I could use some help with a math problem

[Oeditpus Rex]

My truck has different-sized tires than those it rolled on when it left the factory in 1965, and that, of course, throws off the speedometer and odometer. I want to see what kind of fuel mileage I'm actually getting (and, while I'm at it, an approximation of the number of miles actually on the chassis), but I need a conversion factor, and my stoopid brain doesn't seem to grasp ratios and stuff very well.

I could use an online conversion chart if I knew the height of the original tires, but I don't. I do know that when the speedometer indicates 65 mph, I'm actually doing about 61.

Anyone got the key to unlock this mystery? :shrug:

[Dangerously Amused]

1. 42

[Oeditpus Rex]

2. W00t!

I'm getting 42 miles per gallon! :bounce:

[Elidor]

30. Somebody stayed awake through algebra

I should know how to do this, in fact, I might even be able to set up the equation. Then I'd get bored and wonder if there's any chocolate left.

[Pierre.Suave]

3. I think we can figure the conversion factor

by getting the circumference of the new tires and using that to figure out how far it will go in one rotation.

let me think about this some and get back to you...

[Oeditpus Rex]

4. I don't have the circumference at hand

But the tire height is 25.1 inches.



Thanks, dude. :toast:

[Pierre.Suave]

26. The circumference is

78.85 inches

With each rotation your tires travel 6.57 feet

and it takes 803.65 rotations to go one mile.

[LeftyFingerPop]

5. 65 divided by 61 = 1.0655737, so...

Your odometer reading is too high.

Odometer mileage divided by 1.0655737 = actual miles traveled.

[Bake]

35. Presumably SOME of the miles were driven on the OEM tires.

So you really need to know at what odometer reading the tires were changed, then use the conversion factor for the remaining miles.

Bake

[klyon]

36. all you need is mileage fill-up to fill-up

a previous post gave you the rest, the one about circumference divided by one mile in inches to match circumference. You can measure your tire circumference by put in a chalk mark on the ground and tire and rolling the tire one time until chalk mark on tire gets to the ground again, mark again, and then measure between the marks.

[PeaceNikki]

6. Buy a GPS unit. They show your speed.

Why be required to THINK about it?

Oh wait, that only addresses HALF of your problem. A pedometer for the other?

:shrug:

[Oeditpus Rex]

9. Because thinking is *fun*

'Specially about car stuff. :D



And anyway, I use "maps." :P

[GOPisEvil]

11. The GPS unit could solve that as well.

If he filled the tank (assuming the capacity is known), and drove a pre-set distance (as measured by the GPS), then re-filled the tank, he would know how much gas was used to drive that distance. Voila!

[PeaceNikki]

13. Ah ha!

People must get smarter after they hit 40! ;) (happy birthday!)

[GOPisEvil]

19. Bonus of the GPS route:

BUYING A NEW TOY!!!! :bounce:

Thanks. I think I've magically gotten wiser in my old age. ;)

[Elidor]

31. Your scenario requries him to run out of gas!

:spank:

[GOPisEvil]

34. No it doesn't.

Read it again.

[Ptah]

7. Odometer mileage times .94 gives your actual miles.

odometer x .94 = actual miles.

[LeftyFingerPop]

17. Exactly right...multiply by .94 or divide by 1.06. n/t

[hedgehog]

8. Do you swear you're not a kid trying to get his homework done?

For every 65 miles you travel, your odometer measures 61. You need to multiply the number on the odometer by 65/61 or 1.066.


For example, if the odometer says you've traveled 100 miles, 100 * 1.066 = 106.6 round up to 107 miles you've actually traveled.


On edit - I'm wrong, they're right. I misread your OP. For every 61 miles you travel, your odometer records 65, so the factor is 61/65 = 0.938.

So if the odometer says 100 miles traveled,

100 * 0.938 = 93.8 round up to 94 miles traveled.

[LeftyFingerPop]

12. Other way around...need to divide...odometer too high. n/t

[Pierre.Suave]

14. That is speed

not distance.

he is trying to get to fuel economy for which he needs distance traveled.

[hedgehog]

18. We're all assuming the odometer is off by the same ratio as the speedometer.

For every so many revolutions of the speedometer cable, the odometer records a fraction of a mile traveled. Therefore, if the speedometer is measuring a fraction of actual speed, the odometer is measuring the same fraction of actual travel.

[Pierre.Suave]

27. I think that assumption may be the only way

to do it my way we would have to have the circumference of the original tire.

[Oeditpus Rex]

16. Well, swell

One of ya says multiply by .94, another says by 1.066.



:crazy:

[DarkTirade]

20. ... why not both?

:P

[hedgehog]

22. They're right, I'm wrong, it's 0.94

:blush:

[LeftyFingerPop]

23. See post 17. depends if you multiply or divide. same result essentially for this purpose. n/t

[Oeditpus Rex]

24. Well, then...

I get even worse fuel mileage than I thought.



:banghead:

[pokerfan]

25. multiply your odometer reading by 61/65

Your speedometer is reporting that you are traveling farther/faster than you actually are (smaller tires?) so the result must be reduced by 61/65 to convert to real miles.

[The Velveteen Ocelot]

10. I think maybe

π is involved somehow.

[Pierre.Suave]

15. yes it is

lol.

[skygazer]

21. I suggest pi

Or perhaps pie. I'll have apple. :hi:

[Dr. Strange]

28. Yum!

[Pierre.Suave]

29. YUM

is it chocolate?


I LOVE chocolate.

[Danger Mouse]

32. 1 + 1 = 11. I can do that one in my head.

(old joke stolen from an old math teacher)

[MonkeyFunk]

33. buy metric tires

and divide by 10.

[Dangerously Amused]

37. Here:

Sorry dude. I tried.