actual speed

[ncstreetrodder]

a quick question hopefully someone can answer. Long story short. Model A w/ 40s rear. Mechanical speedo. Couple months ago attached speedo cable. Checking mileage while on highway. For every mile on highway, odometer reads 9/10 mile. This seems to be pretty consistent. My question is how does this relate to the speed I am actually moving? If my speedo reads 50, how fast is it really? I didn't know if there was a way to equate the two.

[James Rogers]

Why not get a GPS and check your speed against it? Without knowing what your gear ratios are, it is probably not possible to give an accurate answer.

[green30coupe]

I agree with James. And if you have an iPhone use SPEED BOX. Its handier than a regular GPS because I have it on me all the time. (learned about it here on Ford Barn) There are other apps that register speed, but this one is exactly what I need.

Ed

[Clydes 31 P/U]

I'm using some brain mass that has lain dormant for some time but I'm thinking you're reading 10% slow so at 50 (on speedo) you are actually doing 55 at 35 (again on speedo) you are doing 38.5 ..........I think I'm sure someone will tell me if I'm wrong.

[Special Coupe Frank]

If the odometer is off, then the speedo drive is not correct for the tire size / rear you are running. The odometer is a direct-geared device, not a magnetic coupling as with the speedo drum / needle itself.

That said, if the odomoter is reading 10% "slow" ( or "short"), then I would expect the speedo to indicate similarly...

Suggest tracking the odometer over 10 measured miles for a better determination of the odometer error.

[Napa Skip]

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Odometer (correct) = Odometer (indicated)/0.9.

For example, if 1.0 miles actually traveled by the car results in 0.9 miles being indicated on the odometer, then using the above formula results in:

Odometer (correct) = 0.9 miles/0.9 = 1.0 miles. Similarly, 52.5 miles indicated = 52.5 miles/0.9 = 58.3 miles actual.

As stated above, while the odometer is direct drive, the speed indication comes from a rotating magnet acting on a spring-loaded speed dial. Thus, the faster the driveshaft turns, the faster the speedometer cable turns, the faster the magnet in the speedometer turns, the greater the influence of the spinning magnet on the spring-loaded speed dial, the more the speed dial is displaced, and the higher the indicated speed.

At least that’s the way it’s supposed to work.

So, PRESUMING there is a 1-to-1 correspondence between the “effective” magnetic field created at various speeds and the odometer reading (that is, presuming there is a linear relationship between the indirect-drive speed indication and the direct-drive odometer), then the above relationship should also suffice; that is:

Speed (correct) = Speed (indicated)/0.9.

Or, to use the example posed in the question:

Speed (correct) = 50 mph/0.9 = 55.6 mph.

However, I would be leery of presuming any linear relationship between the odometer and speedometer. As an example, if the speed dial spring were broken or the speed dial shaft were frozen in its pivots, then there would be no linear relationship between the indicated speed and indicated/actual distance traveled (unless you consider "whatever speed/distance = 0 mph or 0 miles" to be linear).

So, absent the very handy (and exceedingly accurate) GPS solution suggested above, I would head back to the road with a stop watch and – maintaining constant speeds such as 25 mph, 40 mph and 55 mph (for example) – compare the indicated and actual speeds for set distances (you can use the roadside mileage markers).

Then you could, if desired, create a table of indicated vs. actual speeds. This is what I did for the otherwise-nicely-operating speedometer in my 1930 coupe, having found that the actual speed was about 2 mph slower at 20 mph, up to 3 mph slower at 55 mph (which in and of itself, points to some non-linearity in the indicated speed).

Of course, all this should be done with the assistance of your faithful companion and not attempted (as some of us may be inclined to do) by yourself. Besides the obvious advantage of accuracy, there are some of us who find it difficult to stay in the center of our lane at a safe distance from surrounding traffic in the normal course of driving, much less while attempting to maintain a constant speed, operate a stopwatch, record the speedometer readings and keep track of the roadway mileage markers.

