Speedo Calibration

[66gt350]

Ok, first for the disclaimer...this is not your typical speedo calibration question.

I've replaced my guages with all electric autometer guages, including the speedo:

To calibrate the speedo, you need to drive exactly 2 miles. I know that my truck's speedo is off due to tire size, so I used my work vehicle to get my 2 miles. Well, it's speedo is off. I timed it out on the freeway, and when it says that I'm doing 60, I'm really doing 62. I know not that much off, but I since I can calibrate it exactly why not. I don't want to do this on the freeway, since you have to come to a complete stop before and after.

For the life of me, the logic is escaping me at the moment and I'm drawing a blank. Do I need to drive a little further or drive a little less for the calibration.

thanks
rob

 

[AtlantaSteve]

If you're going 62 when your spedo reads 60, then your speedometer is slow by a factor of 3.322%

That means that for every mile your odometer says you have traveled, you have in fact traveled 1.0322 miles. So at two miles on the odo, you have traveled 2.0644 or 2 miles and 340 feet.

To accurately make your car travel exactly two miles, you would want the ODO to clock 1.934 miles, or 1 mile, 4931 feet. That is 349 feet SHORT of where your car says two miles.

Ergo, mark the start point on the road, and start your work vehicle there, Drive until it clocks exactly 2 miles and stop. Set your end point 349 feet SHORTER than that.


This will actually be 2 miles

Good luck!

Steve

 

[66gt350]

Thanks Steve,

That's exactly what I was looking for. For the life of me I couldn't wrap my head around the logic. I guess I haven't had enough coffee today. It's crap weather here today, I'll give her a try when it clears up.

Thanks

 

[AtlantaSteve]

Well, sit tight..I'm sure someone will be along shortly to point out a flaw in my math

 

[Sluggo]

There is a slight problem with the math. I think.....

You state to end 349 ft short.

I see how you arrived there but I think you are dealing with an exponential here. No?

349+349=698. So He would actually be 698 feet short of where the speedo says two miles.

That's almost an eighth of a mile. 12 feet shy to be precise.

I think he would want to stop 1/2 of 349 short (ie. 174.5) which would be 523.5 short of where the speedo says 2 miles.

 

[AtlantaSteve]

not following, Sluggo.

Here's where my calculations came from,

60/62 = 0.9677 : IOW his Speedomer shows 0.9677*ActualSpeed

Distance he wishes to travel:
2Miles
2Miles * 0.9677 = 1.9354 Miles

(I said 1.934 in my earlier post, which was a rounding error...I can explain how I got there but it's only a thousanth off, so I won't bother)

1.9354 miles is:
5280 feet + 4938.912 feet = 10219 feet

Two actual miles:
10560 feet

10560 - 10219 = 341 feet

Therefore he needs to travel 341 feet backwards after his car registers 2 miles on the clock.

341 vs 349 is the rounding error I discussed.

Why you would double or cut that in half isn't registering with me yet.

Steve

 

[Sluggo]

Not really sure why I am thinking that. I think that the 2 mile thing is making me want to further divide it.

I see it now with the breakdown. I'm going to agree with you. :vic

 

[daveSanborn]

To calibrate the speedo, you need to drive exactly 2 miles.

I'm not following....

And then what? Are there pin switches on the backside of the speedo that need to be moved in order to calibrate the unit?

 

[AtlantaSteve]

Dave, I've never done the calibration, but my understanding is that with a programmable Electric Autometer, you measure out a two mile distance, do some magic code on the trip/reset button to put it into calibration mode, go to the start of your 2 mile distance, push the trip/reset button once to start the sequence, drive exactly two miles, and stop, push the button again and it's calibrated.

it counts the number of pulses it gets over that two mile distance, so it knows how many pulses per mile to expect. During normal operation, it counts those pulses per second, and does the math to translate that into MPH.

I believe they use 2 miles instead of 1 mile to reduce the margin of error. If you're 100 feet over or under on a 1 mile sojourn, that's almost 2%, which means your speedo starts to show wrong even at 50MPH. over two miles it is reduced by half (of course) to .9% (9 tenths of one percent), which keeps you accurate almost up to 100MPH.

Steve

 

[66gt350]

Steve is exactly correct. Before starting the car, you have to hold down the trip/reset button. That puts the speedo into calibration mode. Drive to where the 2 miles start and then hit the trip button, and finally one last time at the end of the two mile mark. Then in theory you're speedo should be calibrated -- unless you use a vehicle with a speedo that's off for the two miles....like I did.