General Question

NostalgicChills's avatar

Could the calibration of my speedometer be wrong?

Asked by NostalgicChills (2779points) April 7th, 2013

I have a 1995 Geo Prizm and its a great car. However I’ve noticed that I get passed A LOT, especially when I’m in New York for my horseback riding lessons. As a disclaimer, I’m honestly not a bad driver at all- and I know that especially in New York it’s better to go a little over the speed limit because they get really pissy really fast. So anyways, yesterday on my way home, these old people passed me and crossed the double yellow lines all while beeping the horn and making faces at me. I was going 43 in a 35. What gives? Could my speedometer be off, or were they just typical New York assholes?

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11 Answers

Lightlyseared's avatar

Yes. There are a whole number of reasons a speedo could be reporting the wrong speed. The most common cause of errors is when the tyres (or wheels) have been changed to ones with different diameters to stock but tyre wear, temperature, vehicle load can all effect the accuracy. Also manufactures usually calibrate the speedo to over indicate speed by the average error of the speedo which can be +/- 10% so in the worst case scenario it could be out by 20%.
Probably worth getting it checked out.

whitenoise's avatar

If you have a smartphone with gps, you can easily check how fast you are really going.

Or time a set length of road at a steady speed and verify your speedometer against the time the stretch took you.

NostalgicChills's avatar

@Lightlyseared
I did not know that. I’ll certainly get it checked out.
@whitenoise I had no idea you could check the speed of your car using your phone either.

Thanks for your answers!

hearkat's avatar

@NostalgicChills – I have the Waze App that tells you your speed – I don’t know how accurate it is, but it is within a couple miles of what my car’s speedometer says (as are those radars at the side of the road that tell you how fast you are going). I agree with @whitenoise that it could give you a pretty close idea to compare with your car’s speedometer.

jaytkay's avatar

Expressways have mile markers

If you drive at 60 mph it will take 1 minute between markers.

If your speedometer says 60 and it takes more than 1 minute, your suspicion is correct.

SadieMartinPaul's avatar

Putting aside the fact that there are plenty of @$$hole drivers in New York, your speedometer could need recalibrating. I once saw someone get a speeding ticket completely negated because his speedometer had been wrong. He brought a mechanic’s statement and repair receipt to court, showing that the speedometer had, indeed, been faulty, and that he’d taken care of getting it fixed.

LuckyGuy's avatar

If you don’t have one, borrow a GPS from someone. The speed they indicate is perfect.

Tropical_Willie's avatar

Are thires on the car properly inflated, also the tire size should be 175/65–14. If it is 165/65–14 that would cause the speedometer to read higher than the actual speed.

Judi's avatar

I have a GPS that tells you your speed. It is usuall a few miles per hour slower than I’m actually going.

jerv's avatar

They generally are a bit off as they are designed to accommodate being used with non-stock tires without ever reading low. That means they often read a little higher than your actual speed, but never below.

I generally trust my GPS’ speeedometer far more than I trust the one in my dash. If nothing else, when I drive past a speed limit sign with a radar gun, the number it gives is the same (+/- 1 MPH) as my GPS gives, both of which are 5–10% lower than what my dashboard says. In other words, I will be doing 64 and my dash says 70–75.

Also note that few people drive the speed limit. I-5 in Seattle is a 60 zone and NH-9 is a 55 zone, but average speed on both is ~62 MPH whereas MA 2 around Boston is a 55 zone with an actual average of 74. Don’t assume that the speed of traffic matches what the signs say it should be ;)

@hearkat Waze is accurate.

Mariah's avatar

Yep, I’ve got the same issue going on. I first noticed it because I had a GPS in the car that measures speed and they weren’t matching up.

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