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higaran

Speedometer Inaccuracy?

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Hi all:I just recently bought a 2008 Passion Cabrio from a friend with just 15,000 km on it. I've had it since September, and I'd been noticing that other motorists seemed to drive with a great deal of "attitude" around my Smart. They'd pass me on the highway and floor it as soon as they were beside me; one dude even gave me the California Wave.I just shrugged it off and figured they were all jerks, until it happened a couple of Saturdays ago for the umpteenth time. That's when I pulled out my smartphone and flipped on the GPS function. Turns out my speedo is 10% over. In fact, I have to drive 111 km/h according to my speedometer - about 3100 RPM in 5th gear - in order for my GPS to show that I'm actually doing 100.A couple of weeks after getting the car, I ordered new winter tires and rims from Tirerack.com. I was careful to specify the factory tire sizes so there wouldn't be any noticeable difference in my handling and speedo reading. So today I went into the shed with a tape-measure and physically checked the diameter of the factory tires and rims against the aftermarket winter tires and rims. The back tires are virtually identical in diameter, but the front tires, for some reason, are about half an inch smaller than the factory front tires.Does anyone know which wheel(s) the Smart uses as input for the speedometer? And has anyone else had this problem with the speedo being so far off? I could tolerate a difference up to 5%, but as far as I'm concerned 10% is unacceptable for a car with just 15,000 km on it. If I can establish that this is a defect and not something resulting from my aftermarket tires and rims, I'm going to try to make Mercedes fix it.Your thoughts?

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My speedometer reads about 8-10% high - just like yours, against a GPS and a Scangauge. Bigger tires will help - but then the Scangauge will be off.If you look at the spedo dial - the spacing changes suddenly at about 80 km/h - I am sure the computer doesn't know how to compensate for this. My odometer seems to be accurate.

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According to my phones GPS, with my intermediate tires (all OEM size) my speedo is off by about 3%, with my 16" summer tires my speedo is bang on.

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The cdi for 2005 and 2006 are 3 km off the mark for sure, but I am surprised the 451 reported above is that much out, and confirmed by GPS, so how can it be doubted. BUt my 451 is not out that much. I should check it again.

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Wow, yikes! Is the 451 really that far off?!Could it just be the US cars? I noticed their speedometer is different; it doesn't have the variable-pitch gauge like the rest of the world does. So that means there's a unique program for it's needle motor which may not have been as carefully developed.For legal reasons, it obviously has to over-read just a bit, but only a few km/h.-Iain

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My car was bought new in Edmonton, so it's definitely not a US vehicle. I wonder if the US and Canadian Smarts use the same programming for the dashboard? That would explain at least some of the inaccuracy.

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Yeah, my 451 is the same. Odometer seems to be accurate but the speedometer reads 110 kph when my gps reads 100. I've always hated it and also agree that it's truly unacceptable but never did talk to the dealer about it thinking that route would be futile.

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The speedometer on the 450 at least displays speed from the average of the 4 wheels.

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Maybe programmed high because they know the stock Contis wear so quickly? My speedo was reading low by the time my rear 175s were bald at 47,000 Km.

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It should always be programmed a bit high, otherwise you could get a ticket for doing the speed limit.

-Iain

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It should always be programmed a bit high, otherwise you could get a ticket for doing the speed limit.

-Iain

Say what you will but all other vehicles that I've driven in the past 2 years (about 30 different makes & models) are within 1 or 2%. The 451 is the only one that's close to 10% high. (and it's not related to tires, mine are now almost bald too and its still currently 9% high)

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Say what you will but all other vehicles that I've driven in the past 2 years (about 30 different makes & models) are within 1 or 2%. The 451 is the only one that's close to 10% high. (and it's not related to tires, mine are now almost bald too and its still currently 9% high)

Obviously have never driven a Honda then....... :P They are notoriously optimitic. There was even a recent lawsuit because of it, but that had more to do with the odometer being off due to the speedo being off.

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@ Jolicouer:

I wasn't saying I agree that 10% out is right; I was saying that it being a BIT higher (notice how I italisized that particular word) pretty much has to be done.

It can never read 100% perfectly; that is simply a fact of mechanics.

Therefore, somewhere along the design process, you have to decide "Which way shall we get our programmers to make the system err?"

And so you have two sitautions:

1. You make it read high, so when you think you're going 100 km/h, you're really going 97 km/h, and life is good.

2. It reads low, and then you get someone going 100 km/h, but it's really 103 km/h, and they get a speeding ticket, sue smart, and the company goes out of business, and 12000 kittens die, and Iran blows up the world with Nuclear bombs.

Jus' sayin'.

-Iain

Edited by Duck

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my other 2 vehicles are 2 kph out by gps, this is acceptable. the smart is 4-5kph high at 100 kph. can they change the way the speedo reads with the starscan?

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...and 12000 kittens die, and Iran blows up the world with Nuclear bombs...

