Speedometer Error on Purpose?

[javacontour]

That is true. The tests for cafe are doing by measuring the components of the exhaust gas over a known course on a dyno. So they don't actually measure the volume of fuel or read the speedometer/odometer. They simply sniff the tailpipe and calculate how much fuel was burned.

 

[cchase]

My truck and my car are within 1 mph at 65 mph. My fathers GMC truck is also within 1 mph at 65. My mothers Accord is within 2 mph at 65. My sisters BMW is within 10 mph at 65. No joke. BMW's spec for speedo accuracy is apparently quite loose.

 

[bradepb]

my ford escape is same as my gps, have to check the others

 

[opus1]

My Malibu's speedo is spot-on with both my GPS' and a local "reminder" radar set up near the high school in my area.

My Dakota was spot-on also, until I replaced the tires. I have a +0 size, but there's still a bit of a change that I was able to compute based on a tool on Discount Tire's website; the tool said my speedo would read 2MPH slower than my actual speed, and I've also confirmed this with the GPS' and the radar mentioned above.

 

[d00df00d]

Originally Posted By: cchase
BMW's spec for speedo accuracy is apparently quite loose.

I've heard this as well. Mine seems to be pretty good but I know I saw this discussed in a magazine at least once. They had to go quite a bit faster than 65 MPH to get a 10 mph error, though. Are you sure your sister's car has the right size tires?

 

[Kestas]

SAE standards (#J1226-1983-02) require manufacturers to build and install their speedometers to read within +3% and -1% of actual speed.

If your car isn't reading withing those limits, you have a warranty claim for your vehicle.

I had a new car where it was obviously WAY off. I got out of a speeding ticket by showing that warranty work was done afterward to correct the speedometer.

 

[willix]

The electro mechanical anolog speedo in the older Cadillac I drive seems to read +1 mph when above 45mph. I have a driver info center that I can hit a button and get a digital average speed read out every time you reset, it displays the current mph at that moment. That display seems to be very accurate. I use this to set my cruise 0.5 to 0.9 mph over the speed limit.

 

[Gary Allan]

Originally Posted By: ARCOgraphite
Originally Posted By: crazyoildude
do you own a Honda?
Also some gps units are extremely accurate
You beat me to the Honda thing. A great way for a manufacturer to boost its fuel milage! Then again, I would assume the feds test dont utilise the car speedo in its CAFE test though, so it would only be a customer perceived false gain. Sneeky!


Yes, but as XS650 related, the odo and speedo aren't integrated. You can have the speedo read whatever you want with the right scan tool and the OEM proprietary software. The odo is based on 8000 pulses per mile (at least on my jeeps and iirc).

I still have speedo gears before the VSS. Both jeeps have taller than stock tires. My wife's and my jeep read within a few 1/10's over her normal 50 mile commute. Her speedo, however, is off the reservation above 65mph. I would need to see a dealer and have it dialed with the DRB-II scan tool.

 

[cchase]

Originally Posted By: d00df00d
Originally Posted By: cchase
BMW's spec for speedo accuracy is apparently quite loose.

I've heard this as well. Mine seems to be pretty good but I know I saw this discussed in a magazine at least once. They had to go quite a bit faster than 65 MPH to get a 10 mph error, though. Are you sure your sister's car has the right size tires?


The car is on OEM wheels and tires. After she discovered how far it was out she brought it back to the dealer, they acknowledged that it was off and re-programmed the computer which improved it to the point where it was off 6-8 mph off at 65 and consistently off, which was better than before when it could register multiple values at a constant speed. She brought it back again to say that it was still off quite a bit and they said it was within the spec BMW sets forth which is some percent of reading plus/minus x mph. Her solution was to buy a GPS and mount it in the corner of the windshield with the speed displayed.

I drive the car frequently and never even look at the speedometer anymore because it's so annoyingly innacurate.

My cousin has a 2009 135i and his also appears to be off 5-8 mph at 65 mph (also reads faster than actual like my sisters).

