Odometer accuracy

plainvanilla · January 12, 2006

I recently went from the factory 245/70/17 size to a AT 265/70/17.

I noticed the economy from the computer went down about 3-4 mpg.

I used my GPS to verify actual speed and I figured I was off about 1.7 mph on my speedo at 60.

I went to a friends shop and he recalibrated the computuer to the new tire size.

Immediatley, I did not see a change, then after the next time I shut the engine off and started driving again, I noticed the estimated economy was back where it had been originally smile.gif smile.gif smile.gif

SO, I took my GPS again and did a test of two different drives semi-constant speed and traveled 10 miles each time. Comparing the truck trip meter to the GPS, it shows that the GPS shows I have traveled about 1 to 1.2 tenths of a mile more than the trip meter shows, an average of both 10 mile test.

Where I live here in Texas, the WAAS feature does not provide better accuracy, so I keep it disabled. I know the GPS is not perfect nor is the truck's odometer.

But at the rate of about a 1% error, or 1 mile per 100 miles traveled, I think I can say with confidence that both ways of measuring are as accurate as can be expected and I should just call it good and don't worry about it. cool.gif

What do you guys think??

az'03Silverado · January 13, 2006

I just went from 235/75/16's to 235/85/16's and I noticed that my odometer is off a little. I don't really worry about it. I drove from Phoenix to San Bernardino (CA) the other day to visit family and the 330 mile trip only showed 323 miles. So, I lost 7 miles somewhere and I took the route I always take. Not sure if it's worth getting the computer reprogrammed. The tire diameter is about 2 inches bigger but the same width.

plainvanilla · January 13, 2006

I just went from 235/75/16's to 235/85/16's and I noticed that my odometer is off a little. I don't really worry about it. I drove from Phoenix to San Bernardino (CA) the other day to visit family and the 330 mile trip only showed 323 miles. So, I lost 7 miles somewhere and I took the route I always take. Not sure if it's worth getting the computer reprogrammed. The tire diameter is about 2 inches bigger but the same width.

<{POST_SNAPBACK}>

I would not have had mine reprogrammed either had my friend not offered to do it for free.

I just wonder how accurate mine was to start with

05Quadrasteer · January 13, 2006

Here is a site where you can put in your old tires and your new tire sizes. It then gives you the percentage of change you would need to calculate in for accurate mpg if you don't get your pcm reprogrammed.

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

Hope you find it helpful. Have a great day.

Bill

redvett · January 14, 2006

I would not have had mine reprogrammed either had my friend not offered to do it for free.

Not reprogramming after a tire change or gear change will affect fuel economy, engine performance, trasns shifting, ABS & SDM/airbags.

DanMan_S · January 15, 2006

From the factory these speedometers aren't too accurate. Infact, my dad just put 265/75-16's on his 2500HD to replace the 245/75-16's and his speedometer now is reading closer to the GPS value than it was before.

dewfpo · January 15, 2006

The speedometer/odometer on my 05 is reading about 5% off and I have the stock tires on it.

Not very accurate at all!

DEWFPO

plainvanilla · January 15, 2006

The speedometer/odometer on my 05 is reading about 5% off and I have the stock tires on it.

Not very accurate at all!

DEWFPO

<{POST_SNAPBACK}>

Darn, I wonder why that much off?

dewfpo · January 15, 2006

I really hope it's not because someone figured out that they could ring up the miles 5% faster than they are actually acrruing getting owners out of their warranties 5% sooner thereby saving GM $X on 1 million trucks a year. I really hope that's not the case. But from what I've heard on this forum, nobody's speedometer/odometer reading are correct from the factory. And they're all reading in GM's benefit, none the other way. Makes you wonder.

DEWFPO

67ChevyRedneck · January 16, 2006

I really hope it's not because someone figured out that they could ring up the miles 5% faster than they are actually acrruing getting owners out of their warranties 5% sooner thereby saving GM $X on 1 million trucks a year. I really hope that's not the case. But from what I've heard on this forum, nobody's speedometer/odometer reading are correct from the factory. And they're all reading in GM's benefit, none the other way. Makes you wonder.

DEWFPO

<{POST_SNAPBACK}>

It would also show better fuel economy, not saying that's true, but if it is...... You also have to remember that the tires on these trucks have thick tread. If the speedo is calibrated with brand new tires on it will be different than if the tires were bald. Lets say the tires are 32" with full tread. If you figure an inch of tread loss 1/32 * 100 would equal 3.12%. The tires would be smaller so they would spin faster and cause your speedo to show a faster speed/more miles than stock.

redvett · January 16, 2006

The variation allowed by the government is +- 7 mph and due to tires not being consistant in size (air pressure) there will be some differences in MPH. That is why law inforcment cannot legally give you a ticket for speeding for less than 7 MPH. My current truck has 30000 mi and GPS says its right on at 70 mph. Different tire brands will make a difference too.

lrymal · January 16, 2006

Comparing my '05's speedometer with my GPS and OEM tire sizes at 35 PSI, my readings are dead-on.

dewfpo · January 16, 2006

It's nice to know they slip and get it right every once in a while.

My 2500HD is stock with only 3,900 miles and two different radar guns have me running 52 mph when the speedometer is registering 55 mph on the dash.

DEWFPO

67ChevyRedneck · January 17, 2006

The variation allowed by the government is +- 7 mph and due to tires not being consistant in size (air pressure) there will be some differences in MPH. That is why law inforcment cannot legally give you a ticket for speeding for less than 7 MPH. My current truck has 30000 mi and GPS says its right on at 70 mph. Different tire brands will make a difference too.

<{POST_SNAPBACK}>

That's right, I forgot about that too!

toneman · January 21, 2006

The variation allowed by the government is +- 7 mph and due to tires not being consistant in size (air pressure) there will be some differences in MPH. That is why law inforcment cannot legally give you a ticket for speeding for less than 7 MPH. My current truck has 30000 mi and GPS says its right on at 70 mph. Different tire brands will make a difference too.

<{POST_SNAPBACK}>

Is that a federal vehicle code that applies to any and all law enforcement jurisdictions? I ask because I'm sure I've read/heard about folks getting radar tickets for 5 mph over...