Speedometer Accuracy Check

Cracker39

Cracker39

Senior Member
I've been curious about the accuracy of my '31's speedometer so I took it for a ride using my Garmin GPS. My results were as follows: At 23 mph on the speedmoter, the Garmin read 25 mph; at 32 mph on the speedometer the Garmin read 35 mph; at 37 mph the Garmin read 40 mpg, and finally, at 42 mpr the Garmin read 45 mph. That said, it looks like the 3 mph error is fairly consistent above 30 mph. I wonder if the speedometer was any more accurate in 1931 than today?
 
I rather doubt it, Cracker. "Maybe" that's why Cops give us a little LEEWAY?
"Maybe old speedos were designed just to let a DRUNK know if he's MOVING or NOT??
Even today, we have lots of KOOKY reading GAS GAUGES.
Bill W.
 
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Ah heck Bill - what car could have a more accurate gas gage than a Model A? You can see the gas in the gauge window!
 
Don't know where you are located, but if the police in your area occasionally set up one of those 'speed trailers' which show your MPH, you can drive past it a couple times and cross reference your Garmin.
 
I wonder if the accuracy of a GPS differs going up and down a hill.

I would guess the satellites only refer to the moving object in a horizontal plane in reference to flat ground.

The positioning would be accurate but the speed maybe would not.

Maybe the satellites take up and down into their computations ...dunno
 
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I would say the speedos were fairly accurate when new. It is not hard to be make a reasonable speedo, even then.

The speedo works on eddie currents witha spinning magnet. So over time the speedo magnets will loose some force and start reading a little lower. If you had the right tools there is a little hole in the back of the speedo where you could change the gap in the magnet and get the speed a little closer to correct.
 
I couldn't help myself. I did a couple of quick and dirty calculations and it looks like it would take a 2" difference in diameter to make a difference of 3 mph between the old tires and todays tires. I just installed new tires and they measure roughly 29.5" in diameter. Using that figure, the old style tires would have measured 27.5" in diameter. Is there that big of a difference - or were my calculations too quick and dirty? Let the professional engineers jump in here and get us straightened out? I'm retired and I've already forgot everything I thought I knew.
 
pooch said:
I wonder if the accuracy of a GPS differs going up and down a hill.

I would guess the satellites only refer to the moving object in a horizontal plane in reference to flat ground.

The positioning would be accurate but the speed maybe would not.

Maybe the satellites take up and down into their computations ...dunno
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Damn - that's interesting! I never thought about that. I'll bet you're right about the horizontal plane. That said, it would mean that an airplane in a near vertical dive may only measure 2 or 3 miles per hour via the GPS, while the airspeed could be in excess of 500 mph (---ignoring the "knots" convention.) Perhaps another algorithm comes into play to account for the speed of descent? HELP! I don't want to go to sleep tonight thinking about this!
 
Cracker39 said:
Damn - that's interesting! I never thought about that. I'll bet you're right about the horizontal plane. That said, it would mean that an airplane in a near vertical dive may only measure 2 or 3 miles per hour via the GPS, while the airspeed could be in excess of 500 mph (---ignoring the "knots" convention.) Perhaps another algorithm comes into play to account for the speed of descent? HELP! I don't want to go to sleep tonight thinking about this!
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Well, let me help you, Cracker, go to sleep thinkin' about the INTERNATIONAL DATE LINE or WHITE PUFFY CLOUDS, shaped like a DOG:confused::D (works for me)
Bill W.
 
the GPS operates in 3 dimensions when interrogating 4 or more satellites. with only 3 satellites they can only operate in 2 dimensions.
 
monto said:
Don't know where you are located, but if the police in your area occasionally set up one of those 'speed trailers' which show your MPH, you can drive past it a couple times and cross reference your Garmin.
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I have found those things to be very inaccurate, we have a few permanently mounted ones that flash if your going 5 over the limit in a 50mph zone, 2 of my vehicles it cant read correctly and says im going anywhere from 37 to 42 when im really going 55 as verified by spedometer, cell phone GPS, vehicle GPS and hiking GPS...

if its raining it doesnt even register me half the time or im going 15...

the trailer type are a tad more accurate but still bounce around a hell of alot.

either way if your in 3mph i would not worry about it at all, if you do get a rookie cop that gives a ticket for 3 over, fight it and the judge will prolly throw it out due to the age of the car/spedometer/technology of model A years.
 
First thing is to make sure the odometer reads correctly, then check and adjust the speedometer. I've checked my modern car by using highway markers over a 10 miles stretch, then when I find that car is accurate to within 1% I mark to spots on a local road and see if the Model A hits the same two spots at one mile on the odometer. I also used highway markers to check my 28 Phaeton when I drove it to South Dakota 5 years ago, and it is very accurate.
 
I also think that any difference in a speedometer reading might be because the Gear ratio in the rear end is not matched correctly to the gears in the drive line which the speedometer cable attaches to.
 
SeaSlugs said:
I have found those things to be very inaccurate, we have a few permanently mounted ones that flash if your going 5 over the limit in a 50mph zone, 2 of my vehicles it cant read correctly and says im going anywhere from 37 to 42 when im really going 55 as verified by spedometer, cell phone GPS, vehicle GPS and hiking GPS...

if its raining it doesnt even register me half the time or im going 15...

the trailer type are a tad more accurate but still bounce around a hell of alot.

either way if your in 3mph i would not worry about it at all, if you do get a rookie cop that gives a ticket for 3 over, fight it and the judge will prolly throw it out due to the age of the car/spedometer/technology of model A years.
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If you can, check the construction and use regulations (or whatever they are called in your jurisdiction) for the date that your vehicle was manufactured. Years back I successfully dodged a big fine as the regulations at the time my motorcycle was manufactured called for a speedometer accuracy of +/- 10%. Therefore I could legally drive at 77 mph in a 70mph zone even though the speedometer of my 1954 BMW bike was spot on accurate.
 
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