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Big tires and odometer question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by mattman, Apr 6, 2001.

  1. mattman

    mattman 1/2 ton status

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    OK, stupid question time. I know that when you put large tires on, the speedometer shows less than what you are really going, i.e. it shows 65 and the radar gun shows 72, but what about the odometer. For example, if the odo reads 100 and you know you have a 7% difference in old and new tires, then is the "real" mileage 107 or 93? Kinda curious what my gas mileage is and can't keep straight if it is better or worse than I think.
     
  2. Pugsley

    Pugsley 1/2 ton status

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    With 33" tires and 3.73 gears, mine loses exactly 1/10th per mile, or 10 odometer miles = 9 real miles. Take a spotter out on the highway (not Interstate), and watch mileposts. It'll give you a baseline to figure out how far you're off.

    Note for AZ guys - If you go way the hell out Grand Ave. toward Wickenburg, ADOT was nice enough to put up a calibrated, 10 mile odometer test area.

    [​IMG]
     
  3. gassman

    gassman 1/2 ton status

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    If I'm not mistaking your speedo cable goes into a gear that goes into the trany that is setup for the stock gear rato and tires. So if you change to bigger tires, you change the rato.
     
  4. Chewy

    Chewy 1/2 ton status

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    Mine is all screwed up... it actually indicates HIGHER than I am actually going. I know for a fact because I had my girlfriend follow me on the road and we tested it at different speeds while in contact via cell phones. The worse part is it gets exponentially worse the faster you are going. Makes it real difficult to figure out my actual speed. Oh well, I guess I'll just go with the flow of traffic.[​IMG]

    '87 Beast
    Colorado, USA
     
  5. pcorssmit

    pcorssmit 1/2 ton status

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    This will happen if you regear for bigger tires, and overcompensate with the gearing. Mine does this with the 35's on it. I also use the mile marker method, get on the interstate and note the odo reading at a given mile marker, then do it again 10, 20, or more miles down the road. The farther you go, the better the results.

    Pete
    '83 K5, 350 TBI (ex 6.2), 700R4, NP208, Dana 60/14 bolt, 4.56s, Detroits, 3" lift, 15-39.5x15 TSLs
    '97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd
     
  6. gokartergo

    gokartergo 3/4 ton status Staff Member Moderator

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    Does anybody have any idea how much a new speedo gear would cost?????

    Become one with nature.........Then marinate it..
     
  7. pcorssmit

    pcorssmit 1/2 ton status

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    They're just plastic, they couldn't be too much. I recently bought one for the Dodge after putting on 255/85R16s (~33" tall), it was around $12-13, and my experience has been that Chrysler thinks very highly of their parts.

    Pete

    '83 K5, 350 TBI (ex 6.2), 700R4, NP208, Dana 60/14 bolt, 4.56s, Detroits, 3" lift, 15-39.5x15 TSLs
    '97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd
     
  8. gokartergo

    gokartergo 3/4 ton status Staff Member Moderator

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    Buy that you mean chrysler parts are expensive??

    Beccome one with nature........Then marinate it..
     
  9. Storm Trooper

    Storm Trooper 1/2 ton status GMOTM Winner

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    I have 38.5's and I hate to say it but 3.08 gears (4.56 to come soon) At 65 my speedo is pegged. I'm about 10-15 mph over the actual. Go with the flow and only run over small cars in the way. Wish someone could hook us up with some part #'s of tranny cable gear changes to get close.....
    Mileage is all screwed up. need a GPS

    [​IMG]
     
  10. gokartergo

    gokartergo 3/4 ton status Staff Member Moderator

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    OUCH. I have 3.08 gears with 33's. and it takes me 11 miles to get to 60 mph..LOL I bought my 4.56's last week and am putting them in next week.

