Speedo reads too optimistically, too fast

[X JBM X]

Originally Posted by aoncurly

I thought about the wheel size as well. Although my 81SC came from the factory with 16-inch wheels, the PO or dealer changed my speedo - I think my car came with the idiotic 85 MPH speedo. What I have often thought about was when the speedo was replaced, were there two speedos available? A 15-inch version and a 16-inch version, each calibrated for the appropriate wheel size? It just seems like my speedo is calibrated to 15-inch wheels, not the 16's I have.

I wonder how NHS "calibrate" their speedos? Is it done on a bench? If it is, how do they know if they are calibrating for a 15-inch wheel or a 16-inch wheel, because when they "calibrated" mine, they did not ask, although they had my car at their shop. I simply assumed they used the dyno rollers because I saw them there. That is why I was quite surprised that after my speedo was calibrated, it still registered higher speeds than I know I am going. And for me, I just didn't like the fact that my mileage is probably higher as a result of the incorrect speeds.

I agree with the mileage point.

However, it shouldn't matter regarding wheel size. The ONLY differentiation is tire size. So if I run a 15" wheel with 27" tires (I'm making things up) or a 16" wheel with 27" tires, the speedo should read exactly the same. The outermost circumference is the relevant measurement to accurately calculate the true speed - it's what determines the speed of rotation.

[manbridge 74]

Most cable driven speedos I've encountered get more optimistic after 55mph. Higher top speeds, even if not accurate, impress certain owners.

[scarceller]

Originally Posted by manbridge 74

Most cable driven speedos I've encountered get more optimistic after 55mph. Higher top speeds, even if not accurate, impress certain owners.

Tire changes shape at high speeds?

[manbridge 74]

Originally Posted by scarceller

Tire changes shape at high speeds?

Maybe a little. But back in the day guys at Cycle World, Motorcyclist, Cycle News etc often noticed this. Up to 15mph off or more. And manufacturers need only be concerned with accuracy around legal limits.

[scarceller]

A few years back I created a custom circuit so I could record speed in MPH with my data logging on my laptop. I crunched the following numbers:

----
The very first thing you need to do is obtain the following information for the given vehicle.

Tire Diameter = 25” Rear tires are 245/50 16”
Tire Circumference = 78.54” PI * Diameter (3.14 * 25”)
Pulses/Rev = 8 Number of pulses per final drive rev
For ‘Tire Diameter’ simply measure this with a tape measure, you don’t need to be 100% accurate here just to the nearest ½ inch is fine.

----
Now for the calculations: (important part!)

First, how many times does the rear tire turn per mile?
Inches per mile / Wheel Circumference = Tire turns in 1 mile.
63360 / 78.54 = 807 turns per mile

You can double check the math above by marking the rear tire and the pavement then push the car forward 1 complete tire rotation and measure the distance traveled in 1 tire rotation. I recommend calculating both ways. The 2 results MUST match!

On the Carrera the sensor gets it’s signal from a large disk that has 8 magnets and the disk turns at the same speed as the wheels. So, we can calculate the number of pulses per mile like this:
Tire revs per mile * 8 magnets = number of pulses per mile
807 revs * 8 = 6,456 pulses per mile

----
Given the above we can now express these:
So at 100MPH you will see about 645,600 pulses per hour
- or -
At 100MPH = 10,760 per minute (645,600/60)
At 100MPH = 180 per second (10,760/60)
Or 180hz signal produces 100MPH on the speedo.

360hz=200MPH (2.778millseconds)
180hz=100MPH (5.556ms)
100hz=60MPH (10ms)
1.8hz=1MPH (556ms)

Or 1.8 signals per second for each mile per hour
Or 1.8 x 60 = 108 per minute for each MPH

The important value is the last one: 108 pulses per minute for each MPH.

[sky1jord]

Must be something wrong with my '88. I just checked it against my Garmin, and it was spot on, at least within +/- 1 or 2 mph from 30 to 80 mph.
Also good agreement with the "Transmission Diagram from my owner's manual.

[shrtshck]

This past winter I had my speedometer rebuilt by NHS, the odo had quit, speed reading was over 10% fast, and they converted it to kilometers.
The work took literally a couple days between them receiving it and having back in the post to me, admittedly the lady said they were slow at the time.
Today I had a chance to verify the reading against my GPS and it is dead on.
Would recommend their work to anyone looking for a reliable repair at a reasonable price and quick turn around.

Blair

[X JBM X]

Sent my Speedo to NHS this afternoon for calibration.

Anyone know if driving without the speedo installed can create any problems? (aside from with local officials)

[rs6er]

No

[Richard C2S]

If you look at the specs posted on Tirerack's site, for example, you will see that overall diameter and revs per mile for identically sized tires can vary. Usually not by very much, but there can be some difference.

[buzdrum]

We do speedos as well! We have all the factory VDO calibration equipment.

Seattlespeedometer.com

[Reiver]

Gear Ratio Speed Calculator

put in your tire diameter, your gears and rear end and make a cheat sheet...eventually you'll have it memorized...I never look at the speedo anymore.
BTW, checked the results with GPS.

[PeterRT]

Originally Posted by Telerding

I believe the German government has VERY BIG FINES on a manufacturer if any of his cars are found to UNDER report speed. Thus they design the stock speedo to OVER report speed by 5% or so. Not sure when this practice started.

If the speedo read low, the vehicle simply wouldn't be road legal.
Until 1990, German licensing regulations allowed a deviation of up to 7% of the max scale value, at any speed higher than 50 kph. Indicated speed must never be less than true speed. So if the speedo of a 3.2 goes to 260 kph, it would be allowed to read 18.2 kph (11.3 mph) high, but never low.
Current EU regulations allow a deviation of 10% + 4 kph, tested at 40, 80 and 120 kph (25/50/75 mph), so at e.g. 80 kmh (50 mph) the speedo could read up to 12 kph (7.5 mph) high. Again indicated speed must never be less than actual speed.

[Breeang]

Mine reads about 4 mph faster than my GPS at all speeds. I'll take that 'extra' speed thankyou very much, it's the easiest tune up item I can think of, and cheap too !