new car speedometer off 4 mph

[1 FMF]

can someone run the numbers for me,
I want to know how off the odometer is since the speedometer is off. Speedometer is 4 mph fast-
if I'm doing 60mph on the speedometer, I'm really only doing 56mph. And it seems to be constant from 50 to 75 mph real speed. I've noticed it since day 1 with the car and verified it this weekend with gps.

What's worse is, correct me if i'm wrong, a fast speedo resulting in higher odometer readings would also result in higher calculated fuel economy than there really is? So if i'm calculating say 28 mpg highway, I'm getting less than that?

2005 nissan altima, 4-cyl.

if someone can get me drivetrain ratios, I can also use fbody.org/gears to dblcheck stuff. RPM's at 70mph speedometer were 2700 I think.

What do you think the odds are, with the SAE publications that come out recently claiming japanese cars are overrated for power (and economy maybe) that those automaker build the speedometer/odometer such that it is off just a little to increase fuel economy for marketing purposes? It's all electronic, so I can't imagine it would be hard to do. And my understanding at least on my camaro is it's just a magnetic pickup/pulse sensor off the driveshaft nowadays, which is accurate unless you change rear axle ratio or tire size.

 

[surfstar]

I'd first double check that your odo is off as well. Use that GPS again on a long-ish drive and see if your miles are significantly off.

I'm running slightly larger than stock tires and my speedo and odo are right on. I imagine they were off 1-3% before.

 

[brianl703]

The speedometer on my Mustang is not very accurate but the odometer is dead-on balls accurate (that's an industry term).

(I tested it by comparing the reading to mile markers on a 10-mile stretch of road).

 

[JohnBrowning]

GPS is not the way to check that speedo!!! GPS is not that accurate in ground track speed calculations when used alone.

 

[XS650]

quote:

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Originally posted by JohnBrowning:
GPS is not the way to check that speedo!!! GPS is not that accurate in ground track speed calculations when used alone.

---

GPS is extremely accurate in ground track speed.

If you don't believe it, stand still and read your GPS. If it says 0.0 mph, it's spot on.

There is the typical 1 second update time on low end GPS units, so it's not good for rapidly changing speeds.

 

[brianl703]

As I recall the cars with the digital dashboards updated once a second, maybe twice a second at most.

 

[Virtuoso]

In a lot of the car review articles I've read; they tested the speedo - IIRC almost all of them were off by a few mph over the test 'spectrum'.

 

[Gary Allan]

On my vehicles that have speedo error, it appears that the error is only a factor in speeds in the highway realm and always on the side of over reporting. Now sure any error will spread as you get higher in mph, but it seems that the mans always err on the side of over reporting the indicated vs. actual. I've never had it the other way around in a car that hasn't been modified (tires, etc.).

My SE was off 5 mph at highway speeds. I had to do 70 to follow all the big rigs that were governed to 65 mph. Remote radar units confirmed it. I just got a speedo gear with one more tooth on it to slow my speedo down (34 to a 35 tooth). I could have had the dealer mod the speedo output from the PCM with the DRBII scanner and leave the 8000 pulses per mile the same ..but that would have been $75 min ..while the speedo gear was $15

 

[Robbie Alexander]

quote:

---

Originally posted by XS650:

quote:

---

Originally posted by JohnBrowning:
GPS is not the way to check that speedo!!! GPS is not that accurate in ground track speed calculations when used alone.

---

GPS is extremely accurate in ground track speed.

If you don't believe it, stand still and read your GPS. If it says 0.0 mph, it's spot on.

There is the typical 1 second update time on low end GPS units, so it's not good for rapidly changing speeds.

---

depends on your unit, but many read better when going faster

I think many cars are flat and high in certain speed areas and the overall calibration is what counts... at least to be off on the low side

 

[Dark Jedi]

you can try to find those mobile radar trailers they setup along side the roads. I know in my town I can check it and see how far off my speedo is. and so far its dead on. if you know its off you could go to your local police and see if they could clock you and see. make sure they have the laser gun as thats the most accurate. if its off by that have him write it down for you. that way when you goto the dealership you have proof from local law enforcement that the speedo is off and dont try and blow smoke.

 

[Kestas]

Like brian, I too calibrate the odometer and speedometer of each car I own using the freeway mile markers. Results are averaged over 10 miles.

I once got out of a speeding ticket in a new car because I found that my speedomoeter was off. I got it fixed under warranty, showed the paperwork to the judge and it was case dismissed.

SAE manufacturing standards allow odometers and speedometers to be off between +3% and -1%.

Keep in mind other things can affect speedo/odo accuracy such as tire wear and inflation pressure.

 

[DriveHard]

the goverment requires that the odometer by almost perfectly accurate, but allows up to 10% (I think) for the speedo. This is common. I would put my money on the fact that the odometer will be almost perfect.

 

[JHZR2]

A GPS is 3-5% off anyway...

In the older MB and BMW cars, the speedometers were always off by ~10%.

JMH

 

[TR3-2001SE]

I hate to be the bearer of bad news but the milage markers on the side of the road are not accurate. In Virginia it shows my odometer as too slow over 10 miles. In North Carolina is shows too fast over 10 miles. I guess the guys that put up the mile markers have different standards.

And I always thought a mile was a standard measurement.

 

[brianl703]

I've noticed the occasional mile marker in the wrong place, but usually the next one is where it's supposed to be.

 

[T-Keith]

There was a good article about this in Carand driver once. The government controls how off a speedo can be. It can not be low so most manufactorers aim higher to give themselves some margin of error. American cars tend to be the most accurate, followed by Asian and then European cars. GM cars were the most accurate, BMWs the worst. There are also other factors such as your own judgement of the needle and the accuracy of the measuring device. It could be worse. I think the BMWs were like 10% off or more.

-T

 

[road_rascal]

Ever since the Ford dealer replaced the dash in my '01 Windstar my speedo has been showing lower than normal speeds. My Magellan GPS shows 75 mph while the speedo shows 71. I've checked the GPS against several radar trailers in the area and it has always been spot on (55 on GPS/55 on trailer, etc).

 

[mechtech]

The speedo and odometer may be OK, or either one can be off independently.
Check then separately.
The odometers are usually correct, but the speedos are still [usually] regulated by a clockspring, which can be adjusted for tension.
Hot and cold make a difference. Different speeds can be OK at one, and not at another.