Testing Speedometer?

[PJC]

Ok, I have gotten 2 speeding tickets in 3 months in my '92 EX. While I usually drive the speed limit ± 5 mph, one ticket was for 20 mph over and the other was for 15. I am absolutely positive that I was not that far over according to the speedo, so I am at a loss to explain it unless the speedomenter is off. I am running stock tires and the truck never has had oversize tires on it, so I am pretty sure the speedo gearing is stock as well.

So, the question is how does one check the accuracy of the speedometer in an accurate fashion? (Ie. I thought about timing between mile markers on the Interstate, but there are a number of inaccuracies that can creep in.)

 

[tgertz77]

get a friend... have them dirve next to you, talk to them on the phone, call out speeds.

 

[Chew_12]

Did you ever notice you going quite a bit faster than the other cars?

 

[MrShorty]

Best two ways I've used to check my speedometer:

GPS

Timing between mile markers on interstate. Not sure what inaccuracies you're afraid of, but as with any other experiment, a certain amount of care needs to go into getting an accurate measurement. If you have a decent stopwatch, and can maintain a constant speed over a couple of miles, you should be able to get a pretty good feel for how accurate your speedometer is.

 

[MalcolmV8]

Actually the mile marker test on the interstate is a pretty standard test every speedo / driveline shop I've ever spoken too seems to use. You'd be surprised how accurate they are. The so called standard test is to drive 10 miles and watch your odometer vs the mile markers. This is easiest when traveling north as the numbers will be incrementing where as traveling south the mile markers are decreasing. You can still do it, just makes your brain work twice as hard.

When I was in the early stages of putting a v8 into my 94 Ranger I had a 4r70w & AWD transfer case with no cable speedo options yet my 94 Ranger had a mechanical speedometer. I decided to try a product called Cable X (don't waste your money). It's an electric motor that spins your speedo cable based on the VSS. You can fine tune it via 12 dip switches to calibrate your mechanical speedometer. You can see more details of my experience with it over here
http://www.v8-ranger.com/blue/cablex/cablex.php

Anyhow I ended up ditching the Cable X box and swapping out the back cover and rear output shaft of my transfer case to get the speedometer cable options.

If you're seeing as much as 20mph off you could do a simple test of driving down the road with someone and get on a cell phone or walkie talkies with them and see what speed each of you are doing at various speeds. I say check at various speeds because if your speedo is off it's not linear. If say at 35mph you only read 30mph you think oh no big deal, only 5mph. Well check again when they are doing 70mph, yours will say something like 55 or what ever. The amount your speedo is off increases with speed.

If that checks out then do the 10 mile test for fine tuning. For example both my 94 Ranger and my wife's 04 Explorer will both read 70mph on the interstate when driving side by side. However there is a very slight difference because after 10 miles our odometers are 1/10th of a mile off. By the time we've driven 100 miles there's a whole 1 mile difference between our odometers.

Ohh I should stop now sorry for the long post but I've spent many hours playing with speedo options and tests.

Take care
Malcolm

 

[gijoecam]

If you're seeing as much as 20mph off you could do a simple test of driving down the road with someone and get on a cell phone or walkie talkies with them and see what speed each of you are doing at various speeds. I say check at various speeds because if your speedo is off it's not linear. If say at 35mph you only read 30mph you think oh no big deal, only 5mph. Well check again when they are doing 70mph, yours will say something like 55 or what ever. The amount your speedo is off increases with speed.

Actually, this statement is half-correct.... yes, the amount your speedo is off changes as the speed increases, but it IS a linear relationship. If you're off 5 mph at 35, you'll be off 10 mph at 70, and 15 mph at 105. Your speedo will be a fixed percentage off at any speed:

5mph/35mph *100% = 14.29%
10mph/70mph *100% = 14.29%
15mph/105mph * 100% = 14.29%

Driving 10 miles down the freeway is a good way to get the percentage error because the distance will also be off the EXACT same percentage that the speedometer is off. Therefore, if your speedometer is off by 15%, and you drive 10 mile markers on the freeway, your odometer will read 8.5 miles traveled (or 11.5 miles depending on whether the speedo reads slow or fast).

Does that make sense?

The advantage of checking against a fixed distance is that the distance error is not dependent on the speed at which you travel that distance.... you can drive 10 miles at 100mph, or 10 miles in rush hour traffic, and the distance will be off the same amount.

As was mentioned above, a GPS is, by far, the easiest way.

FWIW, both my Explorer and my F-150 speedos read 2mph faster than I am actually traveling at all speeds. The odometers are dead-on, so the error there is in the needle position on the shaft, not the electronics.

-Joe

 

[PJC]

No, I haven't noticed that I am travelling faster than traffic around me. I generally drive with the flow of traffic and don't zoom in and out to get around cars. Sometimes traffic is flowing a bit faster than the speed limit and I WILL drive with the flow, but usually it is not flowing THAT much faster.

