Odometer Calculations

Average Joseph · **Posted:** Tue May 02, 2006 9:46 pm

I just got new 31's (they actually measure 31.5 and rub like crazy but that's another story) and I wanted to figure out what it will do to my mileage. Below are the calculations I went through, below that is the reasoning.

The short version (all in inches):

1. New Circumfrence minus Original Circumfrence
2. Old Rev per mile minus New Rev per mile
3. Line 1 x Line 2
4. 63360 / Line 3
5. Answer from line 4 will give you how many miles you will have to travel to lose one mile at the odometer.

I took the circumfrence of the oem tire (28.7"xpi=90.1) and then subtracted it from the circumfrence of the new tire (31.5xpi=98.91") and came up with a differece of 8.83".

Then I divided the number of inches in a mile (63360) by the old and new circumfrences and figured how many revolutions per mile (at the odometer) for each size (703.08 old, 640.58 new) which came out to be 62.5 less revolutions per mile on the new tires.

Then I figured since the new tires go 62.5 revolutions farther than the old tire before the odometer registers an mile, I multiplied the difference in revolutions by the difference in circumfrences (62.5x8.83=551.875") to find out how much farther I now go for every one mile.

Finally, to make it tangible I divided the inches per mile (63360) by the excess distance traveled per mile(551.875") and figured... <b>I will lose one mile on the odometer for every 114.81 miles it registers.</b>

Hopefully this is correct. If not, let me know so I can fix it!

P.S. My calculated circumfrences were a little off from the Tire Size Calculator I initially used so I stuck with my own figures.

Cowcatcher · **Posted:** Wed May 03, 2006 1:06 am

So...........was your odometer accurate before the 31's :roll:

Average Joseph · **Posted:** Wed May 03, 2006 1:40 am

I assume it was accurate when tires were stock, which is what I based my numbers on. Stock tires are 215/75's right?

2006KJSPORT4x4 · **Posted:** Wed May 03, 2006 7:06 am

225/75/16....at least on mine

Se7enLC · **Posted:** Wed May 03, 2006 9:46 am

Average Joseph wrote:

I assume it was accurate when tires were stock, which is what I based my numbers on. Stock tires are 215/75's right?

Not sure what year they changed it, but they used to be 215/75R16 for stock, and now they offer both the 225/75R16 and the 235/70R16

Pretty sure that if you get the dealer to recalibrate the speedometer, it will also fix the odometer, mpg, tripmeter, etc, since they are all tied together. From what I've read, it looks like it has options to change the tire size and gear ratio (since there are different options available for different models). I've yeat to hear of any 2005 or newer getting their speedometer recalibrated correctly, though, so if anyone has, please speak up!

Cowcatcher · **Posted:** Wed May 03, 2006 10:06 am

2006 Limiteds are 235/65R17, Sport and CRD are 225/75R16. Users in the CRD Forum are indicating about a 3 MPH error on stock tires compared to GPS readings and other alternative testing.

Littlehorn · **Posted:** Wed May 03, 2006 10:10 am

The salesman friend I bought my wife's Matrix and my Jeep Liberty from told me that some folks are getting their warrenties voided because they have larger than stock tires mounted upon service visits. Something about DC claiming an Odometer discrepancy. I trust him pretty well and I believe he was trying to warn me before showing-up with larger tires. Maybe a ring of truth due to 2005s not really being "adjustable".

Now, this made a bit nervous because I need to have my engine performance checked/serviced. I get about 12 MPG without converting to the true MPG with the 255s I have mounted.

Has anyone heard of this happening? I mean...No Larger Tires?..On a Jeep?

Chris

Mutt · **Posted:** Wed May 03, 2006 10:11 am

I'm more interested in your definition of "Rub like crazy?"

Average Joseph · **Posted:** Wed May 03, 2006 10:48 am

Got mine in '03 so I have 215's. I was trying to figure out my actual miles per gallon. As for miles per hour, I think you have to come up with a percent, as opposed to a solitary number. The tire calculator I mentioned above stated that I have about a 9.3% difference in diameter (had to use the 265/75R16 for comparison 'cause the overall diameter was closer to what I've got) which means my speedo reads 10% too slow, so 30=33, 70=77, etc... As long as my process was right anyone can use the calculations to figure out their own distance/MPG discrepancy.

As for the "rub like crazy," I'll post some Pics in the Tires section today or tomorrow.

SavageUrge · **Posted:** Wed May 03, 2006 2:58 pm

I bought my Liberty Renegade brand new in '03 and it came with 235/70/16 tires, (those crappy Goodyears) not 75's. I recently upgraded to Firestone Destination A/T's in 75 and now my speedometer reads about 2-3 mph faster than actual. So if I'm going 60, my speedo will tell me I'm going 62/63 mph. Unfortunately, my mileage has also gone down about 1 - 1.5 mpg.

