[eonac]

I have about 1500 miles on my fiesta, 5sp. Have noticed that compared to the GPS my speedometer reads about 3MPH slow @ 55MPH, and about 5 MPH slow at 80 MPH. Talked with some folks, who seem to think that my car speedo is more accurate, however coming from a survey background, and the fact that the discrepancy is constant (Always occurs when driving) leads me to believe, that something isn't quite right the programming, of my cars speedo. Any one else notice this????
That may be able to be programmed, because i think the days of a mechanical tach cable are long gone.

 

[golf strom]

I have about 1500 miles on my fiesta, 5sp. Have noticed that compared to the GPS my speedometer reads about 3MPH slow @ 55MPH, and about 5 MPH slow at 80 MPH. Talked with some folks, who seem to think that my car speedo is more accurate, however coming from a survey background, and the fact that the discrepancy is constant (Always occurs when driving) leads me to believe, that something isn't quite right the programming, of my cars speedo. Any one else notice this????
That may be able to be programmed, because i think the days of a mechanical tach cable are long gone.

This isn't an abnormal error at all. It is done purposely. A margin of error is built in for legal reasons. All makers in the USA do this.

I'm sure you can program it to be dead accurate if you want though, if Ford doesn't disable the channel in the scan tool for it most likely.

 

[eonac]

Did some reading

I guess I shouldn't have found that odd that it is done deliberately. Being such a litigious society. It struck me odd as the '93 I drive was dead on with the gps...
(Where is the spell check???)

 

[MegEtc]

I have about 1500 miles on my fiesta, 5sp. Have noticed that compared to the GPS my speedometer reads about 3MPH slow @ 55MPH, and about 5 MPH slow at 80 MPH. Talked with some folks, who seem to think that my car speedo is more accurate, however coming from a survey background, and the fact that the discrepancy is constant (Always occurs when driving) leads me to believe, that something isn't quite right the programming, of my cars speedo. Any one else notice this????
That may be able to be programmed, because i think the days of a mechanical tach cable are long gone.

I think folks here were talking about one possible scenario in the tire thread - the speedometer is calibrated for a certain tire size (say, 16-inch tires) but if your tires are smaller than the speedometer thinks they are (like the standard 15-inch tires), then the mileage looks faster than it really is, because they are revolving faster than they would if they were bigger. Or I might have it totally backwards. Geometry class was a really long time ago....

Also, your GPS could be assuming bigger tires (like 17+ inch tires like SUVs have) and the Fiestas are a lot smaller than that.

 

[mrbirdman330]

The best GPS to use is a standard latitude and longitude with speed reader which is non biased against tire sizes or anything. Certain Smart Phones have this basic GPS utility in them. My Incite had one and I was able to verify the speedo on my Focus is pretty much dead accurate.

 

[golf strom]

I think folks here were talking about one possible scenario in the tire thread - the speedometer is calibrated for a certain tire size (say, 16-inch tires) but if your tires are smaller than the speedometer thinks they are (like the standard 15-inch tires), then the mileage looks faster than it really is, because they are revolving faster than they would if they were bigger. Or I might have it totally backwards. Geometry class was a really long time ago....

.

Not a bad thought, but you'll find most likely that ALL different factory tire/wheel combinations have a very similar margin of error if not the same degree of reading "slow".

It prevents the manufacturer from having litigation due to drivers being stopped for speeding and then claiming that it was the speedo that was in error...(at least reading low compared to actual clocked speed.

I've seen that over the past decade at least that all makers are having the speedo read around 5 mph above the "certified actual" speed. I know that there was a pretty big flap at that big German car company because their models were reading 7 mph fast compared to the actual speed.

 

[Marc]

An SE has 195/60R15.
If you go for 205/60R15, then it's reading 2% slower.
205/55R16 would read 2.7% slower than stock.
205/60R16 reads 6% slower: 60mph is actually 63.6mph. Theoretically this would compensate a bit for your car reading faster, I think ...

My car is about 5% slower on the GPS than on the speedo @ 120 MPH.

 

[Chan]

It's done on purpose in every vehicle. The margin of error varies from car to car but its about a 5% difference. Its meant to slow you down.

 

[Corvair]

There are laws that require the accuracy of the speedometers. They no longer use a cable going from the transmission to the speedometer on the dash. The computer tells the needle where to point. The computer would be programed with the tires that come with the car.

