tkrispin

On my last 4wheeling trip I made a point to note that for 100 highway miles that my GPS recorded, my odometer registered 94.1 miles. I am trying to figure out what percentage difference that is from stock tires, and thought there was a formula for that. I have 32" procomp mt's, by the way. Stock tires are 235/75/R15.

And if I wanted to find out how many miles I'm *actually* going with the big tires on, what does that formula look like? Do I take miles logged (between gas fill-ups) times 94.1%? So if I went 300 miles, how far did I really go?

These formulas always confuse me so I wanted to see if I was calculating things correctly before I make a big mess of things.

Tom

Dr. Zhivago

The difference would be 5.9%. So, to figure your actual mileage, you need to multiply the odometer reading by 1.059.

For example: 300 Miles x 1.059 = 317.7 Miles

Hope this helps.
G

tombothetominator

hmmm.... actual miles/odom. miles = 100/94.1=1.062699 . Multiply the miles your odometer tells you are going by that, then divide it by the galons you put in. So if you went 300 mi on the odometer you actually went about 318.81, divide that by gallons (probably what like 16 gal?) and you got about 19.93 mi/gal

At least, i think this works....

mike_d

first, let's look at the difference in size of the tires:

235/75/R15 = 2*(235*.75)/25.4+15 = 28.87" diameter
32" procomps = 32" diameter.

(actual milage)=(reported milage)*(new tiresize)/(stock tiresize)

or

(actual milage)=(reported milage)*32/28.87

(actual milage)=(reported milage)*1.108

so that means you're going about 10% farther than your odometer reports, and you're going about 10% faster than your speedometer reports.

of course i'd probably trust your gps measurements more because the 32" tires may not be exatcly 32" and your odometer may not be correct to start with. so go with your emperical data.

so if your odometer says you're going 94.1 miles for every real 100 miles, then you'd go with this factor:

(actual milage)=(reported milage)*1.063

or about 6.3% more milags and 6.3% faster than reported.

same goes for calculating gas milage:

(actual mpg)=(reported mpg)*1.063

because you're actually going farther than your odometer says you are.

note: this all assumes that you're still running the same gear ratio's in your differentials too! if you're not, then we need to adjust for that also.

Dr. Zhivago

How do you guys figure he's traveling 6.3% further? Are you using some new math that I'm not aware of?

100 - 94.1 = 5.9

5.9 miles out of 100 miles = 5.9 %

Doing it your way, adding 6.3% to 94.1 = 100.0283

What I'd like to see is for tkrispin to travel 100 miles by the odometer and see what the GPS says. I bet it says 105.9 miles, not 106.3...

My .02
G

mike_d

you're doing your math wrong...adding 6.3% to 94.1 is 100:

94.1*.063=5.9

94.1+5.9=100

i'll bet you a shiny quarter that if tkrispin drives 100 miles by the odometer, the gps will say 106.3 miles.

mike

Fink

After you two figure out who's right, let me know the final equation because I'd like to know what it would be for me going from 31x10.50 to 33x12.50.

So whoever prevails in the math challenge...hook me up with the percentage for my new tires.

Thanks,

Fink

Dr. Zhivago

If I multiply 94.1 x .063 I get 5.9283, not 5.9

If I subtract 5.9% from 100, I get 94.1

Maybe your calculator is broken?

AFAIK, we're not looking at a ratio, which is what you seem to be looking at, but a difference in miles traveled versus miles displayed by the odometer.

I gotta go to work. Show me some more math that proves you're right and I'll shut up.

Peace.
G

dcampen

No, you are getting things confused. Perhaps by the rounding error. the factor of .063 was rounded to two significant figures. A more precise value is .0627

The starting information is that his odometer showed a change of 94.1 miles while his GPS showed an actual distance driven of 100 miles.

The man wants to know how to calculate how far he has actually driven using the change in miles on his odometer. To do this he should, as Tom said, multiply the change in miles shown by the odometer by 1.0627. (1.0627 is approximately the value of 100 divided by 94.1)

mike_d

that's only because i rounded the percentage to 6.3. the real percentange is what tombothetominator said (6.2699%)

94.1*.062699=5.899975 (about 5.9)


ok, i'll try to prove this, but we are, indeed, looking at a ratio between miles traveled and miles recorded. if it wasn't a ratio, then the difference between miles traveled and miles recorded would be a constant, but we all know that that's not the case, the farther you travel the bigger that difference gets.

so let's go back to the beginning. everyone here should agree with this statement: "all the odometer knows about is number of turns of the driveshaft"

at the factory, they do a cailbration to turn "number of turns of the driveshaft" into "miles" (or "kilometers")

so, let's start with the 235's in the origial poster's question.

the diameter of those tires is 28.87", so the circumference is 90.70"

one mile is 5280'*12=63360"

so in one mile, the wheel will turn around 63360/90.70=698.6 times.

so at the factory, they adjust the gearing in either the transfer case or the odometer (probably the transfer case) to make sure that the odometer turns 1 mile for every 698.6 turns of the wheel (of course the differential gearing comes into play here, but since the origial poster didn't mention any gearing changes, that's not coming into the equation)

now, if we travel 100 miles, the wheel wil turn around 69860 times

but now let's change our tire size, if we put 32" tires on there, the odometer still doesn't know, so it'll record 100 miles after 69860 turns of the wheel, but we will have gone a different distance:

32" tire = 100.5" circumference.

