speedometer not properly calibrated

How can you check if the speedometer is not properly calibrated and if I ask a mechanic to do it how much should it cost?

Thanks, W

Reply to
sh-aly
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If you belong to the Auto Club, they have a roving speedometer check vehicle. They post its location in their magazine, which is usually at one of their local offices. I believe it is a free check.

The GPS idea is also good.

B~

Reply to
B. Peg

I checked my Tundra against a GPS unit and the Tundra was 2 mph fast at 60 mph, but only 1 mph fast at 30. Not enough to worry about.

Reply to
offen rong

Besides the GPS idea that badgolferman posted and the auto club speedometer check that b.peg posted, you can try driving at 60 MPH and watching the mile markers. You should do this over at least a 10 mile stretch to average things out.

Besides asking someone with a speed measuring gun to measure your speed, the only way to check for sure is with a chassis dynamometer. These are very expensive machines and not many shops have them.

Reply to
Ray O

But how accurate is the GPS?...I have a Magellan Meridan Gold...not very expensive (or is expense a factor?)

Reply to
Gord Beaman

If the GPS is WAAS enabled, then it is probably accurate to within 10 or 12 feet. Without WAAS, it is probably within 30 feet.

Reply to
Ray O

Um, drive 110MPH in a 50MPH zone?

They'l let you know!

Seriously what I do is look for one of those radar signs they put out once in a while. My speedos are all within 5% of what they say, even after monkeying with tire sizes.

Or, just go down to the PD and ask them to clock you! They will.

Reply to
Hachiroku

I have the cheapest GPS unit around, but even it is very accurate. I used to do the 'drive 60 and check the mile markers" method, but GPS has replaced it.

Reply to
Sean Elkins

It claims to be, and it claims 'accurate to within 3 meters' so perhaps it it...but that means the 'speed' is accurate does it?...(Yes I suppose it does) :)

Reply to
Gord Beaman

Thanks...this one was just under 500 about six months ago...lots of fun...

Reply to
Gord Beaman

I don't get lost enough to spend $500!!!

I know my way EVERYWHERE! (or so it seems...)

Reply to
Hachiroku

"Accurate to within 3 meters" is about 9 or 10 feet. I assume that the clock function on the GPS is also pretty accurate so the distance/speed equation should be pretty accurate.

Reply to
Ray O

Except he needs the accuracy measuring the speed, which isn't done by evaluating the difference between positions, at least not typically:

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As noted on the link, Garmin (a popular manufacturer, I have one of their units) claims .1 mph accuracy. This probably depends on pretty good skyview and satellite selection. On a highway in a fairly level rural location, with no obstructions and the GPS placed where it can easily see the sky (or at least the satellites currently in the sky), you'll get that accuracy. I have a Sienna. If I put the GPS towards the front of the dash, it gets a pretty good skyview. If it's reporting good positional accuracy, speed measurement will probably be pretty accurate, too.

Got no money? Set your cruise to 60 mph and time mile markers over a straight-ish 10 mile stretch of road. A stopwatch and I-80 in Nebraska are handy for this.

Got $100 and an interest in radio, electronics, hunting, fishing, camping or hiking? Get a cheap GPS and check your speed that way (have a passenger actually work the GPS) and the GPS will provide entertainment later on. You can use them in aircraft (after the pilot says electronics are OK - just hold it up to the window), boats, woods, etc.

Got lotsa money? Get a mapping or navigating GPS unit.

To be sure you're using it right, check gpsinformation.net for more info and be sure to read your product manual. GPS is not magic (it just seems that way) and it does have its limitations. Look over gpsinformation.net BEFORE you buy.

YMMV.

Reply to
DH

Why would you even consider asking a mechanic to check this? Get out on the freeway and drive a measured mile at a fixed speed and see how long it takes. Then divide 3600 by the time (in seconds) that it took to travel the mile, the result should be your set speed within a couple of mph. The displayed speed, 70 for example, will be about 2 mph faster than the actual speed.

TIMES NEEDED TO GO 1 MILE

60 mph, 60 seconds 70 mph, 51.42 seconds 80 mph, 45 seconds

You can divide 3600 by any MPH to arrive at the time in seconds that it takes to travel 1 mile, then you can go out and set the Cruise, or carefully use the gas pedal to maintain a constant speed, and verify the time. The Interstate Highways all have mile markers along the right shoulder that are spaced within a few feet of precisely 1-mile intervals. There will be a slight delay in your timing to start and stop your stop watch, but you can easily confirm if your speedo is accurate or not. Obviously, if you got a speeding ticket doing 50 in a 35, you will find that your speedo is much too accurate to let this happen. If you car has oversized tires on it -- generally this is something that only affects trucks -- then you _might_ have a speedo that reads 35 when you are really doing 50, but all passenger cars in America -- and the rest of the world for that matter -- have a speedo that reads something in the range of 3 mph faster than your actual speed. The purpose of this is to keep you from getting speeding tickets.

Reply to
Jeff Strickland

GPS is close enough to check a speedo.

Reply to
Jeff Strickland

How do you know?

Reply to
Gord Beaman

Because I sold GPS systems, and have a pretty good idea of how the system actually works. It's all based on known mathematical formulas and a minimum of 4 (four) satellites. It's easy to guage the speed. The location is an easy task as well, but the early systems were not allowed to be accurate on the location, but the rules have changed since then. Now, the GPS systems are accurate to within 3 meters (about 10 feet), but this degree of accuracy has nothing to do with the speed.

Reply to
Jeff Strickland

The main issue with speed was sampling rate, or the number of times in a time period that the unit calculated a position.

My old Garmin GPS II and III (early and mid 90's vintage) would lag the speedo quite a bit during spirited acceleration and deceleration. The eTrex Vista and aviation GPSMAP 196 I have are better, while my GPSMAP 60CS has an almost non-existent lag. The lag is evident as the GPS displays below the speedo while accelerating, and above the speedo's indicated speed while decelerating.

Slamming on the brakes with the older units might have you at a dead stop with the GPS reading 4-5 MPH for a second or two, or maybe a 5 MPH reading, then a 2, then a 0. Accelerating down a runway in a jet might see it lag behind 10-20 MPH.

These examples were all with selective availability turned off.

At a relatively constant speed, I've never seen a GPS that wasn't dead on compared to a timed distance.

Reply to
Bonehenge

Well of course it would lag under those conditions, but the point was that the OP wanted to know if his speedo was telling him the truth. When the speed is rapidly changing, the truth is a variable and anything the speedo says is useless. The way to get relaible information from a speedo is to level off on the speed changes, and a GPS works fine to display constant speeds.

Reply to
Jeff Strickland

I TOTALLY agree.

Sorry if you took it any other way. I just though the information might be useful to the OP.

Reply to
Bonehenge

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