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Hi,

I've noticed that my speedo reads about 7% faster than my actual speed. I checked the site below for expected differential due to tires changing from 185 65 R14's to 205 40 R17's and there's practically no change.

Speedometer Calibration Calculator And Information
http://www.csgnetwork.com/speedocalibcalc.html

I contacted a speedo shop here locally and when I told him I had a 2000 Civic Dx Coupe with an analog gauge, he said nothing can be done to calibrate it and that that is the problem with Japanese cars, they all do that. He then referred me to the dealer.

Personally, I think he's full of it because I've had several other Toyota's and Honda's and have never had this problem. Perhaps I should have have pointed out that although it's an analog gauge, the system is electronic, deeming it adjustable/repairable.

Any input on this would be appreciated.

Thanks
Selvan

 
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yeah, according to http://www.miata.net/garage/tirecalc.html, you're absolutely right, there's .1% difference in the speedo with the new wheels/tires. I'm curious, tho, how do you know the speedo is 7% off?? If it is off, i'm not aware of a way to calibrate it, short of taking the needle off and moving it. That's about impossible, tho, and even if you did get it to read dead on at say 65mph, it would still be off at every other speed. You could swap out the gauge cluster with a replacement from ebay, or whatever, but that may not fix your issue. You could be having an issue with the vehicle speed sensor itself, which is obviously what senses the speed of the vehicle. I have not seen any type of DIY for replacing that yourself. In short, i think you may have to have this issue diagnosed professionally, unless you want to just start throwing parts at it. I'm not sure how useful my rambling here has been, but I wish you the best of luck on this one.

 
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I have a portable GPS unit in my 04, and find that the speedo and actual speed as reported by the GPS is very, very close. Is that how you know your speedo is off? If not, how did you determine the 7% difference?

 
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Hi and thanks for the input, it does help.

There's major freeway constructions going on around here and they've put up those radar signs that show your speed on both sides of the freeway. To check their accuracy, I drove by them in another vehicle and both radar signs were right on.

 
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Do you think a failing alternator would have any effect on the speedo?

 
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^^That is a surprise. Those things are normally way off.
I would use a GPS or some other form of testing.
Perhaps a stop watch and the mile markers.
For example: Set your cruise control to 60mph. Now have you passenger hold a stop watch. As soon as you pass a mile marker, start the watch.
When you hit the next one stop the watch.
At 60 mph that is exactly a mile a minute. So go from there.
This test assumes you are on a level piece of ground with no turns and you passenger has a fast trigger finger.

 
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Yes, a failing alternator would cause your gauges to do funny things.
The plug that goes into my alternator came loose once, it took me forever to figure it out.
My srs light would flash the radio would just reset while going down the road just odd random crazy stuff.
All I had to do was unplug it and plug it back in.

 
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Ya, I suspected the alternator because I'm experiencing dimming/brightening dash lights and the SRS light blinks occasionally while at high speed and going up hills only. But, at least not yet, my sppedo needle does not randomly read crazy MPH. I've been searching and reading lots of the same issues caused by a failing alternator but none mentioned it relating to a constant inaccurate speedo.

Ya, I've been looking for those mile markers I often see while traveling but there aren't any here that I've seen. I live in Sacramento, does anyone know where they are?

Okay, I guess I'll go have my alternator checked first.

Also, I was thinking, wouldn't changing my tire size alter the speedo? Am I correct by saying that if I decrease the outside circumference of the tire I would have to travel slower to obtain the speed of 65? Like, for example, if I have to drive 75 to do 65 now, then decreasing the size would mean I would have to travel less than 75 to do 65?

 
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i think cuz u have 17 inch rims on thats y... factory speedmeter only reads that factory 15 inch or whatever that came wit the car.
i leard that **** in my Mechanic class.. it might be true.
Arc leangth / radius = radian i dont know how to apply these to real life situation yet..is new lesson .... bascially sayin .. if u have bigger size of tire, if u are goin 60 mph.. speedo might reads 64 mph...(depends on tire size)
i dont know if this would help u out or not

 
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No offense but is that really a fact because it seems like it should apply to all speeds and not just the one I pick to set it at.

