Speedo seems to be off?
Speedo seems to be off?
Aright gent's, I got nicked the other day. Officer said he got me at 85 in a 60... I will admit I was not paying attention at the time, I had a passenger who was taking my eyes off the speed if you get my drift....he was ncie enough to let me off with 15km over, so no points, just a fine.But that got me thinking, I couldn't have been doing 85.. So i started using my GPS to check my speed, and it seems that my speedo is always 5km/h faster than what my GPS is telling me...I'm not sure which to believe?
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Densetsu
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Re: Speedo seems to be off? (Mariach)
On the highway I take to my parent's house, the CAA (Canadian version of AAA) put up six signs, with a km in between each, for an odometer check. In the summer on my all-seasons, my odometer was almost perfect. This winter I bought brand-new tires, and when I checked my odometer against the signs my odometer was reading about 5% short of the actual distance (odo says 4.7 - 4.8km, actual distance is 5km).
My winter and all-season tires are both stock size, but my winter tires were brand new and the all-seasons a couple years old. I just concluded that the odometer/speedometer sensors probably make the assumption that the tires are 50% worn, to try and best accomodate all tread situations (brand new / kinda used / completely used). Of course, I don't know the actual figures that Nissan uses, but it's a fair guess.
(Geometry time!)
In short, if you have more than 50% tread left, the diameter of your tire will be larger than a tire with just 50% tread left. With each revolution of the tire, you will be travelling more distance on >50% tread tires than 50% tread tires. As such, your speedometer will read slower (and odometer shorter) than your actual speed.
On the other hand, if you have less than 50% tread left, everything's vice-versa and your speedometer will read faster than your actual speed.
My winter and all-season tires are both stock size, but my winter tires were brand new and the all-seasons a couple years old. I just concluded that the odometer/speedometer sensors probably make the assumption that the tires are 50% worn, to try and best accomodate all tread situations (brand new / kinda used / completely used). Of course, I don't know the actual figures that Nissan uses, but it's a fair guess.
(Geometry time!)
In short, if you have more than 50% tread left, the diameter of your tire will be larger than a tire with just 50% tread left. With each revolution of the tire, you will be travelling more distance on >50% tread tires than 50% tread tires. As such, your speedometer will read slower (and odometer shorter) than your actual speed.
On the other hand, if you have less than 50% tread left, everything's vice-versa and your speedometer will read faster than your actual speed.
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K03sport
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Re: Speedo seems to be off? (Mariach)
Speedometers tend to run high. I'm not sure if it is a gov't law thing or what, but every car will show a speed higher than you are actually traveling. EX: speedo shows 35mph, but actual speed will be 33mph.
Some cars are only off 1 or 2mph, while others may be off 3-4 mph. Again, I think this is mandated in some federal law to help save people from themselves.
The only way to get a true speed is to have/use a GPS.
Same size tires from different brands will read differently on the speedo. Just like running shoes, a size 8 by Nike is not the same as an 8 by Brooks or Mizuno.
As tires wear, their circumference changes too. There can be up to a 5-7% difference between when you tires are new and old (saw this arguement on Pete's Boxster board), so that can have an affect on not only speed, but actual miles traveled. 5000 miles was not actully 5000 miles, it was more like 4850 (3%). So in the end, have you actually travled what the odometer reads?
Glad you got off with a lesser offense. You should go play the lottery.
Some cars are only off 1 or 2mph, while others may be off 3-4 mph. Again, I think this is mandated in some federal law to help save people from themselves.
The only way to get a true speed is to have/use a GPS.
Same size tires from different brands will read differently on the speedo. Just like running shoes, a size 8 by Nike is not the same as an 8 by Brooks or Mizuno.
As tires wear, their circumference changes too. There can be up to a 5-7% difference between when you tires are new and old (saw this arguement on Pete's Boxster board), so that can have an affect on not only speed, but actual miles traveled. 5000 miles was not actully 5000 miles, it was more like 4850 (3%). So in the end, have you actually travled what the odometer reads?
Glad you got off with a lesser offense. You should go play the lottery.
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Chuck Tribolet
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Re: Speedo seems to be off? (K03sport)
Believe the GPS.
I have the same experience that speedos run high, but I think it's the carmanufacturers avoiding liability for speeding tickets.
I have the same experience that speedos run high, but I think it's the carmanufacturers avoiding liability for speeding tickets.