Speedo, Odo accuracy?? - Mercedes-Benz Forum

 
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post #1 of 5 (permalink) Old 09-30-2008, 04:11 PM Thread Starter
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Smile Speedo, Odo accuracy??

On my 1992 190E, I replaced the speedometer-odometer cluster as the odometer was broken. I found a new unit with the correct part #, and installed it. I noticed I was passing all other cars when indicating 65 MPH, so I drove through one of those radar mountae on a trailer at the side of the road that advises "Your speed is", at my indicated speed of 30 MPH, the radar said "35MPH", I tried it several times and always got the same answer.

The question here is: If the speedo is off 18%, does that mean the odometer is off by the same amount? When I checked the mileage, I got 22.5 MPG, the same or similar to what I previously calculated before the odometer broke.
:confused
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post #2 of 5 (permalink) Old 09-30-2008, 04:26 PM
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That's way off. Maybe tire size (rolling diameter) is way over OEM size.

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post #3 of 5 (permalink) Old 09-30-2008, 07:01 PM
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Originally Posted by mrtn View Post
On my 1992 190E, I replaced the speedometer-odometer cluster as the odometer was broken. I found a new unit with the correct part #, and installed it. I noticed I was passing all other cars when indicating 65 MPH, so I drove through one of those radar mountae on a trailer at the side of the road that advises "Your speed is", at my indicated speed of 30 MPH, the radar said "35MPH", I tried it several times and always got the same answer.

The question here is: If the speedo is off 18%, does that mean the odometer is off by the same amount? When I checked the mileage, I got 22.5 MPG, the same or similar to what I previously calculated before the odometer broke.
:confused


Get up on the interstate and find a mle marker. Preferably on a long stretch of highway. Pull over right at a mile marker and zero out your trip odometer. Go at least 5 miles and pull over again. Now see how far it says you drove. Mile markers are set out by surveyors and are extremely accurately spaced.

You can also check your speedometer accuracy with a passenger, a calculator, and a stop watch. I like to do this at 60 mph as it just makes it a little more repeatable. Drive at a steady speed, such as 60 mph, from one mile marker to the next. Start the stop watch when you pass the first marker and stop it when you get to the next. Now have your passenger divide 1, by the time it took to travel one mile. Multiply that by 60 to get your miles per minute. Multiply it again by 60 to get the miles per hour. If it took 55 seconds, you would divide 1 by 55 to get 0.018181818181818181818181818. Multiply that by 60 to get 1.090909090909. That's the miles per second. Multiply that by 60 and you get 65.45 mph.

It's really a good idea to check this when you have a lease vehicle. If it's reading high, you pay for say 36,000 miles but only drive 32,000 miles over the life of a lease. That's 2000 miles you paid for but didn't use.
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post #4 of 5 (permalink) Old 09-30-2008, 09:19 PM
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Sounds like you've got replacement speedo from a car having different differential.
MB speedometer are very accurate. I am driving with GPS a lot and all of my MB show within 1 mile difference.
My Ford truck shows 65 mph, while GPS reads 60.
Odometer is another story. It is very hard to compare since road markers have lot of errors. I have a second hose in 500 mile distance. Different cars would read between 470 and 503 miles.
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post #5 of 5 (permalink) Old 10-01-2008, 06:05 PM
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Sounds like you've got replacement speedo from a car having different differential.
MB speedometer are very accurate. I am driving with GPS a lot and all of my MB show within 1 mile difference.
My Ford truck shows 65 mph, while GPS reads 60.
Odometer is another story. It is very hard to compare since road markers have lot of errors. I have a second hose in 500 mile distance. Different cars would read between 470 and 503 miles.


I'm not sure how the speed sender has ben configured. It may just need a drive gear swap. Most car companies have three or four different gears for different ratios.
On road markers, it depends on which ones you use. The mile markers are set out by surveyor. I think the tolerance is no more than 4 inches per hundred miles. Now, the signs saying a certain number of miles to a certain city are a different matter. I've seen them where you got farther away the closer you got. 50 miles and as I got closer it said 58 miles. These are set where ever is handy. Instead of engineers with surveying equipment, you have a political appointee with a state truck.
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