Oversize Tire Speedo Differential

[snowtrooper1966]

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,

[rowhard]

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 285 is a 6% difference, so you are traveling faster then indicated and you will actually be traveling father then indicated. So, recheck you gas mileage using the .06 times your indicated mileage, then add your indicated mileage to this figure to get the actually distance's traveled.

[snowtrooper1966]

So you are saying that I have traveled farther than my odometer is reading? Damn, I was trying to cheer myself up since I turned 70K this week, now it looks like I was 70K last year.....

[don moore]

ahhh...WHAT????.....

[rowhard]

So you are saying that I have traveled farther than my odometer is reading? Damn, I was trying to cheer myself up since I turned 70K this week, now it looks like I was 70K last year.....

yep, 2 inches every rpm of the tire, and if I did the math right, that would be 1 mile every 31680 rpm's.

sorry

P.S. I didn't take my shoes off when I figured that, so it could be off by who knows how much

[nfpgasmask]

Yup, I haven't done the calculations, but yeah, that's how it works. I guess I actually change my oil every 3500 or so, and not 3000. I still go by the ODO though.

With my tires my speed is 7% off, calculated by using a GPS. I set my Scangauge to reflect the difference and now all is good.

Bart

[snowtrooper1966]

So how could I calculate how far my ODO is off if I have had these tires on for say, 10K miles?
Best,

[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.