RixPix Loc: Miami, Florida

I recently had the speedometer on my van checked and it is reading 2.35 miles faster than I am actually traveling. Since the van has about 67,000 miles on the odometer does the inaccuracy of the speedometer mean that there is an inaccuracy in the odometer?

tlbuljac Loc: Oklahoma

Yes

RTR Loc: West Central Alabama

What make of vehicle is it? My wife's Toyota exhibits just about the same anomaly. I first noticed it when I put a GPS in the car. I then checked the odometer vs mileposts on the interstate for 20-30 miles and the odometer is off by the same amount as the speedometer. We had the dealer check it and they said it was ok. Sure, whatever.

Her car has 75,000 showing but in reality is probably only about 73,000.

OnTheFly Loc: Tennessee

Yep. Have you changed the size of the tires lately. We had a larger set of wheels-tires put on my son's truck and two hours later he got a speeding ticket. 8 miles over.I should add that I got behind him in my car and clocked him and sure enough he was doing 8 over what his speedometer said he was doing.

BW326 Loc: Boynton Beach, Florida

As far as I know, all modern vehicles have an ECM (electronic control module) and it keeps a running count of the total revolutions the tire makes. It has a number programmed into it that shows the nominal revs per mile of the original tire that was installed on the vehicle. If you change tire sizes, the revs per mile number should be changed in the ECM. Also, as the tread wears, the tire becomes slightly smaller although over the long haul, wear alone shouldn't make too much difference because the original rev per mile number is calculated at a midpoint of that particular tire size (between new tread and worn tread). All I know for sure about highway mile markers is that they are not 'absolutely' required to be accurate. Most references I looked up on the web gave an indication that they are accurate but I do know this for sure. Fifteen years ago I was assigned a project to find out exactly how accurate they were and I called half a dozen different state transportaion agencies and they all said that they try to make them accurate but their main purpose is to be a reference marker and not an absolute measuring tool. Sometimes the next mile would end up being in the middle of a bridge over water and they would have to place it before or after the bridge or over a large ravine. What is very accurate though are the painted lines on certain highways that the highway patrol planes use to determine speeders. Those are typically +/- 6 feet and usually about every 1/10th mi. or 1/4 mi.. (depends on the state)

RixPix Loc: Miami, Florida

This van is a GMC Safari 1999. I purchased it used about 4 years ago with 51,000 miles on it. (Orthodox Jewish Family vehicles see little use here in Miami). It has an electronic odometer and I know the family so it wasn't tinkered with. How many miles less would it be...I know there is some math formula I should use but my math side of my brain shut down around 1997.

RixPix Loc: Miami, Florida

I had it checked professionally as I kept getting passed on the interstate when going 75. Turns out at 75 on my speedometer is really about 71.75. At 30 it is really 25.25

BW326 Loc: Boynton Beach, Florida

That's a pretty drastic difference. I'm no automotive technician so I'm just guessing in the dark but it might possibly be a bad speed sensor. Back in the old days (where I hail from) I know that fleet truck drivers who wanted to go faster but at the same time, 'fool' the onboard electronic computers and thus 'fool' the speed governors on the trucks would sometimes crawl underneath where there was a transducer screwed into the flywheel housing and 'back off' the transducers a few turns and put in some spacer shims. The transducer's job was to count the teeth on the flywheel as they flew by and thus get an accurate assessment of the engine's RPMs and, by way of that, be able to calculate the vehicle's speed.... the result would be a vehicle speed governor that thought the engine was doing 70 mph when it was actually going 74 or 75.
It's possible a faulty speed sensor might be having the same effect because the truck driver's speedometer would show him going 70 while the vehicle was actually going faster.

phcaan Loc: Willow Springs, MO

I think you will find the culprit is your tires. My car speedometer was consistently reading 4 mph slower that the actual speed. When I purchased new tires( these were Firestone vs the no-name tires I purchased last time) The speedometer was again accurate.
Both sets of tires were the correct size as listen on my car tire data sticker, but from the get-go my economy tires were 4mph off.

Julianmpb Loc: DurhamUK

Complex problem,here in UK speedos must not be more than 10% over but must not read under the actual speed.

ThePapaJohn Loc: All over the world.. Currently WA

I know that my VW (2001 Golf TDI) speedometer is off, but the Odometer is dead on. On the TDI forums, it is very common now that the speedometers read faster than what your actually doing. Not just VW, but a lot of makes...

Steinmetz Loc: Oregon based

Are your tires stock or are they larger or smaller in diameter ?

It is also likely your speed sensor is incorrect, and your odometer can still be okay and visa versa.
Again I have had a speedometer fail whereas the odometer was correct.

I found this out lately on my jeep and my toyota truck

Sweet Willie Loc: Texas

Nissan truck,Subaru wagon,old Honda all did the same. I think it is a
factory design-liability thing. My Garmin GPS will give true velocity.
Sweet Willie

raynardo Loc: Oceanside, CA

In the general scheme of the universe....

If you're not running the tire size recommended on the inside driver's side door jamb, this could be the problem.

You could spend a bunch of money and get a Superchips Flashpaq to reprogram the on-board computer of your vehicle to the size tires you're running.

Or you could just be satisfied that this is your lot in life.... :)

Clif Loc: Central Ca.

I think you will find that all speedometers are 2 to 5 MPH optimistic by design, it may be for safety sake or just the manufacturers want you to feel their cars are faster than they actually are, or just to error on the side of safety within manufacturing tolerances. Odometers will be a little more accurate as to total distance traveled as it is an averaging over say 100,000 miles. They are all made to be accurate within some tolerances as it is less costly to accept a 5% error factor than to build one to be accurate within 1% or less. Add to that the changing circumference of the tires and their manufacturing tolerances and you have 2 to 5 MPH error. In everything made the more accurate you want something the more it will cost to make, even the shutter speed of your camera will not be dead on at 1/500 of a second. Precision costs money, the more precision the more cost, that last 1% is a lot more costly than the 5%.