Some of the inaccuracies that I am concerned with regarding using mile markers are 1)starting and stopping the stopwatch at exactly the same spot in relation to the mile marker and 2)how precise are the placement of the mile markers? Yards? Feet? Inches? Fractions of an inch?

I can see how some of the possible inaccuracies might be minimized by driving a longer distance allowing the per mile time to be evened out some. But, I would like to test different speeds. Travelling 35 MPH on an interstate is probably a recipe for disaster.

I hadn't thought about a GPS and have never used one. Does it keep track of actual distance travelled or only distance from the starting point? Ie, if I travelled 500 laps at the Indianapolis Speedway, would the GPS show 500 miles or merely the distance I am currently sitting from the pole? Also can GPS's be rented? Not sure I would want to buy one just to check the speedometer.

 

[Blee1099]

I use my GPS. I have a USB receiver mounted on the roof of the truck.

 

[MalcolmV8]

But, I would like to test different speeds. Travelling 35 MPH on an interstate is probably a recipe for disaster.

Like gijoecam said speed at which you complete the 10 mile test is irrelevant. 10 miles will turn your speedometer cable over X number of times. Whether you drive at 120mph or 15mph makes no difference. The cable will still turn over the exact same number of times per 10 miles. Then based on how different your odometer reads compare to the mile markers you can tell how off your speedometer is.

 

[gijoecam]

If you were closer, I'd let you borrow mine. Maybe MattAdams will go for a ride with you and bring along his GPS.

Again, the speed traveled is irrelevant. The milage markers are a lot closer than you'll be able to estimate with your odometer. Your odometer is only marked in 10ths. As you're driving down the freeway, reset the trip odometer as you pass a mile marker. Drive 10 mile markers and look at the odometer. The odometer will only be readable in 1/10 increments, so your reading will be plus or minus 1%.... close enough in my book.

The speed makes no difference. Drive the 10 mile markers and look at the odometer. If the odometer reads less than 10 miles, your speedo is slow. If the odometer reads more than 10 miles, it's fast.

Let us know what it reads and we can help figure out what's wrong (if anything)

-Joe

 

[gijoecam]

Oh, and testing at different speeds is unnecessary. The speedo will be off the same percentage regardless of the speed. Again, if you're off 5 mph at 35, you'll be off 10 at 70, and 15 at 105. It's a fixed percentage of any speed.

-Joe

 

[351wExplorer]

nobody has stated the most obvoius yet, find a police radar gun stand thing on the side of the road and make a few passes, those are the best for finding top speeds of quads dirtbikes and gocarts, the po po used to put one right in front of my old house all the time

 

[gijoecam]

If you live in a small town where the police don't have a lot to do, you might approach an officer when he's not busy and tell him some BS story about how your friend told you your speedo was off, but you don't think it is, could he check it for you with his radar gun, blah-blah-blah.... I know people that have done that and like I said, if it's a small town and business is slow, they just might.

Those radar stands are nice too... They have always confirmed my GPS readings.

-Joe

 

[mattadams]

I live in denver, if you want we can use my GPS to test your speedometer. It's pretty dang accurate.

 

[IBJEDI]

If it were me,I would do some quick research,challenge the speeding ticket. I even had a cop say that he paced me in court......I asked him how he could pace me when he was actually only behind me for 5 seconds at most and how he could do this while he was accelerating and I was slowing down????? Case Dismissed....NEXT!
Also I would print out what ever else your research turns up.
I would also consider printing this thread as part of your "independent research" as well.(Minus my input of course).
Good luck in finding the answer and in court.

 

[PJC]

BTW, I haven't forgotten about this. I have had some issues with ageing parents in the past few weeks and haven't gotten to doing any further testing on this (in fact, I am not even currently in the same state as the truck is).

 

[PJC]

Ok, I finally got around to doing something with this. I had to take the truck on a ~1000 mile road trip. While I was at it, I expanded the test and timed it for 100 miles. At the end of 100 miles of interstate markers, the odometer read 94.15 (it was in between .1 and .2). I assume that I am off by 5.85%. Is that a large error? WRT speed, does that mean I am reading fast or slow?

 

[SkanlaxJMO]

I use GPS and i found that it reeds 3 or 4 under what my speedo says
on most cars speddometers are made to read slightly higher to avoid accidents. It is usually 2 or 3 mph

 

[MalcolmV8]

I had to take the truck on a ~1000 mile road trip. While I was at it, I expanded the test and timed it for 100 miles. At the end of 100 miles of interstate markers, the odometer read 94.15 (it was in between .1 and .2). I assume that I am off by 5.85%. Is that a large error?

Seems a little bit more off than usual. When I do the 100 mile test my odometer reads 99 miles when I hit the 100 mile marker.

 

[Bill Kemp]

Means your going about 66 to 67 when the speedometer says 70.
I have always just used my watch to see if the speedometer is off. I keep the speed at 60 and it should take one min. to go one mile. If it takes less than a min. your speedometer is fast , more than a min. its slow. I do this for about 5 mi. on the Expway useing the mile markers.