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Speed sensor with 18in. rims?

 
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Old 03-11-2008, 02:09 PM
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Speed sensor with 18in. rims?

hey, i got a question out of curiosity. I've heard that if you go from a car with small rims, lets say 15in rims, and them you put 18in. rims, does that effect the speedometer readings? would it read less or more? or have no affect?
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Old 03-11-2008, 02:13 PM
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It does indeed effect the speedometer accuracy. You will need to have your speedo recalibrated someplace... I don't know anything about all that though.. sorry =(
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Old 03-11-2008, 02:15 PM
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I had 16s on and put 18's on and it had no affect on me. But I might just be lucky. I had it tested by GPS and a dyno.
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Old 03-11-2008, 02:42 PM
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Ill try getting it recalibrated than, thanks
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Old 03-11-2008, 02:46 PM
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Quote:
Originally Posted by ljcandmwd View Post
I had 16s on and put 18's on and it had no affect on me. But I might just be lucky. I had it tested by GPS and a dyno.
Then your OD (Outside Diameter, from one rubber surface straight through the center of your rim to the exact opposite rubber surface) stayed the same as stock even though you changed rim size...

Anytime you change the OD of a rim/tire combination, it will affect the accuracy of your speedo/odo. Your VSS polls how many revolutions your axle makes and converts that into mph and miles traveled. Now think of it like this... When you change the OD of your rim/tire setup, you don't also change the OD of your axle... the axle size stays the same. One full revolution of the axle (which is the circumference of the axle) is also equal to one full revolution of the rim/tire (which is the circumference of the rim/tire).

Now... the total circumference of your rim/tire combo dictate the distance traveled in a straight line after one full revolution of your rim/tire.

So... your VSS is only counting the revolutions of your axle... and using that to compute your speed and distance traveled... your vss can only make accurate computations based off of stock rim/tire OD and stock Axle circumference. Change one (or the other) and your VSS is ill equiped to figure it all out =P

Here is an illustration for reference



I don't know the actual OD's off the top of my head, so I just made them nice and even for simplicity's sake.

One full axle rotation on the 18" wheel will propel the vehicle forward 4' 8-1/2"

One full axle rotation on the 15" wheel will propel the vehicle forward 3' 11-1/8"

But remember... your speedometer and odometer have no clue how far forward your vehicle is actually moving... it only knows how many times and how fast the axle is revolving.

So, you need to get it recalibrated if you want to have your accuracy as close as possible =P
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Old 03-11-2008, 02:47 PM
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Old 03-11-2008, 03:01 PM
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Here is an interesting article on the subject from tirerack.com
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Old 03-11-2008, 03:58 PM
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Quote:
Originally Posted by BadMofo View Post
Then your OD (Outside Diameter, from one rubber surface straight through the center of your rim to the exact opposite rubber surface) stayed the same as stock even though you changed rim size...

Anytime you change the OD of a rim/tire combination, it will affect the accuracy of your speedo/odo. Your VSS polls how many revolutions your axle makes and converts that into mph and miles traveled. Now think of it like this... When you change the OD of your rim/tire setup, you don't also change the OD of your axle... the axle size stays the same. One full revolution of the axle (which is the circumference of the axle) is also equal to one full revolution of the rim/tire (which is the circumference of the rim/tire).

Now... the total circumference of your rim/tire combo dictate the distance traveled in a straight line after one full revolution of your rim/tire.

So... your VSS is only counting the revolutions of your axle... and using that to compute your speed and distance traveled... your vss can only make accurate computations based off of stock rim/tire OD and stock Axle circumference. Change one (or the other) and your VSS is ill equiped to figure it all out =P

Here is an illustration for reference



I don't know the actual OD's off the top of my head, so I just made them nice and even for simplicity's sake.

One full axle rotation on the 18" wheel will propel the vehicle forward 4' 8-1/2"

One full axle rotation on the 15" wheel will propel the vehicle forward 3' 11-1/8"

But remember... your speedometer and odometer have no clue how far forward your vehicle is actually moving... it only knows how many times and how fast the axle is revolving.

So, you need to get it recalibrated if you want to have your accuracy as close as possible =P
perfect explanation dude, thanks. i got the answerer i was looking for
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Old 03-11-2008, 06:48 PM
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Old 03-11-2008, 06:56 PM
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How does one go about getting the speedo recalibrated? I just got a new gps (which is amzing btw) and it shows my speedometer is off by about 3-4 mph with stock wheels. I know the gps is more accurate too b/c it is always a perfect match with those police speed displays.
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Old 03-11-2008, 06:58 PM
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I dunno to tell you the truth =\. It might be worth your while to give your mechanic a call and ask about it... they might be able to point you in the right direction.
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Old 03-11-2008, 09:59 PM
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I would tend to think that the GPS would be less accurate. If they're selling pinpoint accurate GPS units to the public what's to stop the public from buying them and strapping them to guided missiles?
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Old 03-12-2008, 03:30 AM
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thats pretty much how they work
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