Se7enLC · **Posted:** Wed May 03, 2006 3:34 pm

SavageUrge wrote:

I bought my Liberty Renegade brand new in '03 and it came with 235/70/16 tires, (those crappy Goodyears) not 75's. I recently upgraded to Firestone Destination A/T's in 75 and now my speedometer reads about 2-3 mph faster than actual. So if I'm going 60, my speedo will tell me I'm going 62/63 mph. Unfortunately, my mileage has also gone down about 1 - 1.5 mpg.

Not only will the tire change effect your speedometer, it will affect your odometer, too. So as a result, it is also affecting your gas mileage calculations (or your EVIC readings). If the calculation above is correct (loss of 1 mile per 114 miles), the difference in mileage can probably be ignored. But if it's off by 2-3mph at highway speed like you say, that means that for every hour you drive, you are actually going 2-3 miles further than your odometer says you are. As a result, your gas mileage will calculate out as lower, since it thinks you went a shorter distance while the fuel consumption stayed accurate. Might not end up being a lot of difference, but 1mpg isn't all that much, you might be getting the same mileage you did before the swap.

Skipperb · **Posted:** Thu May 04, 2006 9:24 am

http://www.miata.net/garage/tirecalc.html

Just put in the old tire and the new it will tell you the difference at the bottom Speedometer reading is

MikeD0106 · **Posted:** Thu May 04, 2006 3:35 pm

Average Joseph wrote:

I just got new 31's (they actually measure 31.5 and rub like crazy but that's another story) and I wanted to figure out what it will do to my mileage. Below are the calculations I went through, below that is the reasoning.

The short version (all in inches):

1. New Circumfrence minus Original Circumfrence
2. Old Rev per mile minus New Rev per mile
3. Line 1 x Line 2
4. 63360 / Line 3
5. Answer from line 4 will give you how many miles you will have to travel to lose one mile at the odometer.

I took the circumfrence of the oem tire (28.7"xpi=90.1) and then subtracted it from the circumfrence of the new tire (31.5xpi=98.91") and came up with a differece of 8.83".

Then I divided the number of inches in a mile (63360) by the old and new circumfrences and figured how many revolutions per mile (at the odometer) for each size (703.08 old, 640.58 new) which came out to be 62.5 less revolutions per mile on the new tires.

Then I figured since the new tires go 62.5 revolutions farther than the old tire before the odometer registers an mile, I multiplied the difference in revolutions by the difference in circumfrences (62.5x8.83=551.875") to find out how much farther I now go for every one mile.

Finally, to make it tangible I divided the inches per mile (63360) by the excess distance traveled per mile(551.875") and figured... <b>I will lose one mile on the odometer for every 114.81 miles it registers.</b>

Hopefully this is correct. If not, let me know so I can fix it!

P.S. My calculated circumfrences were a little off from the Tire Size Calculator I initially used so I stuck with my own figures.

Almost sounds like a formula for doing taxes.

Cowcatcher · **Posted:** Sat May 06, 2006 10:28 pm

I already posted most of this on the CRD Forum as it may be a CRD issue more than a general one but it relates to this topic too so I will double post here.

I went on a 300 mile trip today and was hoping to get a good test of longer range highway miles on my new CRD with just shy of 9K miles. That probably didn't work out as planned since I was heading into a 20>40 MPH wind most of the first half trip and had perhaps only 10 MPH for only part of the way back. I also ran about 5 checks against the state road mile post markers and my EVIC and found that I was consistently measuring 6.5% > 7% slow with stock tires. If the other CRD folks were measuring thier speedometers as 3 MPH slow and that was at 55 MPH that would be about the same difference.

So Joseph, that was the reason I posed the question originally. It would seem that if your odometer were as far off as mine the larger tire size would contribute to an even slower reading. Also the assumption on your calculations was based on the premise that your odometer/speedometer was accurate from the factory which might be incorrect.

Average Joseph · **Posted:** Mon May 08, 2006 7:29 pm

Again, I'm not trying to figure out speeds. What I wanted to figure out was how far I am actually going in relation to what my odometer reads. To figure this out you need to work with the circumfrences because that is how far the tire travels over the ground per rotation. As far as I can tell speed doesn't factor in at all.

Quote:

...if it's off by 2-3mph at highway speed like you say, that means that for every hour you drive, you are actually going 2-3 miles further than your odometer says you are.

This is because you are actually going faster too. If I were traveling side by side with a stock Lib. @ 60mph I would go 551.875" farther by the time my odometer registers one mile. What I'm saying is at any speed (adjusted for new tires or not) you will be going farther than what your odometer thinks. Also, my ratio of 1:114 miles is for my tire size only (31.5"). People with other sizes can figure out their own numbers.

As for the odometer being off in the first place... Ignorance is bliss :lol:

Cowcatcher · **Posted:** Tue May 09, 2006 12:35 am

Joseph,

Unless Jeep has devined some new magic there is a direct relationship between speed and your odometer. If you check you odometer against mileposts (assuming they are properly placed) or a GPS the miles you travel versus the miles you should have traveled is represented by the same percent of error for either the odometer or the speedometer. If you travel 100 miles by the mile post and you only register 90 that is a 10 percent error and since both the speedometer and the odometer are driven by the same source it also means that if you were registering 60 MPH you were really only doing 54.

Odometer Calculations

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