Perhaps DelawareDon can attest to this being that I don't have a GPS unit. In my own experiences they seem to always be right on. And mind you, with my last employer I'd easily put on 100k miles a year and have seen my fair share of those radar signs.

Thanks for that tip, I'll definitely go after the alternator first thing.

Yeah, you're right about the bigger size tire. Just think of the 10 speed bicycle and pretend you peddling is the car driving. Change gears on the rear tire to a smaller gear and you peddle slower, change to a larger and you have to peddle faster to maintain the same speed.

In my setup, I did go to a larger rim but because I went with low profile tires, the overall circumference of the tire did not change from stock(the 14's I mentioned in the original post) so my speedo was not effected. The guy at the speedo shop thinks this has been a problem with my from day one.

Maybe if the alternator doesn't fix this, I should put low profiles on 14's to do it...ha ha ha

 
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It's basic math, sir. Say it is exactly 7% fast. So when you're doing 65, it's reading 69.55. So, say you round up, and figure it's actually 5mph fast. So you turn the needle back 5mph. Now you're dead on, more or less, within 0.5mph, when it's at 65mph. The problem here is you haven't corrected it by a percentage, you've corrected by a set increment. So say now you're doing 80. Your speedo will be reading 85.6mph, as 7% of 80, is 5.6. Now assume you turned it back 5mph. You're still off by 0.6mph. Not such a big deal. But now, say you're doing 30mph. 7% of 30 is 2.1. So where your speedo would read 2.1mph faster than you are actually going, it's now reading 2.9mph slower than your actual speed because you set it back 5. At 30mph, you're actually further off than you would have been had you just left it alone. I say just leave it alone, and watch it to make sure it doesn't worsen, unless you want to look into the alternator issue. My speedo reads about 6-7% low because my tire circumference is slightly larger than stock. Once i get above 40 or 50, i just mentally add 5mph to whatever my speedo shows. So far, that's kept me out of trouble. No sense in my trying to recalibrate my speedo for two reasons. 1)up here in the northeast, i put my stock wheels/tires on for the winter, so my speedo is dead on at least 4 months out of the year, and 2)next time i buy tires, i may get a slightly lower profile, to keep my rolling radius, circumference, and speedo readings closer to stock.

 
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realistically, all speedometers are of by a bit, they just arent accurate, by like 5 mph or less is legal, thats part of the reason u dont get a ticket at 5mph over the speed limit. however, i have called every place imaginable, even talked to honda, there is not a way to recalibrate ur speedometer, it just isnt possible, i wanted to when i got my rims, cuz my tires are 215/45/17, so they are off by quite a bit, but i just dont sweat it, there is just nothing u can do, if u do find a way somehow, let me know, cuz id love to do it, but everyone ive talked to, including 3 diff honda dealers, there just isnt a way to do it

 
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Cluster swap from a police package crown vic?? They actually have calibrated speedos.

 
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hahah, hilarious

 
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^^^^Most of the Chevys do.

 
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That makes sense, thanks. What about changing the tire size, sir, will that have a more consistent effect on the speedo? Not sure if I'll ever go that far but I am curious. It's looking like I'm going to have to get use to adding-to the speedo.

I did read that they are but I've never had a car be so far off before.

As for the dimming dash lights and SRS flicker, I called the dealer and they want $60 for an electrical check-up so I took it to Riebes and they attached a tablet looking volt meter to the battery(in the car) and said the battery is good, started the engine and seeing 14 volts he says the alternator(in the car) is good. And goes on to say that a more thorough test can be done on the alt if I were to remove it first.

He's probably right about that but do you guys think it's worth the effort to remove it for Riebes or just take it the dealer?

BTW, Riebes alt with lifetime warranty is about $100 less than the dealer.

