Speedo off 5 MPH ?
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Fleet Owner
Joined: Jan 2004
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From: Brentwood,CA
Speedo off 5 MPH ?
I have a question I just can't seem to figure out in my head
. My speedo is off by 5 mph to slow in my 76 F250. Does a faster speedo reading affect how slow it reads or does it stay at a constant 5 mph slow or does the gap widen?Regardless of how fast the speedo reads. My last question I really have a hard time with, after a given set of miles driven Ex 100 miles how do i figure out what the true miles are? I have ordered some new drive gears for the speedo but i'm just curious because i have been driving it for about 3,000 miles and wanted to keep track of the new engine.Thanks!
. My speedo is off by 5 mph to slow in my 76 F250. Does a faster speedo reading affect how slow it reads or does it stay at a constant 5 mph slow or does the gap widen?Regardless of how fast the speedo reads. My last question I really have a hard time with, after a given set of miles driven Ex 100 miles how do i figure out what the true miles are? I have ordered some new drive gears for the speedo but i'm just curious because i have been driving it for about 3,000 miles and wanted to keep track of the new engine.Thanks!
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Joined: Sep 2001
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You don't mention the speed at which you have determined it is 5MPH off. It is a percentage, not a constant. 5MPH off at 60MPH would be 2.5MPH at 30MPH. Just figure out the percentage and multiply times the reading. Speedometers are set for 1000 revolutions of the cable per mile. 60 MPH would be 1000 revolutions per minute.
There is a lot of speedometer calibration etc info in the 73-9 FAQ thread.
There is a lot of speedometer calibration etc info in the 73-9 FAQ thread.
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Mine is off by about 10 mph at 60. The speedometer is old, but I also have different size tires than stock. Usually if I add 5 mph in town and 10 mph on the highway, it's about right. I have a GPS that has speed on the display, so I've just gotten used to using that as a reference.
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Fleet Owner
Joined: Jan 2004
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From: Brentwood,CA
Eric,
I have a 17 tooth gear in there now will the 16 tooh get me close to being dead on? If not what else can I do? Because as far as I remember 16 is the lowest of the gears made.
I have a 17 tooth gear in there now will the 16 tooh get me close to being dead on? If not what else can I do? Because as far as I remember 16 is the lowest of the gears made.
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Posting Legend
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That will get you about half way there or about a 6% change.
A speedometer shop can set up a ratio box that can get you as close to dead on as possible. You should be able to find a speedometer shop in your yellow pages somewhere around you. They can also tweak your speedo and clean or repair it as needed. It may be that your odometer is correct but your speedometer is tired.
A speedometer shop can set up a ratio box that can get you as close to dead on as possible. You should be able to find a speedometer shop in your yellow pages somewhere around you. They can also tweak your speedo and clean or repair it as needed. It may be that your odometer is correct but your speedometer is tired.
Last edited by Torque1st; Jun 7, 2007 at 11:50 PM.
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From: Maine
Originally Posted by ford390gashog
IMy last question I really have a hard time with, after a given set of miles driven Ex 100 miles how do i figure out what the true miles are?
So to calculate the true mileage on the truck, I take the current odo reading (237732) and subtract the mileage when the transmission was swapped and the speedo started reading high (221019) and get an indicated 16713 miles. Since the speedo is 18% high, I divide 16713/1.18 and get 14163.6 miles. Add that back to the 221019 and get a correct total mileage on the truck of 235182.6.
Posting Legend
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Originally Posted by ford390gashog
I contacted the nearest shop i could find about 2 hours away and to setup a ratio box and recalibrate the speedo would cost 700.00
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