax, and are getting 21mpg? but then again- when we had 305's on the Suburban, I didnt notice,, and we drove that thing to flroida, from Vermont-- I think we got 17-18 then--? sorry I don't know what to tell ya...

    just a note- GM is still using the 4sp auto- I believe the ali only comes in the HD's...?? but then again I could be wrong

    Brandy

    LOVING the new Duramax!!!
     
  5. 82GMC

    82GMC 1/2 ton status

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    Re: 2002 3/4 ton, tires swapped lost gas mileage

    <blockquote><font class="small">In reply to:</font><hr>

    just a note- GM is still using the 4sp auto- I believe the ali only comes in the HD's...?? but then again I could be wrong

    <hr></blockquote>

    Yes GM is still useing the 4L80E, It comes behind any truck (2500 or 2500HD) With the 6.0L engine. Duramax's and 8.1's get the Ali[​IMG].


    I am the hunter Who live's in the darkness, And grows through your fears...... And you are the hunted.
     
  6. speed_deed

    speed_deed 1/2 ton status

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    Re: 2002 3/4 ton, tires swapped lost gas mileage

    For an engine he has the Vortec 6000 V8 SFI Engine. That comes standard with 5 speed manual. But has an optional four-speed automatic overdrive transmission. I was thinking earlier that he had the Big Block Vortec 8100 V8 Engine. I don't know what to say but he IS losing that mileage. Thanks for the reply's anyway.

    '85 k-5 gmc jimmy 4x4 sierra classic. '84 k-5 gmc jimmy sierra. '70 cuda 440 6 pack executive edition shaker hood, steering column shift.
     
  7. Twiz

    Twiz 1/2 ton status

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    What is he comparing it too?
    In the first post/question, it says he "automaticly changed to larger tires". I read this as, He bought a 2002 Chevy (good choice, BTW) drove it to the tire store and put some larger than stuck tires on it. So what is he comparing his gas milage too? The 90-somthing Metro he drove before (for example) ?How is he getting his gas milage comparison?

    How meny miles are on the truck? It needs to be "seated" and broken in, before any comparisons can be made.
    I haven't any proof, other than "seat-of the pants", but I honestly think, G.M. "chipps" the engine down for the first 8-to-10K miles. I have driven them out of the show-room floor, and with some-30K plus miles on 'em. There is a HUGE difference between the two. The higher-milage ones "feel" better. (I have no idea how the MPG would compair, but would guess it would improve as-well).

    I was extreamy impressed with the last 6.0 / 4L80E / 9.5 ring gear (semi-floater 14-bolt) I drove. I'm not sure what the gear ratio was, but I would assume it was a 4.10 by how it felt. I also have no idea what te gas-milage was, but I would guess, something around 13-16 MPG, if you could keep you foot-out of it,( that would be hard-to do.)

    In a nut shell.
    Give it some time and be resonable, it is not a "metro" and it is new and needs to be "broken-in"

    <font color=white>.</font color=white>Eric
    <font color=white>.</font color=white>Twizted
     
  8. speed_deed

    speed_deed 1/2 ton status

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    Re: 2002 3/4 ton, tires swapped lost gas mileage

    I should probably not said automatically. It was a week or so later. My mistake. He was get something like 15 but now is getting like 11-12 Thnaks for the input. Guess he will have to give it some time. Once again thanks!

    '85 k-5 gmc jimmy 4x4 sierra classic. '84 k-5 gmc jimmy sierra. '70 cuda 440 6 pack executive edition shaker hood, steering column shift.
     
  9. Shorts

    Shorts 1/2 ton status

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    how is your buddy calculating the mpg? is he using the electronic reading on the console, or is he manually calculating with a pencil and paper??

    remember, the tranny, tach, speedo, and odo are all electronic and all work iff of each other. since the rpms in the tranny will change (in reguards to going down from 245s to 285s), so will the miles on the speedo and odo.

    if you went from 245s to 285s there will be approximately a 7% difference in the reading of mph(if i remember right).

    what you can do, instead of having the truck's comp flashed(only a temp solution), you can see if TruSpeed has a VSSB type thing for your truck. that will calibrate your gears, tranny, and tire size for you...thus adjusting your tranny shift points, as well as mph.

    <font color=blue>Shorts</font color=blue>

    '94 K1500
     
  10. Suburbanite

    Suburbanite Registered Member

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    Shorts is correct, difference is ~7.75%. Using that figure on the 12MPG given then actual MPG should be around 12.9MPG (1.0775x12MPG), still not anywhere close to the 15MPG but an improvement nevertheless. Remaining difference in MPG could be due to different driving conditions. If the 15MPG was obtained on the hiway &amp; the 12MPG figure in the city there's simply no way the two will ever match.
     
  11. Shorts

    Shorts 1/2 ton status

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    forgot to ask, what's the gear ratio on the truck? the mpg can take hit depending on the gear ratio, even though the tire diameter increase was not huge.

    245 ~ 28.x"
    285 ~ 32.8"


    btw speed deed, you got a pic of that 440 'cuda??? i love those cars. we got a '68 charger r/t 440 that's in need of restoration. one of these days....($$$$) &lt;img src="http://coloradok5.com/forums/images/icons/crazy.gif"&gt; &lt;img src="http://coloradok5.com/forums/images/icons/laugh.gif"&gt;

    <font color=blue>Shorts</font color=blue>

    '94 K1500
     
  12. ntaj*ep

    ntaj*ep 1/2 ton status

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    Re: 2002 3/4 ton, tires swapped lost gas mileage

    Ya, tell your friend to get a JET power reprogrammer. And follow the instructions to the T. That will "fix" his MPG problem and free up some HP! BTW my 5.3 gets (on a very, very good day 14mpg!!) So he may just need to get used to the power. And quit triing to keep up with the hot rods (been there) Hope this helps.

    The Red Beast
     

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