    [​IMG]
     
  11. Truckfan2

    Truckfan2 1/2 ton status

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    At the chevy dealer my dad works at the part man gave me a list of about 6 different gears they had available... they were all colored differently so that if you had to replace a stock on you just order your color... i got one before i lifted the Jimmy and it ran like $6- $7, not much and it was a 5 minute install

    Have been stuck (once) as of 8/12/00
     
  12. pcorssmit

    pcorssmit 1/2 ton status

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    Just figure out what percentage you're speedo is off, the pull the driven gear out of your t-case, count the teeth, and figure out how many more or less teeth you need. Then head down to your friendly Chevrolet dealer, tell them the year, model, etc., and tell them you need a gear with X teeth.

    Yes, it seems to me like Chrysler parts are expensive. I called the dealer yesterday about some OE lift blocks for a '93 3/4 ton (they were 6" tall, like what I'm looking for for my '97), they were $114....each.

    Pete

    '83 K5, 350 TBI (ex 6.2), 700R4, NP208, Dana 60/14 bolt, 4.56s, Detroits, 3" lift, 15-39.5x15 TSLs
    '97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd
     
  13. bigerik

    bigerik 1/2 ton status

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    I have an 81 Chevy that came with 30" tires and 3.08 gears. I run 39.5 Boggers with 5.13 gears. I had a mechanic put in some (gear reducer?) cable, and it set me back $200 (ouch). I, too, thought it would just require a speedo gear.
    <A target="_blank" HREF=http://www.bigerik.coloradoK5.com>http://www.bigerik.coloradoK5.com</A>
     
  14. Wheels

    Wheels 1/2 ton status

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    There are several ways to do this. I changed from 3.07 gears to 4.10. The drive gear on the transfer case is red (#6273992) for the 203/205 t-cases. There are 5 different driven gears that are available from Chevy. There are also 7 different adaptors with ratios to compensate for the difference in driven gear ratio. They range in ratio from .937 to 1.360 to 1. The Chevy parts counter has the computations for different tire sizes and ratio's but they mainly list stock tire sizes (H78-15, etc.) and ratio's. The easiest way I found to calculate the ratio for mine is to hook the speedo cable up to the transfer case (with out the adaptor). Then I take it on the road with the wife in trail, run it up to speed (her doing 70, oh, did I mention have cell phones with you or use hand signals). With her doing 70, I match her speed and get the reading off my speedometer. In this case it was 63 mph. Divide 70 by 63 and it comes up to 1.111. I found with experience that their ratio's are a little low compared to the actual output. I ordered a 1.202 adaptor (#368025) and now at 70 mph indicated, I'm actually running 72 mph. The driven gear goes from 18 to 22 teeth. If you remove the sleeve in the t-case, the driven gear should pull out. You can count the number of teeth on the driven gear, and either go up (decrease uotput speed) or down a tooth (increase output speed) to adjust the output of the t-case and then recalculate for the adaptor. The sleeve runs $15, the driven gear goes for about $18 and the adaptors go for about $55. This is for the NP203 or NP 205 t-cases. I checked around and was quoted $250 (lowest bid) to recalculate my speedo.
     
  15. pcorssmit

    pcorssmit 1/2 ton status

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    going from 30's and 3.08s to 39.5s and 5.13s, it should have only been off by a few percent.

    Pete

    '83 K5, 350 TBI (ex 6.2), 700R4, NP208, Dana 60/14 bolt, 4.56s, Detroits, 3" lift, 15-39.5x15 TSLs
    '97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd
     
  16. bigerik

    bigerik 1/2 ton status

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    With the 39.5" tire/5.13 gear combo, my speedo was reading 20.95% higher than the presumably calibrated 30" tire/3.08 gear stock setup, so that speedo would say 79 if I was going 65. That drove me crazy. My other truck came with 29" tires and 3.73 gears and I didn't have to adjust the speedo at all when I went to 39.5 Boggers and 5.13 gears. I usually concentrate on the tach, anyway--I even removed the good stereo in order to be able to listen to the engine!
     

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