How to verify speed of vehicle

[IIGood]

Hey all...over the weekend I drove past one of those digital road signs that shows "Your Speed" and the sign came up showing I was going 56 mph. There weren't any other vehicles around me at that point, so I'm fairly certain that it was my speed showing. However, when I glanced at the speedometer of my '03 XLT, the needle was on 60.

The Explorer has stock-sized tires, albeit 2 different sets (one brand up front, one brand out back). The rear tires were a bit low on pressure when I checked afterwards; they were each around 28-29 PSI. The truck is otherwise stock, as far as I can tell. It's also a 4WD model.

Is there some way that I can do a test to see if my speedometer is reading properly? Are those signs usually accurate in their readings?

 

[Number Twelve]

Go on a highway. Set the cruise control on 60 MPH. Measure the time between mile markers. Calculate the true speed.

 

[IIGood]

OMG. I was completely overthinking this.

Thanks for that. I was thinking of stuff like plugging something into the OBD2 to get a reading, or if I'd need to take it to a shop for a check, etc. This...so simplistic. I should be able to do this tonight on the way home. Well, depending on traffic anyway.

 

[lincolnshibuya]

have you used the the phone app "Waze"? it has a speedometer display besides showing cops.
I think any GPS app could display the speed.

 

[Number Twelve]

Actually, you don't have to be going 60 MPH, it just makes the math easier.
And while we're at it, trying this on the flat and level will help keep the cruise control speed stable.

I would like to emphasize the fact that lots of verifications can be done from the most primitive of experiments. The charge of an electron from looking at an oil drop??? Way past me! But calibrating a torque wrench, finding the actual speed, calculating how may watts your microwave oven is really delivering? All as simple as a stop watch, a thermometer, and a bowling ball.

There now. Wasn't that nicer than a rant about people becoming so dependent on computers and smart phones that they can't think their way to the curb?

 

[chefduane]

Growing up in California they used to have "Odometer Check" signs on the side of the road. They read "Mile 1," "Mile 2" etc.. up to 5 miles and one could check the accuracy of the odo. Pretty simple method brought to you by CalTrans. As a kid back in the dark ages, I used to get a kick out of doing this.

 

[fordysenior]

I've driven past those things that tell you how fast you're going, they're always wrong. If you're on a rural stretch, you can watch for the road markers that the planes use to check how fast people are going. I've checked my odometer on them and found them very reliable (never have seen a plane though).

 

[IIGood]

Actually, you don't have to be going 60 MPH, it just makes the math easier.
And while we're at it, trying this on the flat and level will help keep the cruise control speed stable.

I didn't get to do the check yesterday...I was originally planning to make a trip up the one interstate (that doesn't actually leave the state) after work but I didn't. I went to another store that took me on my usual route home, and at that time of day, that highway is always backed up.

The other tricky part, at least in the immediate area, is the "flat and level" part. On the way home, the highway does have some dips and inclines. But if I head a bit to the west towards DC, the highway does have more level areas.

 

[TechGuru]

Easiest way is with a GPS (dedicated or phone).

My Garmin GPS shows me going about 1-2mph slower than the vehicle speedometer.

When I first bought my Explorer I kept wondering why people kept tailgating me and going around me, when I got around to installing my GPS I found out I had been driving 2 mph under the speed limit.

 

[swshawaii]

^ Ditto. If no GPS or Smartphone, borrow one. Couldn't be easier or more accurate. Funny thing is GPS says +/- 1/2 (0.5) MPH accuracy.

http://gpsinformation.net/main/gpsspeed.htm

 

[imp]

Easiest way is with a GPS (dedicated or phone).

My Garmin GPS shows me going about 1-2mph slower than the vehicle speedometer.

When I first bought my Explorer I kept wondering why people kept tailgating me and going around me, when I got around to installing my GPS I found out I had been driving 2 mph under the speed limit.

@TechGuru
Get on I-17 north of Phoenix, they will tailgate you at 20 over! We even have a statute allowing citation for "impeding the flow of traffic" if driving in LH lane at any speed over the limit, if a string of cars are behind. Crazy legislators, solving unsolvable problems with more ridiculous laws. imp

 

[drdoom]

Yes imp you are right about that, 90mph is nothing on that run!

 

[drdoom]

Tech I have a tuner and of course you can set your tire revs to correct, but on my tuner you must pick revs from a list of choices, like for instance you can pick 650 or 660 or 670. So you have to round up or down. The closest I can get it is 62mph indicated/60mph GPS. YMMV! Ha

 

[imp]

Tech I have a tuner and of course you can set your tire revs to correct, but on my tuner you must pick revs from a list of choices, like for instance you can pick 650 or 660 or 670. So you have to round up or down. The closest I can get it is 62mph indicated/60mph GPS. YMMV! Ha

@drdoom
2/60 = 1/30 = ~3%. Not too bad. Years back, monkeying with gear ratios, I often got 1%. imp

 

[mountaineer2004]

Drive next to a car with a digital speedo, like Honda Civic, and compare.

 

[drdoom]

Great idea.

 

[drdoom]

I know! Go blow through a radar trap and see what they write on your summons!

 

[mountaineer2004]

That's another idea, problem solved, right? Just a bit expensive, haha!

 

[drdoom]

But so much fun!!

 

[Number Twelve]

Drive next to a car with a digital speedo, like Honda Civic, and compare.

Or a State Police car with a Certified Speedometer.

go blow a radar trap and see what they write on your summons!

Not likely to be the correct number.
Once upon a time, I said to a State Trooper, "When you leave here, you have to go south for at least a mile to the next intersection, so I want to drive behind your certified speedometer to check the speedometer on this car I just bought."
He said, "The Law doesn't apply to me so I am going to do an illegal u-turn through that muddy median strip and you can't follow me."

So I went to a speedo shop and paid $10 to have it checked.
Draw your own conclusions.