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speedometer/odometer correction

7K views 10 replies 7 participants last post by  STM 
#1 ·
After searching I am unable to find an answer to my question. Does anyone konw how to modify the 03 odyssey so that the odometer and speedometer read actual. On my 98 dodge ram I purchased a programmable box that alters the signal from a tone ring in the rear axle to the computer. This allows for correction factors to compensate for different tire sizes. Prior years of ram used a nylon gear, with different tooth counts available from the dealer. Has someone investigated the service manual who can answer this?
 
#3 ·
tmees said:
After searching I am unable to find an answer to my question. Does anyone konw how to modify the 03 odyssey so that the odometer and speedometer read actual. On my 98 dodge ram I purchased a programmable box that alters the signal from a tone ring in the rear axle to the computer. This allows for correction factors to compensate for different tire sizes. Prior years of ram used a nylon gear, with different tooth counts available from the dealer. Has someone investigated the service manual who can answer this?
For what its worth the error you have was designed into the system. It has been a DOT requirement since Liz Dole was the head of DOT that all DOT spec speedo's read 5% high.

Her reasoning was that since most drivers run about 5% over the speed limit making their speedos read high would allow the average driver to drive that 5% higher and still stay within the speed limit. Actually that rule replaced an earlier and far more stupid speed control regulation that mandated that a vehicles speedo could only read up to a maximum of 85 mph. Ever see a 1988 Porsche or BMW with a speedo that only indicated 85 mph? She did however give us the center mounted stop light and that was one of the most useful safety additions in cars since the DOT started passing laws mandating safety equipment.
 
#4 ·
I don't think there is any US law regulating speedo accuracy. Check out this article at Car and Driver, April 2002. It states:
So we sought out the rule book to find out just how much accuracy is mandated. In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.

But wait, there's more. Driving in arctic or desert climates? You're allowed another plus-or-minus two percent near the extremes of 20-to-130-degrees Fahrenheit, and yet another plus-or-minus one percent if the gauge was ever exposed to minus-40 to plus-185 F. Alternator acting up? Take another plus-or-minus one percent if the operating voltage strays two volts above or below the normal rating. Tire error is excluded from the above, and odometer accuracy is more tightly controlled to plus-or-minus four percent of actual mileage.
They also had an interesting comparison of different manufacturers. Hyundai was the most accurate at 0.2% slow, Honda was about 2% fast and BNW was the worst at 4.5% fast!:eek:
 
#5 · (Edited)
dgs said:
I don't think there is any US law regulating speedo accuracy. Check out this article at Car and Driver, April 2002. It states:

They also had an interesting comparison of different manufacturers. Hyundai was the most accurate at 0.2% slow, Honda was about 2% fast and BNW was the worst at 4.5% fast!:eek:
OK I checked it out so I must have been given bad information. That was the explanation I was given for the difference in MPH readings on my 2000 VW A4 when I changed the instrument cluster program from North American @3% too high to UK setting where it was reading much closer when checked by both GPS and road side radar signs. I had originally switched the cluster to the UK to get the clock to read 24 hours and get some functions not available on North American instrument clusters and noticed the change because my speeds were reading a bit slower for a given RPM after the program change.

Thats what I get for accepting an answer without questioning its source. Thinking through the original problem it is obvious now why my readings were different. The cluster when set to UK measurement was reading Imperial miles instead of Statute miles and this answer fits the question I had better because the MPG readings on the trip computer were also off because the fuel readings were also reset to Imperial gallons too. The speedo likely reads high in the UK as well. I need to get further into the program to see exactly how many speedo impulses are used to calculate a mile after the correction for tire diameter is applied.

Thanks for the second opinion! Thats why I never object to being taken to task for a stupid comment in these forums.
 
G
#7 ·
dgs said:
I don't think there is any US law regulating speedo accuracy. Check out this article at Car and Driver, April 2002. It states:

They also had an interesting comparison of different manufacturers. Hyundai was the most accurate at 0.2% slow, Honda was about 2% fast and BNW was the worst at 4.5% fast!:eek:
Yeah, but that's deliberate on the part of the speedo in the BMW. We have the full-function computer in the car and it records the speed very accurately. For instance, you can set it to alarm when the speed exceeds the setpoint, and if you set it to 80, it'll go off when the speedo indicates 83 or 84. If you set it to go off at 30, the speedo basically matches.

The odometer and the computer's trip computer matches perfectly though.

Wayne
 
#11 ·
dgs said:
I don't think there is any US law regulating speedo accuracy. Check out this article at Car and Driver, April 2002. It states:

They also had an interesting comparison of different manufacturers. Hyundai was the most accurate at 0.2% slow, Honda was about 2% fast and BMW was the worst at 4.5% fast!:eek:
Sorry, I just read this thread and there might be something interesting to know about the speedometers in German cars. As far as I remember are speedometers in Germany always adjusted that they never (!) show less than the actual speed. So if you add up all the different percentages you can understand why you end up easily with about +5% total. The most important factor in Germany is the change of tires (e.g. summer and winter tires). This alone produced a few percent in difference in the cars we (my wife and I) drove over in Germany ('84 Mazda 626, '89 VW Golf (Rabbit), '93 Mazda 626, and '97 Opel Vectra Caravan TDI).

Actually I was always happy that the speedometers in the German cars showed never less than the actual speed since in Germany speeding tickets are already possible at 3 km/h (= 2 miles!) :eek2: above the speed limit!
 
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