Speedometer correction

[pchrist]

So currently our speedometer is reading off by a about 3 miles an hour is there a way to correct this?


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[roboto_1337]

Change your tire size

[badger.]

Off at what speed? Mine gets slightly further off the faster I go... At 40mph on the speedo its only off maybe 1mph, if that. At 80mph on the speedo I'm about 77-78 on the gps.

Tire size and wear can also contribute. Example because math:

Standard tire is 235/40-R18. Say new tread depth is 10/32", and your tires are worn with 4/32" left.
new tires diameter = 25.4"
worn tires diameter = 25.025"
difference: (10/32 - 4/32)*2 = 12/32 = 3/8"
circumference new = 79.8"
circumference old = 78.6"
difference: 1.2"; so for each rotation your worn tire actually travels 1.2" less distance. Meaning it needs to spin faster to go the same speed as the new tire.

At 70mph, new tires rotate 927 times per minute. The worn tires would need to rotate 940 times. Since the speed sensor just counts rotations, the car thinks you are going faster with worn tires than you really are.
New tires at 940 RPM would be 71mph, so in this example your speedo would read 71mph when you are actually traveling 70mph.

...aaand that's not as large a difference as I was hoping lol. Oh well, I always wanted to figure that out.


If you had the wrong size tires (smaller diameter than OEM) you can really be off. I had a friend with a truck a long time ago that threw on some small wheels for some reason once and it was hilarious, speedo was reading like 95mph going the highway and we were really only going maybe 70-75.




Also, I've heard before manufacturers calibrate it to be off 1 or 2mph anyway to cover any measurement error/legal reasons, so they should never read slower than actual speed. But I don't know if this is true or not.

[pchrist]

We just put on brand new 235 40 18's and it's still off? I thought it would be closer with the new tires but it doesn't seem to be! I wasn't sure if there was a way to change tire size in the computer of the car?


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[p3u]

There isn't any adjustment that can be done to alter displayed speed.

However if you want a more true reading 245/40/18 would be best.

[pchrist]

Well crap that's would have been better to know last week.


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[luvsspeed]

So currently our speedometer is reading off by a about 3 miles an hour is there a way to correct this?


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Mine is the same and I've checked by radar/lazer. All but one of the vehicles in my household are off 1-3 mph. From my understanding no speedo will be 100% accurate and will be off by 1-3 mph. Anything more than 5mph would be a reason for concern.

[pchrist]

Ok good to know I was running Waze and it came up like 3 mph off made me wonder


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[Concavitor]

Nothing to correct. Designed that way.