|
Sponsored Links (Register now to hide all advertisements) |
|
|
Thread Tools | Display Modes |
07-19-2015, 05:10 PM | #1 |
Senior Member
Join Date: Feb 2015
Posts: 130
|
Speedometer Accuracy Check
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?
|
07-19-2015, 05:54 PM | #2 |
Senior Member
Join Date: Dec 2010
Location: FRESNO, CA
Posts: 12,560
|
Re: Speedometer Accuracy Check
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.
__________________
"THE ASSISTANT GURU OF STUFF" Last edited by BILL WILLIAMSON; 07-19-2015 at 06:17 PM. |
Sponsored Links (Register now to hide all advertisements) |
|
07-19-2015, 06:10 PM | #3 |
Senior Member
Join Date: Sep 2010
Posts: 492
|
Re: Speedometer Accuracy Check
Today's tires are a bit taller than back then. It can easily be a 2-3 MPH difference.
|
07-19-2015, 06:20 PM | #4 |
Senior Member
Join Date: Feb 2015
Posts: 130
|
Re: Speedometer Accuracy Check
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!
|
07-19-2015, 06:29 PM | #5 |
Senior Member
Join Date: Apr 2015
Location: NH
Posts: 182
|
Re: Speedometer Accuracy Check
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.
|
07-19-2015, 06:34 PM | #6 |
Senior Member
Join Date: Feb 2015
Posts: 130
|
Re: Speedometer Accuracy Check
Sponsored Links (Register now to hide all advertisements)
|
07-19-2015, 06:37 PM | #7 |
Senior Member
Join Date: Feb 2015
Posts: 130
|
Re: Speedometer Accuracy Check
|
07-19-2015, 06:43 PM | #8 |
Senior Member
Join Date: Sep 2010
Location: South Coast NSW Australia
Posts: 2,596
|
Re: Speedometer Accuracy Check
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 Last edited by pooch; 07-19-2015 at 06:48 PM. |
07-19-2015, 06:50 PM | #9 |
Senior Member
Join Date: May 2010
Location: South East NJ
Posts: 3,398
|
Re: Speedometer Accuracy Check
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. |
07-19-2015, 07:12 PM | #10 |
Senior Member
Join Date: Feb 2015
Posts: 130
|
Re: Speedometer Accuracy Check
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.
|
07-19-2015, 07:25 PM | #11 | |
Senior Member
Join Date: Feb 2015
Posts: 130
|
Re: Speedometer Accuracy Check
Quote:
|
|
07-19-2015, 07:58 PM | #12 |
Senior Member
Join Date: Dec 2010
Location: FRESNO, CA
Posts: 12,560
|
Re: Speedometer Accuracy Check
Probably, Model Aers, RARELY get speeding TICKETS.
Bill W.
__________________
"THE ASSISTANT GURU OF STUFF" |
07-19-2015, 08:43 PM | #13 | |
Senior Member
Join Date: Dec 2010
Location: FRESNO, CA
Posts: 12,560
|
Re: Speedometer Accuracy Check
Quote:
Bill W.
__________________
"THE ASSISTANT GURU OF STUFF" |
|
07-19-2015, 08:43 PM | #14 |
Senior Member
Join Date: Oct 2012
Location: Sonora desert, Arizona
Posts: 290
|
Re: Speedometer Accuracy Check
the GPS operates in 3 dimensions when interrogating 4 or more satellites. with only 3 satellites they can only operate in 2 dimensions.
__________________
______________ Nothing Short of Right is Right |
07-19-2015, 08:56 PM | #15 |
Senior Member
Join Date: Jan 2012
Location: Phoenix, Oregon
Posts: 661
|
Re: Speedometer Accuracy Check
Man my differential howls at 27mph yet My speedometer says 25mph.
I've got this thing....
__________________
Mike Stitt "A business that make nothing but money is a poor business." -Henry Ford |
07-19-2015, 09:09 PM | #16 | |
Senior Member
Join Date: May 2010
Location: Central, IL
Posts: 3,968
|
Re: Speedometer Accuracy Check
Quote:
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.
__________________
1929 Model AA - Need long splash aprons! |
|
07-19-2015, 10:07 PM | #17 |
Senior Member
Join Date: Dec 2014
Location: Shawnee, Ok
Posts: 3,471
|
Re: Speedometer Accuracy Check
Did the oven trick on my carb today, that seems to work.
Opps...wrong post.
__________________
Keith Shawnee OK '31 SW 160-B |
07-19-2015, 10:23 PM | #18 |
Senior Member
Join Date: May 2010
Location: Mpls, MN
Posts: 27,582
|
Re: Speedometer Accuracy Check
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.
|
07-19-2015, 10:48 PM | #19 |
Senior Member
Join Date: May 2010
Location: Ogden Utah
Posts: 242
|
Re: Speedometer Accuracy Check
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.
|
07-19-2015, 11:10 PM | #20 | |
Senior Member
Join Date: Apr 2014
Posts: 709
|
Re: Speedometer Accuracy Check
Quote:
|
|
|
|
Sponsored Links (Register now to hide all advertisements) |
|