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Discussion Starter #1
Hey everyone,

First time posting after lurking for a while. Big thanks to everyone who's posted here - I've learned a lot and gotten a lot of help just by reading.

Was wondering if I could get some help with a speedometer issue. Background: Bought a 2014 Touring a year ago with a bad automatic transmission for a good deal. Had the transmission rebuilt right away at a local shop. Then moved to a different state. A few months ago I was using a Garmin GPS unit when I noticed the car's speedometer was slower than GPS speed. Up to about 45 mph, the speedometer reads at or slightly faster than real speed. But after that, the speedometer reads slower than real speed. And the faster you go, the more off it is. At 55 on the speedo, real speed is 56. At 65 mph on the speedo, real speed is 67. At 75 on the speedo, real speed is 78 ish. I haven't tested faster than that. I used a GPS speed app on my iphone and got the same results.

I don't think it's the tires. The car had the correct size tires when I bought it (225/45R19). I hit low tread last month, and replaced them with Vredestein Quatrac Pros, 225/45R19. But the speedometer still reads slower than real speed with the new tires.

So I'm thinking there's something off with the transmission rebuild, with the speed sensor, with the speedometer cluster itself, or maybe all 3rd Gen Mazda 6s are like this from the factory.

Does anyone else have an automatic 3rd Gen with a speedometer that reads slower than real speed from the factory? I didn't drive enough on the original, bad transmission to notice before I had it rebuilt.

And does anyone have any other ideas what could cause this?

Thanks!
 

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That's weird. I would say something is off with yours. I don't believe manufacturers are allowed to let speedos read slower than actual speed, so it wouldn't be a widespread problem with the model.
 

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My speedo (2015 A/T) reads at to a touch below GPS speed. 0.5 - 1 mph slower than GPS speed at 65 indicated.
 

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That's weird. I would say something is off with yours. I don't believe manufacturers are allowed to let speedos read slower than actual speed, so it wouldn't be a widespread problem with the model.
Yeah, I think they usually make the speedometer read 1-2 mph faster than real speed, not the other way around.
 

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My speedo (2015 A/T) reads at to a touch below GPS speed. 0.5 - 1 mph slower than GPS speed at 65 indicated.
Thanks for letting me know! Gonna see if there are other people with the same thing. If there are, then it's normal. 3 mph ain't a big deal if not speeding. But if not I think it's the transmission rebuild.
 

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My GPS is usually about 1 mph above actual and my actual is just about dead on (at least it seems so every time I go by a speed reader).

Have you changed your tires at all or are they over/under inflated? Tire size can very quickly alter your speed readings which is why we always recommend getting tires as close to OEM as possible.
 

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Discussion Starter #7
I’m at 35 psi all around.

I’ll try to find one of those radar speed signs and do some tests. And run it by a transmission shop too. It really isn’t that big a deal unless I’m trying to go 10 over. I’m thinking I’ll just live with it.
 

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I’ll try to find one of those radar speed signs and do some tests.
That is a good idea; however, you obviously want to be alone on he road, and they lose accuracy when you get close to them.
I pass one daily and I my speedo generally reads 1-2 MPH high compared to it.
My speedo and GPS also generally agree.
 

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Discussion Starter #10
Hey, thanks for all the replies. Looks like some of our speedometers run slow. I found a radar speed thing and ran by a few times to confirm. Ain’t a big deal, all in all.
 
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