Need a math person help.

Discussion in 'General Bushcraft Discussion' started by gohammergo, Jul 13, 2019.

  1. gohammergo

    gohammergo I like sharp things.... Supporter Bushcraft Friend

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    I need some assistance with a math equation. :) Here is the deal:

    I put some slightly taller tires on my truck. Because they are taller, I am actually driving more miles than the odometer shows because the tire revolution per mile is different. I cannot for the life of me wrap my head around finding the percentage it is off.

    Original stock size tires: 195-70-14
    New tires: 205-70-16

    According to an online tire size comparison:
    https://tiresize.com/comparison/

    Stock tires are 815 revolutions per mile
    New tires are 739 revolutions per mile
    How do I find the percentage difference? Divide the 815 by 739? That comes to something like 1.102 ish, does that mean for every mile that shows on the odometer, that I am actually driving 1.102 miles? Is that how it works?

    The website also states that the speedometer would be off as well, which I knew about. It shows that when the speedometer is showing 60 mph, that I am actually moving at 66.3 miles. I think that would mean that for every 60 miles I put on the odometer, that I am actually traveling 66.3 miles. Am I right on this? 66.3 divided by 60 comes to 1.105, which is pretty close to the figure above in tire revolutions.

    I'm trying to keep track of actual mileage for oil changes and for gas mileage. Even though I use math a lot during the course of the day, I am drawing a blank on this. :) Pretty sure I am right, but looking for second opinions. :)
     
  2. Beach Hiker

    Beach Hiker LB 42 Supporter

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    My head hurts just reading this.
    I am SO not a math guy.
     
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  3. XMP

    XMP Mountain Man Supporter

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    I want to hear the answer too. I have my suspicions, but I'll wait for someone who actually knows.
     
  4. cbrianroll

    cbrianroll Professional Tinkerer Supporter

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    Can you get your truck calibrated for the tire size difference? Or that little gear in the tranny changed for the speedo? My buddies would do that with their big trucks....maybe they cant if it's not a drastic enough change? I'll try the math tomorrow...I'm sleepy lol
     
  5. Broke

    Broke I found my hat! LB #1 of 42 Supporter Bushclass I

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    I would just go to a gear/tranny/speedo shop and get a new gear for the speedo.
    What is the truck? Year etc.
    4wd or 2wd?
     
  6. Duncsquatch

    Duncsquatch Heed the call. Supporter

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    It's basically 10% difference. If you multiply 815 by .9 you get 733.5, so that means 733.5 is 90% of 815. for most purposes that should be accurate enough. I usually don't do math after 11 at night but...
     
  7. camp casey

    camp casey Guide

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    Could have a buddy pace you at various speeds and note the difference, same for distance.
    The reason I build models is that it’s a work around for the fact the I can’t do the math.
     
  8. gohammergo

    gohammergo I like sharp things.... Supporter Bushcraft Friend

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    I don't want to get that far into it, though I should check that out to see what is involved. The truck is an old 88 Ranger and I more than likely will be changing tire sizes relatively frequently. I drive a lot of miles and usually put used tires on rather than spending money on new tires that I wear out fast. I am a deal shopper and pick up used tires when I can get a killer deal on them. :)
     
  9. Broke

    Broke I found my hat! LB #1 of 42 Supporter Bushclass I

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    Throw a cheap GPS on the dash, use it to track mileage, or get a phone app that does the same thing.
     
  10. pellegrino

    pellegrino Much to learn... Supporter Bushcraft Friend

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    I think you're right, gohammergo. Exactly what Dunsquatch said. The size comparison lists a 10.5% increase in circumference, which matches your 1.102 pretty closely. (New circumference is 110.5% the size of the old tires). So just take 89.5% of your new mileage reading or speedometer to get back to true measure. Or 90% for quick math.
    The calculator rounded the rev/mi to the nearest whole number, so that explains the slight 0.3% discrepancy. Not that it matters, really.
     
    Last edited: Jul 13, 2019
  11. Lazarusaurus

    Lazarusaurus Idot Supporter

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    Just slap a flux capacitor on there. Problem solved. Should be able to find a retrofit kit for the 81 Delorean model on Amazon. If not, try Summit Racing.
     
