Oversize Tire Speedo Differential

[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.

[crotchrocket]

Well My Tom Tom is a liar!!! Its always reading too fast and now i have put 32" tyres on instead of 29" i am doing the same speed as the Tom Tom says i am (if you follow me)

So i have a KPH reading from the dash thats wrong
A mph reading from the scan guage thats wrong
and a mph reading from the TOm Tom thats wrong


ARGHHHHHHHHHHHHHHHHHHHHHHHH

nfpgasmask

How did you set the scan guage to work correctly with the new tyre OD ???

[rowhard]

How did you set the scan guage to work correctly with the new tyre OD ???

do you have the little manual that came with the scan gauge? the math was in there, but if not, I can dig mine out

[snowtrooper1966]

"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.

BLASPHEMY!!!!!


Best,

[tom4bren]

"BLASPHEMY!!!!!"

Sorry - the force musta left me for a moment

[snowtrooper1966]

That's OK.
With the offset you roll around on, you have alot of leeway around here in my book
Also, love the pic you posted in the "all proton's check in here thread", with snow on your VX. There is actually little piles building up un your tires!
Priceless!
You da man!
Best,

[tom4bren]

"There is actually little piles building up un your tires!"

That pic you posted a while back with 3 feet of snow on the roof of your Dragon would've been great with my Proton parked nearby.

You should see the 'waterfall effect' while driving in the rain.

"With the offset you roll around on..."

Yah, I had a kit of foxes build a nest in one of the rims last fall

[snowtrooper1966]

If you ever care to experience some of the best sking / snowboarding in the US, you have an open invitation to park next to the Dragon and a place to stay!
If you attended the 4th of July Mammoth Campout / 4x adventure you could park next to a bunch of VX's!

Now if you could have just trained those foxes to run on top of the tires while you were driving....Think hamster wheel on steriods!

Best,

[tom4bren]

"If you attended the 4th of July Mammoth Campout / 4x adventure"

Sorry - takin the wife & kids to Niagara Falls that weekend.