This has been a mostly accurate test for me. Use the mile markers on an interstate or simular road, set cruise at 60 mph, run a few miles watching a watch. A section without exits and on ramps is best. Should be a low traffic time. Safety first and all that.

Just get a speeding ticket and the police officier will be hapy to tell you how fast you were going.

Dan

11 Fiesta SE sedan week from Monday

 

[Nels]

My car is about 5% slower on the GPS than on the speedo @ 120 MPH.

Do I dare ask how you found this out?

 

[Nels]

Just get a speeding ticket and the police officier will be hapy to tell you how fast you were going.

Yes, but first he asks YOU if you know . . . .

I am reminded of hudson & Landry's "Hippie and the Redneck" --
"All right boy, I got you clocked at 80 miles per hour!"
"That's impossible, man! I haven't even been out an hour!"

 

[mrbirdman330]

Do I dare ask how you found this out?

GPS is highly accurate to within a mere couple feet in measuring distance and rate of travel. Odds are if the GPS says you're doing 70mph you're doing 70mph.

 

[Marc]

Do I dare ask how you found this out?

Ha, in my Fiat barchetta (think: Miata size and Ferrari styling) now and then. It's safer to do it in Germany though, as overhere (Netherlands), the speed limit is 75. I used to do it more, but well... congestion, radars, laser guns, unmarked cars all are spoilsports!

GPS is highly accurate to within a mere couple feet in measuring distance and rate of travel. Odds are if the GPS says you're doing 70mph you're doing 70mph.

Don't tell anyone, but I always have a GPS running when traveling by plane. Did you know a 737 will accent at 155 MPH and a 747 at 205 MPH?

 

[anverc]

Don't tell anyone, but I always have a GPS running when traveling by plane. Did you know a 737 will accent at 155 MPH and a 747 at 205 MPH?

I tried doing this on my garmin and it never found satellites... maybe you have to turn it on while stationary?

 

[iamz]

There are laws that require the accuracy of the speedometers. They no longer use a cable going from the transmission to the speedometer on the dash. The computer tells the needle where to point. The computer would be programed with the tires that come with the car.

This has been a mostly accurate test for me. Use the mile markers on an interstate or simular road, set cruise at 60 mph, run a few miles watching a watch. A section without exits and on ramps is best. Should be a low traffic time. Safety first and all that.

Just get a speeding ticket and the police officier will be hapy to tell you how fast you were going.

Dan

11 Fiesta SE sedan week from Monday

I was pulled over in my current car a few years ago.

Officer; "Do you know how fast you were going?"
Me; "My GPS said 64.9"
Officer; "I clocked you at 65."
Me; "Hmmmm, at least I know my GPS is accurate."

He let me off with a verbal warning.

In any case, I have not been in a modern vehicle where the speedometer is off by even 1%.

 

[Marc]

I tried doing this on my garmin and it never found satellites... maybe you have to turn it on while stationary?

I used this device: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=140401567927
Eventhough my phone has GPS, this one is much much more sensitive. I didn't have to keep it in front of the window (which is a bit suspicious), and with the right software (Nokia SportTracker) you can see speed, height, distance, direction, etc. But don't tell FAA!

 

[golf strom]

In any case, I have not been in a modern vehicle where the speedometer is off by even 1%.

Then you must be very lucky because most mfrs today program the speedo with a margin of error ,so even if something else is a bit "off" it won't read in a way that the driver interprets as "slow" .

 

[mrbirdman330]

My wifes Kia is off by 5mph and to accuratize it I have to put on 205/55 R16 tires which I plan to do for her. Mine is accurate which I like.

 

[LMTech]

The speedo spec is +/- 5%

 

[Corvair]

I just got my Fiesta yesterday. There is a radar near where I live. I was going 61 and the radar said 57. I'm very surprised. The odometer after watching the mile posts on the interstate was very close after 15 miles. Too close to call. Plus or minus a car length.

Also the gas gauge doesn't go up to the full mark. Almost, about a needle width from full. Drops to 7/8 in 15-20 miles. It then drops at a reasonalble rate. 1/8 of tank it had just under 4 gallons left. My trip computer said 37.4 mpg and my actual was 37. Five speed, air on 100% of time, mostly highway 55-75mph. A/C is powerful!