69860 turns * 100.5" circumference = 7023087 inches travled

7023087/12 = 58525.7'

58525.7/5280 = 110.8 miles.

notice, that this is the same ratio i came up with in the original calculation. so to go from the reading to actual milage, you'd make sure you add 10.8%.

now, let's say that with our 32" tires, we drive 100 actual miles (as measured by gps for example)

100 miles * 5280 feet * 12 inches = 6336000 inches

6336000/100.5" circumference = 63040 turns of the wheel

but remember that our odometer still is calibrated for 698.6 turns per mile

63040 turns/698.6 turns per mile=90.24 miles

so, the gps says we drive 100 miles, the reported milage is 90.24.

to test our original statement of 10.8 percent increase

90.24*1.108 = 100 miles (actually 99.99, but that's because i only kept 4 sig figs in this calculation because i didn't want to write them all down)

the difference between the actual (100 miles) and the reported (90.24) is 9.76 miles, but since the ratios are based on the reported miles, it's 10.8%, no 9.76%.

the same argument can be make for the emperical data of 100 acutal miles and 94.1 reported miles. the difference is 5.9 miles, but the ratio is based on what the odometer reports.

bulldog-yota

mike's % calculation is correct.

A note for everyone be careful when working with percentages.

DO this quick math and you'll see why:
100 - 10% = 90
90 + 10% = 99

SO if the GPS says 100 and the odo 94.1 it will mean that you need to add 6.26992% to the odo reading to get the correct distance as per the GPS.

Long way of calculating:
100-94.1 = 5.9 miles
5.9 miles divided by 94.1 = 0.00626992 or 6.26992% with which the odo under reads. SO you have to add that percentage to the odo reading everytime to get the true distance travelled as per GPS.

QUick way:
(100/94.1) -1 = 0.0626992 or 6.26992%

ALso always look at a tires rotations per mile spec. Forget about the diameter as it is not the fitted with load diameter of the tire.

PirateFins

But the real question is.....

What is the avg. flying distance of a swallow?
African or European?
I don't knooooooooooooowwwwwwwwwwwwwwwww!!!.........


This has been one of the best post I have watched in a while. Oh and by the way I did learn something.

Dr. Zhivago

On the way home from work yesterday I realized we were talking about ratios. I guess I didn't have enough coffee before posting.

Sorry for the confusion!

Peace.
G

tkrispin

Sweet! Thanks for all the responses and discussion. The confusion above is what would have cluttered up my head if I kept trying to figure this out!

By the way, stock gear ration, 4.30 gears.

Tom

jx94148

Just go by what the GPS says.

All these calculations have too many variables like actual circumference with different pressures and loads. Your 32s might really only be 31.5. If this was an offroad trip, wheelspin will throw it way off.

GPS eliminates all the variables.

tkrispin

I agree and most definitely will stick with the GPS readings, but the main reason I was asking is because I keep detailed maintenance and fuel logs in my vehicles, and it would be nice to know how many miles I actually drove my truck with the big tires, since the odo would be off. I am currently 15 miles away from 100,000 on the odo, but in reality I think I passed that mile marker some time ago...

Tom

astyles

Hmm, ok I am going to make an attempt to guestimate my MPG with bigger tires and you people can tell me how wrong I am? Ok good! Let's go!

*Jeopardy Music*

(note: I assume my stock tire size is 28?) Never had stock tires...

(actual milage)=(reported milage)*33/28.87

(actual milage)=(reported milage)*1.143

So...if I get 250 miles to a 16 gallon tank does that mean that I get

250*1.143=285.75

285.75/16=17.85

*end Jeopardy music*

I would be very happy if I am getting this gas mileage! I drive slow.

mike_d

Looks right to me, but I'm confused why you did 33/28.87. If the odometer is calibrated to 28" tires, then you'd do 33/28.

astyles

I was using your tire size I should just use 28!

mike_d

Heh, that was a long time ago and I didn't go back and re-read the thread. You're (or I guess I'm) right. 28.87 is the size of the 28's on your truck.

I'm dumb sometimes.