Is my speedometer right, or is the speed checker thingy on the highway right?

Impossible to say without having both the speed check machine and your speedometer checked for accuracy. They could both be wrong. haha
Good question, maybe some one of our car gurus like @jerv will answer.

Have you changed the tire size from original, like going to a 55 series tire from a 65? ? (i.e. 265/60–15 to a 265/55–15) If did the new tire spins more time per mile, more MPH on your speedometer. Most of those RADAR speed checker are accurate.

IDK @Tropical_Willie. My thoughts are that the radar things are more accurate too.

Police radars are calibrated periodically. Those on commercial vehicles even moreso as trucking companies and taxis charge by the mile, and trade regulations require accurate measurement. The average car speedometer/odometer, on the other hand, is set “close enough” at the factory and often drifts from there. In the last 30 years, the adjustment on mine has drifted quite a bit.

As for tire sizes, while changing to a different tire size will affect the rolling diameter, and thus the accuracy of the speedometer/odometer, tires are like clothes or shoes in that even tires that say they are the same size aren’t always really the same size. Some 205/60R16s are wider than other 205/60R16s.

It’s worth noting that carmakers know that there will be some inaccuracies, especially if the tires on the drive wheels (the front for most modern cars) are changed, so they set the speedometer to read slightly high. With brand new factory original tires, pegging the speedometer at 55 MPH will probably happen at an actual speed of around 53 MPH.

In general though, you can count on it being 3–5% high with stock-sized tires, and up to 10% with non-stock tires (like my 175/65R13s instead of 175/70s) and/or a few years of wear on the drivetrain.

I’ve noticed that many speedometers seem to be 5 MPH slow. My new speedo in my 91 Nissan is 5 MPH off but should change to about 3 MPH slow when I get the right size tires on the back. That radar thingy is correct. The next time you pass a mile marker, check your odometer. If your speedo is 5 MPH off, your odometer will read 0.9 miles instead of 1.0.
If your car is new enough and has a electronic computer controlled speedo and odo, the dealer or your mechanic might be able to calibrate it via the OBDII or OBDIII computer connection.

If you have a GPS you can use it to measure velocity. That will give you your answer.

My speedometer has been consistently off by about 10% against every speed checker thing for as long as I have had the car, which is 15 years. And I have been in a pack of people scared to pass a Highway Patrolman and all going the same 55 m.p.h.speed, and my speedometer will say 58 or 59.

I heard you’re pretty safe going 3 to 5 miles over the speed limit.

My car’s digital speedometer read consistently lower than the speed that shows on my sat nav. I will be traveling at 30 mph exactly yet the sat nav says 26–27.

My husband says it’s a request from the government to car manufacturers to make us drive at lower speeds.

LOLL! Love dem conspiracies!

That is a funny conspiracy.

As one who understands technology and it’s limits, I’ve known for decades that trying to calculate actual speed by measuring the RPM of the wheels was foolish. Hell, anyone who ever rocked a car out of a snowbank and saw their speedometer hit 70 knows how inexact that mechanism is.

GPS signals really don’t care about your tire diameter though, nor do they rely on approximation. All GPS cares about is your position. That’s it. They can measure your position quite accurately, and by comparing the distances between different positions over a precisely known value of time, they can give you an accurate, objective measure of speed.

I trust the speedometer in Waze, my GPS app of choice, more than I trust a worn out cable-driven mechanism build as cheaply as possible that is taking a wild (and wrong) guess about my tire size. Why? Because engineering.

So I checked out the conspiracy theory and it’s true!! Sort of… UK law says that a speedo must not show the speed as less than it actually is.

@Stinley It’s not just the UK though, but truth is that the conspiracy has slightly less sinister roots.

Cops really get sick of hearing, “Oh, my speedometer must be off, officer!”, as it stands. It simplifies things for police if there is no way that the speedometer could be off in a manner that indicated you were under the speed limit when you were actually speeding. Those who know the engineering (including carmakers and police) know that mechanical speedometers will be inaccurate, so carmakers tweak it until the speedometer is consistent about always reading faster rather than leaving enough wiggle room for some legal nightmares.

..speedos must not show anything at all…we wish.

If someone hits 70 getting out of a snowbank, then somebody doesn’t know how to get out of a snowbank. Anything above 20 mph is a foolish, useless waste of perfectly good snow and energy and tires and time and will get you no where.

@Dutchess_III If you drive a rock-crawler maybe. Those of us who drive lighter vehicles that lack triple-width tires with inch-high lugs use different techniques than those who are unable to skim across the top of the snow like an airboat.

Since moving gets you to more places than not moving, and moving faster gets you there quicker than moving slow, I feel that those who try plowing through the snow instead of riding on top of it waste more time and energy than I do since it takes a lot more energy to push snow than it is to be up above it, and I’m actually getting somewhere. If you wish to refute that then you might want to explain how my little Corolla manages feats like crabwalking it’s way up a steep hill that left multiple vehicles with 4WD (some with chains) stranded. I surely wasn’t actually moving at 70–80 MPH, but unlike everyone else, I was moving!

The real trick isn’t minimizing wheelspin; it’s knowing when to use it and when not to. Those that never spin their tires are absolutely stuck after first snowfall. Those that aren’t afraid to kick up a roostertail will get through places that more conservative drivers will get stuck.

I assumed you were referring to someone who was stuck on ice or snow, and was trying get out from a stand still. Spinning your wheels gets you nowhere in that situation. You can’t get off of ice from a stand still by punching it. You have to tease the car out.

Starting from dry ground, where you can actually accelerate to get over a snow bump, or something, is a different animal. Then you have the advantage of your momentum.

It has nothing to do with being “afraid” of kicking up a rooster tail. It has to do with the useless futility of kicking up a rooster tail when stuck on ice and snow. Some people never seem to get it.

If you actually have the car creeping forward, and feel the tires grab, then punch it to gain some momentum.

And if you have kids-in-training, you head to the high school parking lot to teach them how to recover from slides and spins and stucks and stuff. Spend 15 minutes demonstrating the slides and spins, while they look at you in alarm because their staid, never-break-the-rules mom is yelling “Wheeeeewwww! Go doggie!” ;)

Ah, no. While there is a pretty big difference between getting moving and staying moving, and it seems that the second part trips people up, leading to them getting stuck repeatedly. Starting tends to take a bit of rocking, and is why I prefer three-pedal cars to slushboxes.

Dry ground? I take it that you have never been to the rural Northeast, at least not any time between October and June :p

You mean “teach them the way most New Englanders learn”. On a related note, Finns tend to do well in rally racing simply because what most people call “special racing technique”, they call “commuting”. Many people go to racing school to learn things like the Scandinavian flick that a lot of those who grew up in the Northern latitudes figure out on their own by the time they can reach the pedals.

Jerv, I love you, but this is a ridiculous conversation. I’m sure you’ve had lots and lots of experience getting out off of and out of snow and ice. So have I. I agree, rocking is the best way to get moving. Flooring the accelerator is not.