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  12. Akela

    Akela Scout

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    As already stated, larger diameter tires have a respectively larger circumference.
    So, for each turn of the drive shaft (and axle shaft), the vehicle will travel a greater distance with the larger diameter tires installed, and your speedometer (and odometer) will read slower (and show fewer miles traveled) than actually occurs.
    If you have the speedometer drive and driven gears on the gear box output shaft (transmission output shaft for 2WD, or transfer case output shaft for 4WD), instead of the newer magnetic pulse arrangement, then installing a speedometer driven gear (the gear connected to the speedometer cable) having a lower tooth count will cause the speedometer to "speed up" , and cause the odometer to click over the miles traveled at a faster rate. Conversely, increasing the tooth count on the driven gear will cause the speedometer and odometer to "slow down" .
    Your ring and pinion ratio will also need to be factored into the equations, as that ratio also affects the final speedometer and odometer values you obtain.
    Have fun. ;)
     
    Last edited: Jul 13, 2019
  13. nomad orphan

    nomad orphan Scout

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    Odb2 reader from Ebay for 7 dollars....
    Leave it plugged in and sync with a smart phone for free.
    Or pay the 3 dollars and tune the rig too.
     
  14. Broke

    Broke I found my hat! LB #1 of 42 Supporter Bushclass I

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    Obd2 is going to give the same information the speedo is giving, which is wrong due to he tire size changing from stock.
     
  15. haunted

    haunted Guide

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  16. haunted

    haunted Guide

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  17. haunted

    haunted Guide

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    thats all ill do ..........................me know do math now
     
  18. Tangotag

    Tangotag Field Gear Junkie Bushclass I

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    ODB readers are not going to help. It's an '88 Ranger, with a mechanical speed sensor. It uses a spinning cable from the transmission housing to the speedometer. Maybe you could change out the VSS plastic gear if it really bothers you.
    https://therangerstation.com/tech_library/speedometer.shtml

    When I had my '88 Ranger, I had different sized tires and just paced off a car going the known correct speed to find out how far off I was. That was well before GPS was easy to get your hands on. I always had conformation of my speed on my speeding tickets too.:16:
     
  19. Pinelogcreek

    Pinelogcreek Supporter Supporter

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  20. gohammergo

    gohammergo I like sharp things.... Supporter Bushcraft Friend

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    I have done that before, with other vehicles, and I actually have a mileage tracker to keep track of mileage used for work for tax time. For some reason I can't get a steady reading on this. I can get it up to about 60, and keep it steady, and the mph on the app bounces from around 53-55 up to 69 and higher sometimes. I have tried two different apps for this.
     
  21. CowboyJesus

    CowboyJesus Supporter Supporter Bushclass I

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    i know nothing about cars-but i am a math teacher!



    if x=old circumference and y=new circumference

    815x=1 mile and 739y= 1 mile, therefore 815x=739y

    solving for y gives you y=(815/739)x and (815/739)=1.10288 meaning you are 110.288% of what you were before, which is 10.288% increase in size.



    long version of saying-near as i can tell, you figured right!
     
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  22. Broke

    Broke I found my hat! LB #1 of 42 Supporter Bushclass I

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    The cell phone is not going to be highly effective as a speedometer, it will track mileage. The cell phone guesses your location by ping time from cell towers, so it is averaging the response and guessing where you are, then based upon the last location pinged from it is guessing how fast you are moving. A stand alone el cheapo gps will actually show you in real time how fast you are moving based upon pinging a much more precise satellite. You can also drop a breadcrumb trail in the gps and use it to track mileage but it wont be as easy to assign that mileage to a job. I think a combination of phone app for mileage tracking and a cheap GPS for speedometer use is where you will end up. Or just buy a set of stock size tires and have a coke.
     
