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Old 10-07-2020, 09:19 PM   #1
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Speedometer reads high

2002 workhorse w22, 8.1 vortec , allison 1000 5 speed.

When speedometer reads 65 mph, GPS reads 58 mph. Bought and installed a scangauge2. The scangauge readout for speed agrees with the GPS. Why is the speedometer reading so far off. Where does the speedometer get its info? How can it be different than the obd2 port readout? Originally thought the ecm had to be reprogrammed but that was before I had the info from the scangauge. I am confused. Does the speedometer get its info from the tcm? Is this different info than that being displayed from the obd2 port?

Any help would be appreciated.
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Old 10-07-2020, 10:08 PM   #2
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Welcome to the club! My speedo is 10% optomistic as well.

It also is 10% off on the odometer, so it piles on the milage faster than you actually put it on. Great for the seller regarding warrenty.

I have contacted Brazel's RV, and they said they can fix it with their computer/control modual programmer.

Mine is a 2007 W24 chassis.

I haven't been by Brazel's for a while, so will just live with it until I get it changed.

One plus is the oil gets changed more often than normal!

Happy Glamping.
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Old 10-07-2020, 10:47 PM   #3
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First thing I would look at ia the tires. Are they OE stock size or did a PO put larger or smaller tires on it?
Modern vehicles the speedo is driven from one of the various computers. Again did aomeone reflast a computer? If so you if you suspect thats so then your only choice is to have it reflashed back to original configuration.
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Old 10-08-2020, 12:33 AM   #4
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A slightly smaller diameter tire will make the speedo read higher than it should. Even if the tire size printed on the sidewall is what the factory recommends, tires from different manufacturers can be of a slightly different diameter. It is usually correctable through a computer flash.
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Old 10-08-2020, 06:36 AM   #5
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Mine reads high also. 65 is really 60ish. At least it keeps me in check, but also thought about the added mileage on odometer. Where I'm confused is the speed odometer is high, yet the odometer is dead nuts when comparing to mile markers. Not sure how that is possible. Stock size tires.
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Old 10-08-2020, 06:56 AM   #6
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Pretty much, but not always, since the introduction of the OBD2 data bus the mechanical speedo has been replaced with digital date word from the ECU (or one of the many sub processors) commanding the analog speed display. Which sensor is used to determine speed will vary between chassis manufactures, or even model years. Could be the transmission output shaft rotation speed, maybe sharing data from the cruise control or even the antilock brakes module. Usually the error is in the analog speed display and a correction can be made by reprogramming the ECU.
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Old 10-08-2020, 07:50 AM   #7
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Re: tires. Tires are new, less than1500 miles. Same size as original, different brand. OE tires have a diameter of 33", replacements(hankook) are 33.1". Not enough difference in diameter to reflect an 8 mph overspeed indication. Actually the new tires should read slower than originals.

I guess the real question is; how can the readout from the OBD2 port agree with the GPS and the speedo be so far off?

I want to compare odometer readings with mile markers, but wil not have that ability until next week.

A while back I had some info from Brazels, they suggested they could reprogram the module for a small $100 fee (quite reasonable). I would need to take it out and send it to them, not sure if they would want the engine control module or the transmission control module.
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Old 10-08-2020, 09:35 PM   #8
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My 08 Dolphin with W22 chassis is also reading 10% high(reads 66 mph but is actually going 60mph and recording 10% more miles on the odometer since day one and on stock Michelin sized tires. It can be corrected by reprogramming PCM but not here in Canada.
I would need to remove the PCM and send it to someone like Brazel's RV or have it done while in the US. IN my case my Scangauge also shows the +10% speed error.
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Old 10-09-2020, 06:31 AM   #9
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It’s interesting that when the chassis are ordered, they know what they are going to used for, so why couldn’t the pcms have been programmed correctly from the get go?

The only positive I see is rv drivers will be driving a false slower ( safer and better economy), while racking up additional mileage on some
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Old 10-11-2020, 12:22 PM   #10
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My speedo is off on the slow side. When I 70 it is within 1mph or so. Interesting.
I mostly use my gps for speed but the real kicker was the temp gauge being way off. One summer in Jackson, TN at 98 degrees I lost all the coolant and couldn't find a reason why. I installed a scan gauge this summer I found that when the dash gauge said 210 it was actually at 260. Ouch. Discovered the radiator was covered with crud between it and the condenser.
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