 

[cchase]

Originally Posted By: ARCOgraphite
Originally Posted By: crazyoildude
do you own a Honda?
Also some gps units are extremely accurate
You beat me to the Honda thing. A great way for a manufacturer to boost its fuel milage! Then again, I would assume the feds test dont utilise the car speedo in its CAFE test though, so it would only be a customer perceived false gain. Sneeky!

Hondas problem with odometer optimism was also well-documented, however. I believe there was even a lawsuit which resulted in Honda extending warranties to 39,000 miles instead of 36,000 miles.

 

[Touring5]

I see about 4-5 mph error on the speedo at 60 mph. One way to check is to run a constant speed (as indicated by the speedo), then bring up the AVG. SPEED. display on the OBC - then reset it and watch it update (this will be an accurate speed as it uses the same pulses as used for the odometer).

 

[mechtech2]

Fresh tires vs. old tires makes a difference.
Car speedos are pretty good. Remember that the speedo and odometer are not tied together. The odometer is often more correct than the speedo.
Where you see a lot of 'fudging' is with motorcycles speedos. Seems they are always high!

 

[calvinnnnnnnnn]

On my 01 integra with OEM TIRE SIZE, at 70mph, my speedo reads 73-74

What i also noticed was the ODOMETER reading slower than the GPS.
At 30 Miles on the GPS, my ODO read like 27.7

My 06 tsx is DEAD on with my GPS all the way to 90(fastest i've tested)
GPS is a garmin 255w

 

[cchase]

Originally Posted By: mechtech2
Fresh tires vs. old tires makes a difference.
Car speedos are pretty good. Remember that the speedo and odometer are not tied together. The odometer is often more correct than the speedo.
Where you see a lot of 'fudging' is with motorcycles speedos. Seems they are always high!


Some car speedos are pretty good. Others are not. American cars follow the SAE standards which are a lot tighter than the European standards.

Fresh tires vs old tires probably don't make a noticeable difference unless you're running some very deep treads.

 

[ThirdeYe]

Originally Posted By: calvinnnnnnnnn
On my 01 integra with OEM TIRE SIZE, at 70mph, my speedo reads 73-74

What i also noticed was the ODOMETER reading slower than the GPS.
At 30 Miles on the GPS, my ODO read like 27.7

My 06 tsx is DEAD on with my GPS all the way to 90(fastest i've tested)
GPS is a garmin 255w


Well then it might not be due to my smaller tire size after all, my speedometer is exactly the same as yours in your teg. Mine was verified with a Garmin 285w and another Garmin that my girlfriend's dad let us use. I've noticed discrepancies with the odometer though, I know the route from one of the campus buildings I've been going to for 5 years is exactly 10 miles (round-trip) and in my old Nissan it would be 10 miles on the dot. In this car, it's also 10 miles on the dot so I know the odometer is correct, it's just the speedometer that's kind of wacky.

 

[cchase]

Just for reference some really quick and dirty math says that on a standard-ish car tire, losing an inch in diameter is going to have a 3% impact on speedometer reading.

 

[chevrofreak]

Originally Posted By: mechtech2
Fresh tires vs. old tires makes a difference.
Car speedos are pretty good. Remember that the speedo and odometer are not tied together. The odometer is often more correct than the speedo.
Where you see a lot of 'fudging' is with motorcycles speedos. Seems they are always high!


Both read about 3% lower on my moms Subaru. The in-dash odometer will say something like 327 miles, and the Scangauge will say 316.

In my car, my in-dash odometer, my Scangauge II, and my GPS will all be within 2 miles of each other with 400 miles on the tank.

I may experiment one of these days by loaning her my GPS for a month or so. The cigarette lighter in her car shuts off with the key, so it wouldn't be any kind of hassle for her. That way I could see just how close all 3 odometers are.

 

[crazyoildude]

your 96 vic has a calibrated speedometer if it was a police intercepter

 

[crazyoildude]

there was a class action lawsuit

 

[chevrofreak]

Yes. It's very accurate.