Bigger Tire Odometer Math - Am I wrong??

Your math logic gets weird at the top of page 2. Both of those numbers have already been adjusted for the tire size, and so by multiplying them together your e doing weird things to the ratio.

to fix it, multiply your adjusted inches per mile by your original revs per mile instead.
7.719 * 658.95 = 5086.43 inches/mi = 423 extra feet per mile

(5280 + 423)/5280 = 1.08

I do love me some unit analysis tho

 

Funny, I didn't see 50 unit conversions. MPG comes into it because if you actually went x miles, but your odometer says you went x+5% miles, your calculated mpg will be optimistic. But if you're not into numbers, that's OK too.

 

All that mathematical work you put in to prove the mpg theory just got negated when you installed a lift to run heavier tires.

 

The tire thread is titled “odometer math” so gas mileage has not a thing to do with it. There’s no need to figure out how far one would go on a single tank in order to determine the formula for how an increase in tire size impacts the meaning of the odometer.

If you can’t figure out that “50” is an exaggeration for the purposes of dry humor, I can’t help you.

I do love math though so let’s do this:

As an example with nice round numbers, let’s say I happen to have tires with 50” circumference and I increase the circumference 10%, thus 55”.

Let’s say I drive 1,000 miles according to the odometer. The odo is calibrated to 50” circumference tires, so that’s 1,267,200 revolutions.

For 55” tires, 1,267,200 revolutions is actually 1,100 miles. 10% more.

Thus: a 10% increase in tire circumference over stock requires adding 10% to the odometer reading to determine actual miles traveled.

Q.E.D.

But you’re right, I’m not into numbers.

 

My headache is getting worse.

 

Do you have time to hear about our lord and savior, partial differential equations?

 

Yes, we know all that - no reason to rehash it. And obviously your '50 unit conversions' was an exaggeration, albeit absurdly over the top and to no point. And mpg is calculated using your odo reading, so naturally comes up in this context.

 

Point: keep the math simple and you’re less likely to fuck it up

 

Find one of the free speedometer apps and use it. Millions use some version of a time/distance app every day for walking, running, bicycling.........

I'm not that "tech" guy but I have used Speed Box for years as an occasional check of speedo accuracy and mileage. Works great, free, no math and a surprisingly good heads up display in right conditions.

 

And, all of the above assumes brand new tires. As tires wear down they get smaller in diameter. An All Terrain or Mud tire will be roughly 1", or one size smaller when they have 40,000+ miles on them vs when new. A street tire will do the same, but not quite as much. Your odometer and speedometer are gradually changing as you drive, then dramatically change again when you buy new tires, even if you stay with the same size.

If someone starts out with factory 265/70/16's and drives them to the legal minimum tread depth, then replaces them with brand new 265/75/16's the new tires are going to be significantly larger than the old worn out tires. Even though the charts say only 1", in reality it will be closer to 2". Of course they will eventually wear down too.

 

Who would have thought a little bit of math would be so contraversial haha

While I appreciate the response there has only been 1 person even close to my original question.

Not really doubting if i messed something up but the question is where and why.
Looking to learn from my mistakes.

I am sorry for causing @splitdog a headache lol

 

@0xDEADBEEF already pointed out the flaw. Multiplying the difference in distance traveled per rev by the difference in revs per mile does not provide information that’s of any use to the original question. If you want to use your 250 mile example, determine how many revolutions are made by the new tires over that distance.

Circumference = 103.8704 inches
250 miles = 15,840,000 inches.
Divide latter by former = 152,497.728 revs per 250 miles for new tires.

Vehicle sees that number of revs and multiplies it by the stock circumference of 96.1525 inches = 14,663,037.8 inches = 231.42 miles.

So on the new tires when you travel 250 miles, your odometer only records 231.42 miles. That’s a difference of 18.58 miles.

Guess what 18.58/231.42 =?

8.03%. Your original percentage increase from your work above.

 

Here is another way ... use a GPS unit to measure actual distance driven. Divide miles shown on odometer by miles on GPS; invert it; and that is your multiplier ratio.

 

You weren’t the one that caused me a headache LOL, but your assumption speaks volumes on your pizza cutter problem.

 

Some backup on your numbers here: New tread depths will vary, but 16/32" is common on an All-Terrain tire. In most US states a tire is illegal below 2/32, and I'd be surprised if most people didn't get new ones at 3/32 to 4/32. So you are losing about 12/32 of tread, or 24/32" in diameter...so 3/4".