  23. x39

    x39 Hyperborean Supporter

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    Set it up as a fraction and cross multiply to solve for percentage;
    739/815 = ?/100
    To solve for ? multiply... 739x100= 73900
    Then divide... 73900/815= 90.67
    Your new tires turn at 90.67 % of the speed of the stock tires. Correspondingly, indicated speed would be 90.67% of actual speed. Using the same formula, we find that an indicated speed of 54.4 is shown at an actual speed of 60.
    Using the same formula again, we find that 60 is 110.3 % of 54, so actual distance travelled when the odometer shows 60 miles is 1.103 times greater, or 66.2 miles. Bottom line, multiply your odometer reading by 1.103 for true distance travelled.

    Looks like @CowboyJesus already posted this. Phone rang as I was posting.
     
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  24. Robert Y

    Robert Y Scout

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    You have figured correctly. If you want to be precise (actually accurate would be the better term), 1.1028 is better than the 1.105. The first number was obtained by dividing precise numbers, and 1.105 was obtained from distance rounded to the nearest tenth, and speed rounded to the nearest whole number.

    I won't get an to a discussion about significant figures on this forum, though:11:.
     
    Last edited: Jul 13, 2019
  25. Guttersnipe

    Guttersnipe Supporter Supporter

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    I used to build "real world" 4x4 "monster trucks" before they were all the rage and dealt with this often.
    @CowboyJesus and @x39 are correct and this is pretty much what I did if the customer didn't want to pay for the mechanical/electrical correction, or a programmable speedometer.
    There are other ways too that can really give you a headache, or you could use mile markers, done that too.
    I love math. :22:
    And I hate math... :32:
     
  26. 66drifter

    66drifter Guide

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    ghg, the worry about oil changes is only semi valid :-O

    if you look in the book that came w/ the vehicle you will see that even the manufacturer offers over a 10%(the above^^^ determined difference) variance depending on driving conditions

    over the years and specially since the advent of the hand held/dash mounted GPS's i have discovered darn few automotive speedometers and odometers that were even close to accurate

    there are accurate(within 1/2mph) free apps for for your cell phone

    you surely have a pard who has a hand held unit and one who has a dash mounted unit

    now take the 3 "modern technology" devices and go4 a ride

    once everything is "zero'd" out take off and drive 100 miles(average on the techno units) and see what the odometer reads

    VOILA

    or if you're broke or just don't want to go4 a ride use the above agreed on 10% and forget it

    a btw is in order here, on my vehicles i run synthetic lubricants and only change out the filter @ the factory recommended period and do a oil n filter change @ the next interval for the 2000 Cherokee(270k on the clock) and the bride's 1997 Accord(155k on the clock)

    for the 2002 F-350(355k on the clock) the interval is 10k for the filter change and an oil n filter change at the next 10k(20k on the oil w/ a fiter plus a qt± @ 10k)

    ghg, you are wise to wonder and it'z fun to learn to figg'r this stuff out sewz you can extrapolate in your own mind as you boogie down the boulevard ;-)
     
  27. Foulwind

    Foulwind Guide

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    I bought a tire speed compensator (Plug & Play) plugs in line with a connector under my dash. I had to calibrate to my new tire size went from stock 265 to a 295 on my 2002 Dodge CTD 6 spd. By driving and adjusting a tiny pot on the unit. Used a GPS and highway markers to set the speed. Don't recall the name of the compensator (Maybe Tru-speed?) I'll snap a photo of it (If I remember too) when I leave work this morning and post later.
    IMG_7040.jpg
     
    Last edited: Jul 13, 2019
  28. chansta

    chansta keeper of the flame Supporter

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    I say you pose this very question to guys at the shop, Tell them you'll pay 50 to the guy who can prove it. Then sit back with some popcorn and watch the show.
     
  29. Bushcraft and Brews

    Bushcraft and Brews Supporter Supporter

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    O'Reilly Auto look up part 121G
     
  30. Kelly W

    Kelly W Love the Axe Supporter

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    That trucks too old to get all fired up about trying to do something to make the speedo accurate.. Use your gps speedo to figure out the relationship to the meter in the truck~to actual speed and let it go..You're gonna worry yourself to death and throw $$ at something that I'd be very surprised you can fix.
     
  31. Mr. Tettnanger

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  32. GreyOne

    GreyOne Elder Lifetime Supporter

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    Percentage is the part divided by the whole x 100.
    739 divided by 815 is about 88.5 percent, so the increase is about 11.5 percent.
     
