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Discussion Starter #1
I know this may sound insignificant but this may be worth mentioning. a little comment regarding speed measurements on your vans or trucks. If not all, majority of us here have 13" wheels which is one inch larger than the stock 12" wheels. I believe our speedometers and odometers are not giving us correct measurements.

If for example i'm using 175/70R13 tires, that's equivalent to approximately 4.8" section height, multiply that by 2 and add 13" for the wheel diameter, we get 22.6" total diameter of the tire. This translates to 71" tire circumference. This means that for a tire this size, one rotation means traveling 71".

On the other hand, if the speedometer design was based on the stock tire size of 165/60R12 for example, one wheel rotation translates to only 62" traveled linearly. There's a discrepancy of 9" per tire rotation. That's about 15%.

From this data, if our odometer reads 10km, it should have been 11.5km. Consequently, our speedometers might also give us inaccurate readings. For us who base our km/L measurements on the data given by our odometers, we might consider the % difference especially when we are dealing with hundreds of kilometers.
 

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well i've got bigger wheels on my vitara, so the speedometer is slower than stock, about a 10%.
But the speedometer is stock not 100% acurate, and reads more than actual.
Compare your speedometer with a gps and see how much its different with larger tires. Wont be much i can tell you that
 

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Sorry, you cannot compare your speedometer with a GPS and come up with any kind of an accurate measurement, UNLESS, the road has absolutely no curves whatsoever.

Most highways here have markers. Find one that has markers for 50 or 100 km and drive it, noting the difference between the highway markers and your speedo. With just a small grasp of mathematics, you then know almost exactly the percentage your speedo is off, and then can determine km/ltr accurately.

Larry.
 

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Someone correct me if i am wrong. but that does really count? - i mean the tires size? My speedo cable is inserted on the trannny just somewhere near propeller. so tire size wouldnt matter.

I used to have a volks beetle, on this car the cable is at the front wheel on the left - for this i think tire size matters.
 

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Ok, so your speedo is connected to your tranny, which is connected to your differential, and your differential is connected to your tires. How would it NOT count?

Sorry, I'm at a loss here.

larry.
 

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Discussion Starter #6
we can put it this way; if we just assume that the gear ratio between transmission-differential is 1:1. One turn on the tranny would result to a longer linear distance traveled by a bigger tire compared to using a smaller tire. One turn is equal to the circumference of the tire.

As what larry said, we can just note the landmarks on the sides of the road and compare them to the readings on our odometers. From experience, mine varies by about 15% since i am using a taller tire.
 

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Discussion Starter #9
oops! my odo reading is 20% less than the actual km. just noted it this morning. good for us here in the philippines since highway officers don't have laser speed guns. we might argue that we're just doing 60 but actually we're going 72km/h.
 

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Discussion Starter #11
Cyclometers or cycle-computers (bicycle speedometer) have very accurate readings since you input values of your wheel circuference on the unit.

They do not use laser sensors. A wheel magnet is attached to the spokes and a magnetic sensor is attached to the fork. Everytime the magnet passes the sensor, it is interpreted as one complete wheel rotation thus translates to a linear distance traveled based on the values we input as the wheel circumference. For cyclometers with the cadence meter, a sensor is clamp on the chainstay and a magnet is attached to the crankarm.

I have a friend who attached one one these cyclometers to his motorcycle. His conclusion is that, the cyclometer gave more accurate readings than the OEM speedometer.
 

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Discussion Starter #12 (Edited)
Cyclometers can also display trip distance; average and maximum speed; total distance(odo) and cadence.

It is very accurate since we input circumference values in inches or centimeters.
 
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