Tire size does change, however after crunching some numbers I have reached this conclusion: it is still more accurate to use the ODO verses a consumer GPS unit.

Error associated with changing tire size using the tires I have on my Liberty (Firestone Destination LE's) at OE size.

Original Diameter = 30.5" --> circumference = 95.82"

Worn Diameter (assuming the 12/32" of tread depth worn off on both sides) = 29.75" --> circumference = 93.46"

Assume the wear on the drive train other than tires is negligible.

Percent error = 1 - (93.46/95.82)*100 = 2.46%


This means that you'd need a GPS unit that is consistently within +/- 2.46 ft of accuracy to be more accurate than calculating it using the ODO.

I do not want to carry this discussion too far away from the original topic so I will only add this. It is better to be more precise than accurate. Precision will allow repeatability. My hand held gps unit stated not to use it for navigation (haha right?) and this is because it can not assure accuracy due to the variability in satellites, and to always carry maps since it is not a direct replacement for hard copies of maps. Always remember depending on your location relative to available satellites you will see large swings in accuracy in any GPS unit. I can tell you as I stated before, my hand held more times than not is +/- 18 ft due to only having X number of satellites available to bounce signal off of (now this should get better with the summer coming for us in the northeast) which means draw a circle 18 ft in diameter around me, the GPS can only say I'm within that 18 foot circle. And keep in mind that this error is introduced at every data point taken along your route or path. This is perfectly fine when you're hiking or driving since you don't need a high degree of accuracy since you can use your other senses to locate your self, but it is not a reliable measuring tool unless you spend high dollars to gain the high degree of accuracy and precision. Using your GPS to confirm the mileage against your car does help to fuel the argument that the GPS is an accurate measuring tool, but I'm only speaking from seeing the error on the hand held that I'm not sold that if t says you went a mile, you actually went a mile. The only way to actually check that is to calibrate is against something more accurate than a mile marker on an interstate. One last thing, due to the swings in error, one day your mile could measure 1.05 miles on the GPS and the next it measures 1.10 miles, again based on number of satellites available.

I do not want to carry this discussion too far away from the original topic so I will only add this. It is better to be more precise than accurate. Precision will allow repeatability. My hand held gps unit stated not to use it for navigation (haha right?) and this is because it can not assure accuracy due to the variability in satellites, and to always carry maps since it is not a direct replacement for hard copies of maps. Always remember depending on your location relative to available satellites you will see large swings in accuracy in any GPS unit. I can tell you as I stated before, my hand held more times than not is +/- 18 ft due to only having X number of satellites available to bounce signal off of (now this should get better with the summer coming for us in the northeast) which means draw a circle 18 ft in diameter around me, the GPS can only say I'm within that 18 foot circle. And keep in mind that this error is introduced at every data point taken along your route or path. This is perfectly fine when you're hiking or driving since you don't need a high degree of accuracy since you can use your other senses to locate your self, but it is not a reliable measuring tool unless you spend high dollars to gain the high degree of accuracy and precision. Using your GPS to confirm the mileage against your car does help to fuel the argument that the GPS is an accurate measuring tool, but I'm only speaking from seeing the error on the hand held that I'm not sold that if t says you went a mile, you actually went a mile. The only way to actually check that is to calibrate is against something more accurate than a mile marker on an interstate. One last thing, due to the swings in error, one day your mile could measure 1.05 miles on the GPS and the next it measures 1.10 miles, again based on number of satellites available.

A consumer GPS is much more accurate than your odometer in measuring distances at road speed. You are stating that all the errors in position accumulate over your travel and cause the end reading to be way off. In fact the opposite is true. The further you travel the more the errors cancel each other out. It's called the Central Limit Theorem.

About 26mpg on short trips, between 30-32mpg longer distance motorway. On a stocker 2002 crd

Diesel is about 8% more expensive than unleaded in UK at the moment, equivelent to just over $10/gallon for you.

A lot of talk about our odometers off so today I did a little test.
No GPS involved here. On the interstate passing the mile marker, writing that down
and writing down the trip o meter as well. 30 some mile each way to work.
Based on that, my trip o meter is off 5 1/2%
24 MPG may really be like 25.5 MPG.
Right or wrong it sure looks good to me.
I love my Jeep.
PS: Crude hit a new high today. 113.50.

Steve

I don't know what goes! I get between and 19.2 and maybe 24 on highway. If I take it easy! New MAP and mass air flow. Hopefully summer fuel will help.

I suspect that in order to save money on the few CRDs produced for the US they used parts off the shelf and stuck in a trigger wheel (can't remember the proper name) that sends pulse signals to what ever controls the odometer. I took a trip that was 192.1 miles long and my CRD odometer only racked up 174.3 miles amounting to a 10% error Several other members have reported the same error
Since the rest of the world including Canada is metric and use Kilometers they most likely have the correct number of teeth on their trigger wheel.
I would almost bet a bottle of Beer that they grabbed the trigger wheel for a Ram 1500 with the same rear axle ratio (but bigger tires) as our CRDs and since both use the 545 trans, it kinda works. You could add taller tires and correct the odometer, but since the speedometer reads 5mph fast with stock tires, you would have a speedo error if you corrected the odometer error.


Steve

P.S. I got 28.3mpg corrected on this trip with a cargo carrier on the back and the inside loaded with stuff.