My speedometer was off!
3rd Gear Member
Joined: Feb 2007
Posts: 523
From: Psychiatric Unit
RE: My speedometer was off!
Speed is a simpleequation of distance / time. The quicker in time you cover a given distance, the faster you are traveling. Also understand that 1 MPH = 1.466 feet per sec (fps) and there are 5280 feet ina mile.
So, 60 MPH is figured by taking 5280 feet and dividing it by the 60 seconds it would take to cover this distance. This equals 88 (fps) which you then divide by 1.466 to get 60 MPH.
Knowing this you can figure out a lot with a known distance and some type of timing device.
Do any of you have aircraft speed enforcement marks on the pavement in your area? They most always are a white stripe of a few feet in length somewhere across the lane of traffic. They usually are set up to be 1/4 mile from start of one to start of another. So, there will be five marks that define a surveyed mile start to finish.
Many of us then have the IUP in their Mustang which includes some fancy stuff in the menu. One such item is a nice timer.
If you want to see how accurate your speedo is, or how fast you are really going, simply set your cruise at 60 MPH before you get to the measured mile. Use that timer on the dash display and start it when you cross the first mark and stop it when you cross the last mark. Note the time. Lets say it took you 60 seconds. You now have all you need.
That measured mile is 5280 feet in length and it took you 60 seconds to cover it. You have the distance and you have the time. Now just plug it in.
5280 feet / 60 seconds is 88 .....88 ??!! What the heck!? You must understand that you are dividing FEET by SECONDS so your result is in FEET PER SECOND (fps). To convert FEET PER SECOND to MILES PER HOUR you just divide the fps by 1.466. So, 88 (fps) divided by 1.466 = 60! So, you averaged 60 MPH over that measured mile.
If it took you 50 seconds you are traveling 72 MPH (5280 / 50 = 105.6 (fps) / 1.466 = 72 MPH).
If it took you 70 seconds you are traveling 51 MPH (5280 / 70 = 74.4 (fps) / 1.466 = 51 MPH.
Although not as accurate, you can use the mile markers on the roadside as long as it is out in open country where they are most likely to place them where they should be and not have to account for streets, driveways or other obstructions. You might want to cover a distance of say 5 miles to get a better average because the longer a distance you travel the more accurate you can figure your average speed to. To use the above formula simply multiply the distance of 5280 by the number of miles traveled (lets say 5). This is how it would look.
5280 feetx 5 = 26,400 feet. Say it took you 300 seconds to cover this distance.
26,400 feet / 300 = 88 (FEET PER SECOND) / 1.466 = 60 MPH!
I think you get the point
So, 60 MPH is figured by taking 5280 feet and dividing it by the 60 seconds it would take to cover this distance. This equals 88 (fps) which you then divide by 1.466 to get 60 MPH.
Knowing this you can figure out a lot with a known distance and some type of timing device.
Do any of you have aircraft speed enforcement marks on the pavement in your area? They most always are a white stripe of a few feet in length somewhere across the lane of traffic. They usually are set up to be 1/4 mile from start of one to start of another. So, there will be five marks that define a surveyed mile start to finish.
Many of us then have the IUP in their Mustang which includes some fancy stuff in the menu. One such item is a nice timer.
If you want to see how accurate your speedo is, or how fast you are really going, simply set your cruise at 60 MPH before you get to the measured mile. Use that timer on the dash display and start it when you cross the first mark and stop it when you cross the last mark. Note the time. Lets say it took you 60 seconds. You now have all you need.
That measured mile is 5280 feet in length and it took you 60 seconds to cover it. You have the distance and you have the time. Now just plug it in.
5280 feet / 60 seconds is 88 .....88 ??!! What the heck!? You must understand that you are dividing FEET by SECONDS so your result is in FEET PER SECOND (fps). To convert FEET PER SECOND to MILES PER HOUR you just divide the fps by 1.466. So, 88 (fps) divided by 1.466 = 60! So, you averaged 60 MPH over that measured mile.
If it took you 50 seconds you are traveling 72 MPH (5280 / 50 = 105.6 (fps) / 1.466 = 72 MPH).
If it took you 70 seconds you are traveling 51 MPH (5280 / 70 = 74.4 (fps) / 1.466 = 51 MPH.
Although not as accurate, you can use the mile markers on the roadside as long as it is out in open country where they are most likely to place them where they should be and not have to account for streets, driveways or other obstructions. You might want to cover a distance of say 5 miles to get a better average because the longer a distance you travel the more accurate you can figure your average speed to. To use the above formula simply multiply the distance of 5280 by the number of miles traveled (lets say 5). This is how it would look.
5280 feetx 5 = 26,400 feet. Say it took you 300 seconds to cover this distance.
26,400 feet / 300 = 88 (FEET PER SECOND) / 1.466 = 60 MPH!
I think you get the point
s197 Junkie
Joined: Sep 2006
Posts: 16,088
From: IN
RE: My speedometer was off!
http://1010tires.com/TireSizeCalculator.asp
You can check revs per mile here. I am not sure about this, but i dont think that the tuner would send a default tire size to the computer....unless you specifically change that size, I would hope that it just reads factory info and leaves it as that number. Otherwise I think some people could really get screwed.
I also have seen those roadside speed measurements be off buy quite a bit.
You can check revs per mile here. I am not sure about this, but i dont think that the tuner would send a default tire size to the computer....unless you specifically change that size, I would hope that it just reads factory info and leaves it as that number. Otherwise I think some people could really get screwed.
I also have seen those roadside speed measurements be off buy quite a bit.
2nd Gear Member
Joined: May 2006
Posts: 225
From:
RE: My speedometer was off!
ORIGINAL: Orion_240
Maybe you need to put more air in the tires???
I've noticed that whenever I come across one of the machines that read your speed alongside the road that show you how fast you're going that I seem to be clocked at about 2 mph slower than my speedo shows.
Maybe you need to put more air in the tires???
I've noticed that whenever I come across one of the machines that read your speed alongside the road that show you how fast you're going that I seem to be clocked at about 2 mph slower than my speedo shows.
1st Gear Member
Joined: Jun 2006
Posts: 70
From:
RE: My speedometer was off!
Well, guess what. The speedometer is off from the factory. I auto-tapped my car just a bit ago and compared what the speedometer was reading vs what the computer was reporting and sure as heck, the speedometer reads a few MPH faster than even its own computer is reporting as the correct speed. In fact, it was right on to what I calculated it to be using a stop watch and mile markers on the highway. When the speedo reads 70 you are actually doing about 67 mph.
Thread Starter
3rd Gear Member
Joined: May 2006
Posts: 700
From:
RE: My speedometer was off!
ORIGINAL: cerino2000
Well, guess what. The speedometer is off from the factory. I auto-tapped my car just a bit ago and compared what the speedometer was reading vs what the computer was reporting and sure as heck, the speedometer reads a few MPH faster than even its own computer is reporting as the correct speed. In fact, it was right on to what I calculated it to be using a stop watch and mile markers on the highway. When the speedo reads 70 you are actually doing about 67 mph.
Well, guess what. The speedometer is off from the factory. I auto-tapped my car just a bit ago and compared what the speedometer was reading vs what the computer was reporting and sure as heck, the speedometer reads a few MPH faster than even its own computer is reporting as the correct speed. In fact, it was right on to what I calculated it to be using a stop watch and mile markers on the highway. When the speedo reads 70 you are actually doing about 67 mph.
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