:lol: I've often wondered whether you could make a case in court for getting a ticket if it was within 10%. Like say you got a ticket for 109 in a 100. If the OEM's are only required to be within 10% on their speedometer reading, could you argue that your car is within specs and that you were indicating 100? Would you have to prove that your car was off by 9%? For the life of me I can't think of anyone that's ever gotten a ticket for speeding less than 10% over though, maybe there's a reason?

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:lol:

I've often wondered whether you could make a case in court for getting a ticket if it was within 10%. Like say you got a ticket for 109 in a 100. If the OEM's are only required to be within 10% on their speedometer reading, could you argue that your car is within specs and that you were indicating 100? Would you have to prove that your car was off by 9%?

For the life of me I can't think of anyone that's ever gotten a ticket for speeding less than 10% over though, maybe there's a reason?

Regional word-of-mouth has it here (and it is my experience too) that police won't usually write a ticket if you are no more than 7km/h over the posted limit. And often 10 over if the traffic is flowing nicely.

I think you have struck on a logical reason for this practice.

Bil :sun: .... :drive: @+7

Edited by bilgladstone

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Uh, guys, please give this some thought. If car's engineers wanted to leave a margin of error for tire wear, then they would have programmed the speedometer lower, not higher. A worn tire has a smaller circumference, therefore it will need to turn more often in order to cover the same distance. Once my tires are worn, my speedometer will read even higher when the car is travelling the same speed.

In any case, and regardless of the reason, a ten-percent (or greater) margin of error is completely unacceptable in an (almost) new car. I'd perhaps find it more understandable if I was buying a "lower-end" brand of car, say a Kia Rio, Hyundai Accent, or even a Chevy Cobalt. But this thing was built by Mercedes-Benz, for pete's sake.

I definitely intend to take it up with the dealer in January when the car goes in for service.

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higaran,

Yes, do take it up with your dealer. The consensus on the earlier 450 model was that the speedo reads approximately 3% over actual. Maybe the 451 is more... :dunno: or maybe your computer is a little out of whack :dunno:

Tell us what you discover, eh, cousin?

Bil :sun:

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My speedo reads 5 km/h high almost throughout the entire range compared to the GPS. The odometer is practically bang on, though. I don't mind too much about the 5 km/h, but 10 would really annoy me. My old '91 Cavalier read 3 or 4 km/h too low and I ended up getting a couple of speeding tickets because of it.

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hmm, I had this thought in the back of my mind often but never really gave it too much weight.. I just figured the speedo was off and had an acceptable margin for error... some brands, like BMW motorcycles, are notorious for being WAY off in the higher speed ranges.. (that is, saying you are going faster than you really are).Well, I have a USA car with MPH, Odometer in Miles, though, I always guessed the reading is just a software thing, isn't it a digital speedo getting input from a speed pulse/sensor, and either KPH or MPH reading would be off by the same amount? I don't know, but guess this would be the case.Since I crossed the border from USA into Canada though, I just use the GPS in my navigation system to read back KPH to me.. I may be doing 10 KPH over the limit or so naturally, to keep up with the rest of traffic, but if everyone stayed exactly 80 KPH or 110 KPH or whatever posted limit was, I would be doing the posted limit as well.. I generally go with the flow of traffic and don't look at the speedo too frequently unless I'm slowing to exit or entering a town's school zone or that sort of thing.. The only times I have gotten a speeding ticket was when I was going WELL OVER the posted limit, so that extra 10% wouldn't have helped ;-)Oh.. and almost since day 1 of my car I was on 17" wheels, only during winter (now) do I revert to the stock 15" rims with all-season continental tires.. I must say after having wheels so big for so long, it looks awfully strange to see my car sitting on stock rims again... Eibach springs and all..

Edited by StarLord

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StarLord:I know this is slightly off-topic, but you may want to ditch those all-season Contis in a hurry if you plan on driving at any length in Canada during the winter. I drove on the Contis during our first snowfall of the season on a country road, and my knuckles are still white.I just recently bolted on a nice set of four Pirelli Winter 190 Snowcontrols in factory tire size from TireRack.com, and they make all the difference in the world. I drove to work 45 minutes one-way both yesterday and today on very icy roads, and felt perfectly safe zipping past 4WD half-tons and SUVs creeping along the highway at 40 km/h. I almost felt embarrassed and I'm sure other motorists wondered about my sanity, but I simply had good enough control of the car that I wasn't worried in the least.Bottom line: this is one investment no one will regret if they live in an area with any significant snowfall.

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Well, I have a USA car with MPH, Odometer in Miles, though, I always guessed the reading is just a software thing, isn't it a digital speedo getting input from a speed pulse/sensor, and either KPH or MPH reading would be off by the same amount? I don't know, but guess this would be the case.

It's a bit more complicated than that. In the USA, it's the only market where there's a fixed-pitch speedometer installed. In all other markets (including Canada), the speedometer pitch (incrementation) varies. It jumps from 5 km/h increments to 10 km/h increments after 60 km/h.

You can see that in this image:

Posted Image

The USA cars don't have this feature, and just go at a steady increment up through their full range - therefore, there's some different software running in there, and that's why I was proposing earlier that perhaps the guy who sat down and did the USA programming didn't do a very good job.

-Iain

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