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Old 10-06-2022, 10:39 AM   #1
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speedometer is off

I have a 2008 Tiffin FRED/Freightliner. The speedometer is reading a bit slower than my actual speed. If I'm actually traveling at 65 MPH, the speedo shows 62 (or close to it).

A few questions:

1: Could this discrepancy cause any performance/MPG issues due to the ECU getting incorrect information?

2: Can the speedo be recalibrated or adjusted on a Freightliner?

3: Could this be happening because I am running Toyo tires instead of the Michelins that came with the coach?

THANKS!
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Old 10-06-2022, 10:42 AM   #2
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Different diameter tires will definitely cause speedometer error.

Shouldn't affect the engine computer.
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Old 10-06-2022, 10:58 AM   #3
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Different diameter tires will definitely cause speedometer error.

Shouldn't affect the engine computer.

ECM looks at engine rpm and TPS signal ( Throttle Position Sensor ) . to determine amount of fuel to inject , not vehicle speed.

Check your tire specs against those of the original Michelins, look for the RPM ( Revolutions Per Mile in tire talk ) to see if that's the issue .

If Freightliner can adjust the speedo for the correction , they'll need the tire RPM to make the input .
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Old 10-06-2022, 11:02 AM   #4
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Did you put slightly larger new tires on your coach? If so that's the cause for the difference. However a 3 mph error at 65 mph is nothing to worry about.
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Old 10-06-2022, 11:04 AM   #5
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I'm curious how you are measuring the actual speed.
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Old 10-07-2022, 02:49 AM   #6
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I'm curious how you are measuring the actual speed.
Two different GPS units that have been checked against two other cars.
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Old 10-07-2022, 07:56 AM   #7
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Our 2008 is off by 3 mph - verified by our Trucker's GPS. Easy to live with.
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Old 10-07-2022, 08:17 AM   #8
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Spedometer is a measuring device, and all measuring devices have some error in them. My rigs soedometer is off by about one mph. My car (a mini cooper, which is actually a bmw in disguise) reads low by three mph at 55mph, and four mph at 70.). I asked atdealership, and they said all BMWs read low on the speedometer, and that it is *not* adjustable. … which makes you wonder that all those fast aggressive bmw drivers out there actually *think* they are going faster than they actually are
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Old 10-07-2022, 09:00 AM   #9
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Spedometer is a measuring device, and all measuring devices have some error in them. My rigs soedometer is off by about one mph. My car (a mini cooper, which is actually a bmw in disguise) reads low by three mph at 55mph, and four mph at 70.). I asked atdealership, and they said all BMWs read low on the speedometer, and that it is *not* adjustable. … which makes you wonder that all those fast aggressive bmw drivers out there actually *think* they are going faster than they actually are
I have nothing to base my comment on but common sense. What the dealership told you about all BMW ‘s being off is a ridiculous statement. Sounds like the service department is just poorly trained.
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Old 10-07-2022, 11:08 AM   #10
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I have nothing to base my comment on but common sense. What the dealership told you about all BMW ‘s being off is a ridiculous statement. Sounds like the service department is just poorly trained.
Speedometers are not calibrated to be exact. Car magazines used to publish speedometer error in their data and the speedometers were always off by 2 or 3 mph at 60 mph.
There are some aftermarket speedometers that folks put in their hot rods that use gps to determine the indicated speed.
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Old 10-07-2022, 11:21 AM   #11
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GPS doesn't know if you're going up or down a hill. It's measuring your distance moved between measurements. The steeper the grade, the bigger the angle, the more inaccurate it is. It's not reading your distance moved continuously either. It has to extrapolate from time to time so you're seeing more of an average than an exact measurement at any moment. It's never perfectly accurate but usually closer to your actual speed than the speedo, at least on flat ground. A lot of speedos are also just off. Many come from the factory that way.
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Old 10-07-2022, 04:29 PM   #12
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I have nothing to base my comment on but common sense. What the dealership told you about all BMW ‘s being off is a ridiculous statement. Sounds like the service department is just poorly trained.
It might just be a reasonable statement. We have Gold Wing motorcycles, 1800's. We have a feature which is called Honda Miles where the speedometer is off by about 10%. It also appears that Harley Davidson has the same feature, at least in the two our friend owns. He says it is widespread.
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Old 10-07-2022, 07:42 PM   #13
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I have nothing to base my comment on but common sense. What the dealership told you about all BMW ‘s being off is a ridiculous statement. Sounds like the service department is just poorly trained.
Ever owned a BMW? We have two at present, and had a few before that. *All* of them have had speedometers that indicate speed above actual speed, as measured using GPSs and time between mile markers. Friends with BMWs report the same.
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Old 10-08-2022, 08:52 AM   #14
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Ever owned a BMW? We have two at present, and had a few before that. *All* of them have had speedometers that indicate speed above actual speed, as measured using GPSs and time between mile markers. Friends with BMWs report the same.


That is what the dealer reported to me. Confused the daylights out of me when i first got my mini … i was driving the speed limit, and *everyone* was passing me. Then i checked against a gps and realized what was going on…
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