Oversize Tire Speedo Differential

[VehiGAZ]

Greetings!
Just pondering as I was driving the Dragon today....
If the oversize Nitto tires (285/60/18) affect the speedometer reading, that is I am actually travelling faster than the dash indicates, how does that affect the mileage reading? Do I have less than is on the readout? I wonder what the culminative affect of that discrepancy is over the years?
Best,

The Great Goog answers all... http://www.miata.net/garage/tirecalc.html

[Riff Raff]

The Great Goog answers all... http://www.miata.net/garage/tirecalc.html

VehiGAZ--- Thanx!!! What a cool website. According to the calculator, ROWHARD's example should have been 6.4% (not 6.0%). However, ROWHARD was right there in the ball-park and gave a great 3rd grade explanation (I liked 3rd grade). Therefore, thanx goes out to you ROWHARD as well (AIM HIGH!!!).

[rowhard]

VehiGAZ--- Thanx!!! What a cool website. According to the calculator, ROWHARD's example should have been 6.4% (not 6.0%). However, ROWHARD was right there in the ball-park and gave a great 3rd grade explanation (I liked 3rd grade). Therefore, thanx goes out to you ROWHARD as well (AIM HIGH!!!).

Thanks Brian. That web site is the one I used to get these sizes.

Yes, the point 4.

Kind of like, measure it with a optical micrometer, mark it with a piece of chalk, and cut it with a axe.

and excuse me, that is 4th grade fingering

[Baxman]

Beware of the handy online tire calculators. They are acurate for passenger tires but not necessarily for off-road tires which are taller than same-size passenger tires.

After putting new, oversized BFG ATs on fresh new American Racing chromed alloys I made my way to Moab for ZuZoo. Before leaving, I checked the online tire calculator which told that the speedo would read 3% slow. As I was cutting across the NE corner of Oregon a very nice state cop used laser to show me the inacuracy of the online calculator. After months of letter-writing to the court in LaGrand, the very nice judge changed the charge from speeding, to "faulty equipment" but it still cost me something over $100.

Borrow or buy a GPS, then you'll know for sure!

[tom4bren]

Your ODO is off by the same amount as the Speedo. GPS is the easiest way to determine the change. Running 33's, my speedo is off by 'zackly 10% (according to TomTom). So my ODO is off by that amount as well.

If you don't have access to a GPS, try using the mile markers on your next road trip. Reset your trip ODO at the 34.0 mile marker (or whatever location you're actually at). Check the reading at the 44.0 mile marker. You should do this several times to make sure the survey crews weren't drinking when they laid out the markers.

For those anal retentive Peeps who have to know PRECISELY what the correct reading is - you should check this periodically during the life of the tires as the correction factor will change slightly as your tires wear. It's all based on the rolling diameter of the tire.

BTW, if you regear, you'll have to do this same sort of correction as well.

[VehiGAZ]

Beware of the handy online tire calculators. They are acurate for passenger tires but not necessarily for off-road tires which are taller than same-size passenger tires.

Good point - the tire size specs for off-road tires work differently, but they would not even work with the tire size calculator I linked to.

That said, don't forget that any particular speedometer may be off to some degree - even two different cars of the exact same model & year. Baxman's speedo may have been off before he switched tires, and the new tires only exacerbated the problem.

Garmin GPS says that my VX's speedo is reading exactly correctly at highway speeds. All of our other vehicles are off by up to 4 mph at 80 mph.

[snowtrooper1966]

Yikes!
So if I am understanding this correctly, my ODO is off by (+-)640 miles per 10K?

[tom4bren]

"my ODO is off by (+-)640 miles per 10K?"

Yup - but that's a good thing. If you ever decide to sell, you'll be showing less miles than actually accumulated.