 
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yes, changing the tire size would change the speedo by a percentage, rather than a set interval, like moving the speedo needle would. Since i did all the math for my last post, i realized that i think my tires are causing my speedo to read about 6% low. When my speedo reads 70, i'm actually doing about 74 to 75 according to my gps. Four things to consider regarding changing the tire size to compensate for the speedo inaccuracy. 1)obviously, tires are a bit pricey, especially in a lower profile, 17" size 2)if your VSS, or your gauge itself is going bad, it may get progessively less accurate/worse, meaning eventually, you'd need to change the tire size even more in the future, to continue to compensate 3) it is recommended that you keep the tire circumference and/or rolling radius (the distance from the center of the hub to the part of the tire that contacts the road) within 2-3 percent of the stock size. I am well outside this range with 215/45/17 tires, and i don't notice any major issues, however. Keep in mind tho, that changing your tire size is almost like changing the gear ratio in your transmission, to a degree. by going to a larger profile tire, you would increase the tire circumference/rolling radius and bring your speedo closer to accuracy (at least until whatever is causing said inaccuracy worsens). you could also, however, lose a slight amount of torque (probably negligeble) when taking off from a stop, but, at a set speed your engine RPMs would be lower than with stock tires, at least with a manual transmission. The reason for this is that to maintain the same speed as it would with stock tire, the larger tire actually spins
slower, requiring your half shafts, everything in the transmission, and ultimately the engine to also spin more slowly. I don't notice much if any loss in power with a larger tire, but someone once posted a "post your rpms at 80 mph", and i was 100 to 200 rpm slower than the average with my larger tires. and lastly 4)a larger tire puts a little more cushion (rubber) between your rim and potholes, etc., protecting your wheel.

I don't know that i would advise changing the tire size, tho. I am most concerned about the possiblity of your speedo becoming less accurate, and having the new tires not end up fixing your problem. It was a good thought, but I personally can't recommend acting on it.

 
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From my previous post, my alternter was not bad it was actually charging the battery! My battery was good also. I did the same thing as you, took it to Autozone for the free battery and alternator test. They said I was all good.

So after much thinking and searching and tearing my car apart several times. The problem is the plastic clip on the side of the alternator, it is not the charging wire. There are 4 wires in the little clip, they send info back to the ecu about the status of the alternator, voltage amps stuff like that. I just unhooked the little plug and made sure all the wires where good. Then I plugged it back in and the problem has been fixed for a little over a year now. So with that I say check it first before you start to get really mad, like I did.

 
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Mine does this also. We have a '97 Civic DX and a '97 Accord. They both are off by a percentage. When i am going
45, the GPS reads 42
55, GPS reads 51
70, GPS reads 65...

I also have dimming dash lights and headlights. However, this is not caused by a bad alt; it's been doing it for about 6 years.

 
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Well, as I mentioned, I doubt I'd go that far and change the tire size to correct a speedo problem, it's definitely not the cause. My changing from stock to 205 40 17's is only suppose to make me off by no more than .05 mph, that's not even a 1/2 mph. Strangely though, the speedo shop I quoted before did say that he suspects my speedo has been off like this from day one, I doubt it.

I finally got the time to do that today, took it off and slid it back in and out several times before plugging it back in. I repeated the steps a couple times but it didn't help. At least not for me.

Really weird, when I'm doing 45 on the speedo, I'm actually doing 42 also, my lights dim occasionally, and my SRS light also flickers from time to time.

I feel like I should have the sensor replaced but I've read a lot of people have done so only to find that the problem came back after just a few months.

With a problem like this, I'm surprised Honda hasn't come up with a fix. what about the odometer, how does this effect it?

 
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Perhaps DelawareDon can attest to this being that I don't have a GPS unit. In my own experiences they seem to always be right on. And mind you, with my last employer I'd easily put on 100k miles a year and have seen my fair share of those radar signs.




You're right. Those signs, the speed for which is driven by radar, are typically "right on" according to my GPS.