Does it get it from the rotation speed of the wheels or from the GPS measurement?

The reason I ask is that I was concerned that since the Honda EX wheels and tires that I put on my Civic Hybrid have a slightly smaller outside diameter than the stock wheels that the speedometer accuracy might be compromised. As you can see, even though the stock rim is 15" (top tire), the tire is actually taller than the 16" rim/tire combination on the EX.



Today, I went out and took some comparision measurements between the indicated speed on the speedometer and the speed measurement using a GPS receiver.

I was very pleased that the speedometer was dead on with the GPS data, with a variance of only - .1 mph.

That level of accuracy even with the "wrong" set of wheels and tires got me wondering how the car measures its speed.

Anyone care to speculate on the method of measurement the car uses, or if the "stock" rim/tire combination speedometer combination is accurate?

 

I would think Honda wouldn't use the GPS for speed seeing as if something were to happen to the satellite, the speedo wouldn't work. If the overall diameter of your EX wheels is just slightly smaller, you'll be going slightly slower than the speedo reading. The size difference has to be somewhat marginal to notice a huge difference. For example, on my Mazda MX6 I have 225/45/R16 (compared to 205/55/R15 stock) and the difference in speed was only 2.25mph faster at 60mph.

EDIT : for anyone else curious here is a good calculator.
http://www.discounttiredirect.com/di...foTireMath.jsp

 

my older hondas had a cable that went into the transmission, it was a mechanical link. I am pretty sure thats not the case in the new cars. But it could be an optical sensor in the transmission, or it could meter off the ABS rotation sensor.

someone who has the service manual could look it up.

 

Especially for traditional mechanically driven speedometers. But the Honda's is electronic, so where does it get its signal and how does it calibrate itself to such accuracy?

This is the thing that has my curiosity up. A smaller tire should give a slower speed indication, but on my civic, the variance in indicated speed vs. the actual speed measured by using GPS triangulation is essentially null. The Honda speedometer only measures in whole units, while the GPS unit measures in 1/10ths of a mile.

For example, the Cars speedometer shows 60 mph while the actual speed of the car moves between 59.9 and 60.9 mph. This is pretty accurate, and that is what brings up the question, how does the Civic speedometer achieve such accuracy?

 

What is the size of your EX wheel/tire combo? Also, I know around here where I live the police dept. puts up un-manned radar posts that display your speed to you as you drive by. Just a different way to test the accuracy out.

 

The EX rims/tires are P205 / 55 R16 89H vs the OEM Hybrid set-up which is P195 / 65 R15 89S

 

Most new cars have a completely electronic speedometer without any mechanical parts, except possibly for the dial - the HCH has an LCD. And they all use the signal from the ABS sensors, on the drive wheels, i.e. the front wheels on the HCH. The sensors are Hall type (Hall effect) - magnetic. Exactly how the signal reaches the speedometer varies, most common nowadays is that it is sent over a serial bus (CAN). The displayed speed is the mean of the front wheels' speed. The accuracy is basically limited only by regulations on what tires and wheels are allowed as replacement. Over here, the national registration allowed up to 5% circumference deviation from OEM, but the EU-wide homologation now restricts it to around 1%, so speedometers on newer cars are normally that accurate, within 1%. I think it is similar in US.

At higher speeds, the circumference of the tires change, so accuracy is worse. The speedometer normally compensates for this somewhat.

Only alternative solutions I have heard of are radar or optical type of sensors, but I don't think they have made it into serial production yet. They would be more accurate and independent from tires, slip, etc.

 

That doesn't tell the diameter of the tires.
You can look it up on Tirerack or Discount Tire, but even then, diameters vary for each specific tire brand and model for a given tire size. Even width can vary slightly between models for a given tire size. You can't just say that P205/55 R16 89H ALL have the same diameter.

 

Steve:

I'm not sure what your point is. Can you elaborate?

 

My point is, that stating "The EX rims/tires are P205 / 55 R16 89H vs the OEM Hybrid set-up which is P195 / 65 R15 89S" - does not tell the needed info to determine the diameter of the 2 sets of tires being compared.
I suspect that the 2 sets are quite similar in diameter. Otherwise, I think you would notice some difference in your speedometer. If the EX tires are not much smaller, you *might* notice a 1-2 MPH max difference @ 60MPH. Maybe. Looking on Tire Rack or Discount Tire you can get revolutions per mile. Then, you can see just how much (or little, probably) the different tires are in revs/mile.
You didn't do the GPS comparison on the original tires, did you?