Ask me how I know…

[HoarseWhisperer]

Originally Posted by green30coupe

I agree with James. And if you have an iPhone use SPEED BOX. Its handier than a regular GPS because I have it on me all the time. (learned about it here on Ford Barn) There are other apps that register speed, but this one is exactly what I need.

Ed

Good suggestion. I use GPS and an IPhone app called "Speedometer" for checking accuracy. Works well.

[BrianBurkert]

It seems to me that because the odometer is gear driven and the speedometer is magnet-driven, and adjustable by variable tension on a spring, you cannot assume that if the odometer is 10% off that the speedometer is 10% off. GPS is the way to go.

For example, if I pulled the speedometer out of your car and excessively "tightened" the spring tension (working against the magnet), your speed would falsely read much slower, but your odometer would still read 10% short.

[Fred K-OR]

Originally Posted by BrianBurkert

It seems to me that because the odometer is gear driven and the speedometer is magnet-driven, and adjustable by variable tension on a spring, you cannot assume that if the odometer is 10% off that the speedometer is 10% off. GPS is the way to go.

For example, if I pulled the speedometer out of your car and excessively "tightened" the spring tension (working against the magnet), your speed would falsely read much slower, but your odometer would still read 10% short.

I agree with Brian. I had the same problem as he talks about. My odometer was almost right on but the speed was off by 10 or so miles at 40-50MPH. Thus had to have the spring tension adjusted.

[MCHinson]

And if you have an android phone, there is a free app called "Speed" that will enable you to find your speed.

[wtrueman]

Newbi here: Hi all: This is what I did 40 years ago with a wild motor in a small car. Find a cop checking speed. Talk to him, her. Make a pass at what you think is, let's say 40 mph. Do it twice. Are the results the same? The cop was not busy at the time and actually helped me "dial in" an honest 60 mph speed. I then was able to use the tach or speedo to make sure I was legal. My .02, Wayne. BTW, I have my A speedo hooked up to a 350 auto trans, 3 speed and it is right on.

[okiegreaser]

For the problem of trying to figure out how fast you're going, I think some of you guys are really over complicating it! The easiest way to figure out how fast you are actually going (using math if you don't have GPS) is to time how fast you cover a measured mile in seconds at a constant speed. Then take 3600 divided by that number (in seconds) and that gives you your speed.

For example:
If you cover a mile in 75 seconds 3600/75= 48 mph

3600 represents how many seconds in a hour and X is how many seconds in the mile
Seconds per hour/ seconds per mile= miles per hour

An old farmer that was my engine rebuilding instructor in college taught me that little trick!

[Napa Skip]

Or create something like this (from "Speedometer Calibration or 'How fast is this thing going?'" by Wiley Higgins, pages 8-9, "The Restorer" Volume 35 Issue 5, January/February 1991).

MPH table.jpg

[mccsix]

Or have your wife set her car at fifty and read what yours says.

[Kurt in NJ]

I drove without a speedo for many years driving the MG ----if you pass 1car for every 3 that pass you you are driving a safe speed

Say you are reading 10% low, the gear in the turtle is about 20 teeth,--10% smaller-- find one with 2 less teeth

[Napa Skip]

Well, as long as we’re coming up with methods to “calibrate” your speedometer, there is another possibility: use the portable digital-readout speed monitor devices occasionally placed along the road side to indicate how fast you are driving.

I’ve found these to be fairly accurate so all you need is to find one and drive past at various speeds. Finding one may be problematic however, although here in California, where – in my experience – most drivers interpret the speed limit signs as being the MINIMUM speed allowed (and where an equal number of drivers seem to view the center dividing line as nothing more than assigning responsibility for running whatever pedestrian is bold enough to step into the crosswalk back onto the sidewalk) they seem to be used frequently, generally just before a speed-enforcement action or just after a particularly bad accident.