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MPH off?

C

csvt2200

Veteran CEG'er
Joined
Mar 21, 2007
Messages
511
Location
Lawrence, KS
So a buddy of mine and I went on a lil road trip today and he wanted to bring along his portable GPS. Well the GPS he has tells you how fast your going. it was dead on in town but cruising 80mph on the interstate, the GPS said I was only doing 75mph. Well in his 06 Grand Prix GT it's dead on no matter what. So I just figured his car being 7 years newer it's probably correct that mines 5mph off at that speed. Not that it's a bad thing really, since i'll be less likely to get pulled over cruising 80 on the interstate...:laugh: But is anyone else's CSVT off like this with factory wheels?
 
I've used my GPS in 2 cars they have been off my 3 mph. At hwy cruising if it indicated you were doing 80 it was really 77.
 
Don't most auto makers rate or test the speedometers up to 60 or 65 mph? I read that in most cars, the faster you go, past 60 mph, the more inaccurate the speedometer is going to be.
 
the inaccuracy is percent based, so if its 2 mph off at 50 it should be 4 off at 100mph.

Its inaccurate for a couple of reasons, namely tread wear and tire inflation being the big two. Also the speedo's aren't calibrated to be exact from the factory because of the above two reasons (and its more expensive to calibrate them to be more exact.)
 
Whenever I have to drive between St. Louis and Detroit on business I like to play a game with the mileage markers to keep me busy for an hour or so. I set the cruise on exactly 70 mph and use the stopwatch on my wristwatch and the right A-post to count off exactly 5 miles. When the A-post reaches the specific mile marker I start or stop the timer.

I always do 5 sets of these and I write down the results after each set. Counting off 5 miles instead of 1 mile reduces any errors in measuring. I've found that increasing the distance beyond 5 miles usually results in having to abort the run due to a slow moving vehicle in my lane or some other hazard.

After I get to the motel or back home I use a calculator to determine the actual speed. I've been doing this for years and the result always comes out to 68.9 mph. I figure 1.1 mph off isn't too bad. I'm using stock tires.

BTW, if anyone is using aftermarket size tires and wants to know how they affect the speedometer, there are several online calculators for that purpose. Here's one that I first learned about on CEG many years ago.

http://pw1.netcom.com/~sgalaba/tiresize.htm
 
catfish said:
the inaccuracy is percent based, so if its 2 mph off at 50 it should be 4 off at 100mph.
Click to expand...

Heh... back when my brother in law was still my sister's boyfriend, he would drive around in his parent's 1990 Oldsmobile Silhouette minivan.

The whole van had major issues, with the speedometer being one of them. The speedometer seemed to climb faster the faster you went (kind of like if the computer was squaring the actual speed)...

At 30 mph it would read over 70 mph!

No idea why, but the only guage that worked properly was the fuel guage (it was more than likely the only guage not computer controlled). The tach would randomly fall to 0 and then pop back, the other guages would seem to read normal at idle or when decelerating, but if you hit the gas, they would peg.

Not to mention other things like the door panels and the passenger door pin (thing that the latch grabs onto) falling off randomly...
 
When I have the factory size with 205/55/16 (mine is E0 for early 98), my speedometer and GPS (Garmin) showed the same speed. After I changed to E1 spec (215/50/16), as a smaller tire than the OEM, the speed is off a little bit. I found the real speed of 67 when the speedo says 70.
 
I'm not sure how accurate those signs are that tell you your speed, but I managed to find out how much I'm off (roughly) by running past one of them.
 
speed variance

speed variance

wuz also wondering about that. Tha otha day, i decided to max out ma car at 145mph according to tha speedometer. Ford says max speed is 143. i couldn't really have been doing 145, could I?
 
You'll have to take into consideration all these measuring devices have a margin of error.

Speedo +-3mph
GPS best accuracy on a level straight road at a constant speed. and at that its +-.5mph
Radar accuracy can vary +-3mph or more depending on the angle it hits you at.
Then of course there is human error on the install of your speedo someone
could have put the needle in wrong... ;)
 
Big Daddy Kane said:
1990 Oldsmobile Silhouette... No idea why, but the only guage that worked properly was the fuel guage (it was more than likely the only guage not computer controlled). The tach would randomly fall to 0 and then pop back, the other guages would seem to read normal at idle or when decelerating, but if you hit the gas, they would peg.
Click to expand...

No doubt the electrical work in some of those was a bit lacking. One of the worst I've seen was also from GM -- the S-10/Sonoma/Blazer/Jimmy dashes from the late Eighties and early Nineties. The ones with the hidden needle trick, where only a small portion of the needles are visible, giving the impression that they are moving through a slot or track. A buddy had a '91 Blazer in college. Oil pressure would read perilously low -- maybe 10 pounds or so if I remember correctly, once the engine reached operating temperature, even though there were no oiling issues. The "check gages" light was always lit, etc. Never did figure out why GM didn't spell it "gauges."

On a totally different note, what always fried my eggs was having a separate key for the ignition as well as the door. And unlike Ford, who provides a convenient two sided key for fumble free operation, the General didn't see fit to do this.
 
EricF said:
You'll have to take into consideration all these measuring devices have a margin of error.

Speedo +-3mph
GPS best accuracy on a level straight road at a constant speed. and at that its +-.5mph
Radar accuracy can vary +-3mph or more depending on the angle it hits you at.
Then of course there is human error on the install of your speedo someone
could have put the needle in wrong... ;)
Click to expand...

Close. In one of my classes I actually just did an analysis of a speedometer. The standard for American cars is +/- 4% error. If the max resolution of the speedometer is 5mph, a design stage uncertainty analysis reveals that at 60mph the speedometer could be off by up to +/- 3.46 mph.

In personal experience, a cop's radar read me at 85 when I had the cruise set right on 85.
 
i've gone by a lot of those speed detection devices that show your speed. Mine seems to be a consistent 2-3 mph off no matter what speed.
 
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