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  33. J. Pierce

    J. Pierce Perpetually Off Topic, Sorry. Supporter

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    As far as speeding, you know how it works around here.
    All you gotta do when you see a cop is let off the gas, hide your beer, and turn the radio down. :)
     
  34. cbrianroll

    cbrianroll Professional Tinkerer Supporter

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    Wow....I missed alot...I came up with 10.28 also....
    But now I'll recheck it and come up with something different lol.

    I thought rock auto was the best deal on flux capacitors...
     
  35. gohammergo

    gohammergo I like sharp things.... Supporter Bushcraft Friend

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    Thanks for all the input folks. I think I have it figured out close enough for what I need. It's been great seeing the "hive mind" work and everyone came up with pretty much the same answer so I knew I was on the right track.

    Well done everyone! :35::35::35:
     
  36. Sawdustdave

    Sawdustdave Supporter Supporter

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    Since others have already done the math...

    But, I'll add: If you're heading north on a state hwy at 55 mph, and a semi 100 miles north of your position is heading south at 60 mph, how long will it take for the sun to set...

    I remember back to my teaching days - yup, all of 4 years teaching 5-8th graders. None understood that most math in life IS "story problems"...
     
  37. Togus

    Togus Echo of the Loon Supporter

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    All this for an 88 Ranger! I struggle with OCD but you all got me beat. Joking aside, how much does it really matter? If you’re a few hundred miles or even a thousand miles over or under on oil changes it won’t harm anything. Today’s engine oils, even conventionals have much greater longevity than they did in 1988. Filters are better as well.
    Also, how would you account for tread depth loss, which could also mean a 1/4-1/2” diameter loss with road wear? Wouldn’t you also have to consider tire weight in MPG lost. Larger tires weigh more.
     
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  38. M.Hatfield

    M.Hatfield Midnight Joker #42 Lifetime Supporter

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    I listened to that wacky pair this morning. :D

    @gohammergo I am happy to see so many folks come together and help you.



    It's sort of like Joe Cocker once screamed: "We get by with a little help from our friends!!!"
     
  39. gohammergo

    gohammergo I like sharp things.... Supporter Bushcraft Friend

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    Right on! :) And I truly do appreciate all of the response. :)
     
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  40. OutdoorsFamilyMan

    OutdoorsFamilyMan Supporter Supporter

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    Have you tried Waze to check your speed?
     
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  41. Oldguy59

    Oldguy59 Supporter Supporter

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    +1 on the Waze app.
     
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  42. gohammergo

    gohammergo I like sharp things.... Supporter Bushcraft Friend

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    I have not, but have a feeling I will be. :)
     
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  43. chansta

    chansta keeper of the flame Supporter

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    great scott!
     
  44. x39

    x39 Hyperborean Supporter

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    Your formula is correct, the math isn't.
     
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  45. RobOz

    RobOz Guide

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    Just unplug the speedo cable. That way you will not be putting any miles on the odometer and will never need the oil changed.
     
  46. highlander

    highlander Veni Vidi comedit lardum Supporter

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    Purple because ice cream doesn’t have bones.
     
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  47. Bushcraft and Brews

    Bushcraft and Brews Supporter Supporter

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    Agreed
     
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  48. TheGeoSquirrel

    TheGeoSquirrel Supporter Supporter

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    I was just going to post this same thing.
    I just put my garmin on the dash and go for a drive. I set it for vehicle and check the speed the gps shows vs the speedometer. That should get you very close to whats really hapoening.
     
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  49. gohammergo

    gohammergo I like sharp things.... Supporter Bushcraft Friend

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    Genius! Why didn't I think if that? That right there is brilliance. :) And, I also can never get pulled over for speeding. :)
     
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  50. bacpacjac

    bacpacjac Queen of the Cups Supporter Bushclass I

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    Math and I do not get along. I don't know why I clicked on this thread but I've read the entire thing so far and.... We are not getting along any better. I am so confused! LOL!

    math.gif
     
    Last edited: Jul 13, 2019
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