Testing Speedometer?

[Aussie Explorer]

Can you get out of speeding fines in the U.S if your Speedo is out?

In Australia it makes no difference if you Speedo is out, that’s your problem and you will still have to pay the fine.

Those guns’s accuracy is debatable any way. In Victoria (State in Australia) they had lawsuits against the state government when this story made the news where someone was booked for doing 150k/ph in a 100k/ph zone or something in some old POS car. The media got a racecar driver to take this POS car for a few laps on a racetrack and they couldn’t get the speed to go above 120k/ph.

As a resalt this state had to pull every radar gun and recalibrate it, reinstate like 2000 drivers licences and pay back millions in revenue from fines. LMAO.

 

[gijoecam]

+/- 5% isn't really too bad, but that doesn't tell you how accurate the speedometer is, just how accurate the odometer is.

On mine, with OEM-sized tires on it, the speedo reads 2mph low across the board, but the odometer is within .5 mile per tank of gas according to the GPS. Ditto for my F-150... on an 800 mile trip, the odometer was off by about 5 miles even though the speedo read high by 2mph at any speed.... close enough for me.

Oh, and PJC, it means that your odometer runs a little bit slow. Assuming your odometer and speedometer are in synch (which mine are not) your speedo will read just a little bit slow: 95mph when you're actually doing 100mph, and will read 47.5 when you're doing 50 mph.

-Joe

 

[MrShorty]

Joe, The big difference, as far as this issue goes, between your '98 and PJC and my '92 is that your speedo/odo are electronically driven where ours are cable driven. For example, on my '92, when the speedo says I'm doing 70, I'm doing 70. When the speedo says I'm doing 35, I'm doing closer to 30. All this with an odometer (which is gear driven by the cable) that is only about 1% off (can't remember if it's low or high).

 

[gijoecam]

True re: the electronic versus mechanical setup, but the relationship between the speedo and wheel speed should still be a linear one. If your speedo is 5mph slow at 35, it should be 10mph slow at 70, 15mph slow at 105, and 0mph slow when stopped. Because of the way the signal is interpreted on the electronic ones, and converted in the back of the dash on a mechanical one, the speedo can still be off even if the odometer (which only measures the number of turns of the screw) is spot-on.

There are always exceptions, but the relationship of any mechanical system has to be linear. Start throwing electronics into it and all bets are off.

His odometer being a bit slow could be caused by his tires being *slightly* over-sized, or even slightly over-inflated. The rolling circumference is what will determine that ultimately. With some of the electronically controlled speedos, you can compensate in the computer that interprets the signal. Mechanically, you have to alter the gear in the trans that drives the cable to compensate.

-Joe

 

[PJC]

Ok, I are confoosed again.

(obviously an easy thing to do!)

If I am driving along with the speedometer dead on 70mph, how fast am I actually travelling?

 

[Bill Kemp]

If there is a direct relationship betwqeen the odometer and the speedometer your are going 5% slower than your speedometer registers. So speedometer says 70 you are going appx 66 mph.
However it has been postulated that there is NOT a direct relationship in the amount the odometer & speedometer differ.
So the answer is: We dont know. Dont have enough info.
Really the easiest way if you live near an Expwy w/mile markers & own a watch is to see how long it takes to go a mile at 60 mph. (a mile a minute) If it takes more than a minute your speedometer is slow. If it takes less than a minute your speedometer is fast. Adjust your speed to see what speed takes exactly a minute for a mile and let us know.

 

[gijoecam]

5.85% more than 70. (Can't do the math in my head.... roughly 73 I think...)

If'n he was 5.85 miles under 100 miles, and if we assume that the odometer is mechanically linked to the speedo, the speedo will be off by the same percentage, the same way. The odometer read slow, therefore the speedo will read slow.



-Joe

 

[ExplorerDMB]

when my speedo was reading slow my odometer was off. I fixed my speedo and now my odometer is back up. I use to get around 220 out of a tank when I had a slow speedo, but now I have fixed it and I get around 250/260 out of a tank. So, with that information, I would say the speedo and odometer are tied together

-Drew

 

[sandflea]

LOL, That's Hella Funny, Aussie Explorer... Thanks for Sharing, Got A Good Laugh!