Mpg calculation
Super Moderator
Joined: Oct 2005
Posts: 24,308
Likes: 42
From: SW Virginia
That's a spot on perfect post Ray. I don't know what you have to do on the '99s to change the speedometer calibration. Maybe just a flash....not sure. I know on my '89, when I went from the OEM 235/75-15 tires to the 33x12.50-15s, I had to change the little nylon gear in the side of the tranny that hooked the speedometer cable to the transmission. The OP probably needs to check with the folks in the 7.3 forum or at his local dealership. I know on the later trucks, the tire calibration can be corrected via the computer and I would assume it's the same for the '99 but not sure.
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
Ford started with the electronic speedometers in trucks in 1992, so there must be a way to re-program the PSOM in 1999 as well. Try searching the superduty forum, it must have been covered at some point.
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
I gotta see a pic of those. To my knowledge all '92 & up f-series have a digital odometer.
Thread Starter
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Elder User
Joined: Aug 2009
Posts: 583
Likes: 0
From: SOUTH TAMPA
this is not mine but heres what it looks like exactly
http://www.truckfindersinc.com/Tract...)/Odometer.JPG
http://www.truckfindersinc.com/Tract...)/Odometer.JPG
Postmaster
Joined: Nov 2006
Posts: 4,517
Likes: 3
From: SE Wyoming
They went back to analog when the changed both the F150 and SD bodies for a few years then back to digital again. If you have the analog you have to change the VSS but I doubt there is one the right size for those tires.
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Hotshot
Joined: Feb 2003
Posts: 11,537
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From: Long Beach, Ms.
Let me get this straight;
You are running on 42" diameter tires
Stock size being 235/85R16.
That makes stock diameter 31.7".
10" diameter difference.
42s have a circumference of 132".
32s have a circumference of 100".
Difference in circumference: 32" or 32%
So when your speedo reads 70mph, you're actually traveling at 92.4mph. (70 x 1.32 = 92.4)
That means that the 280 miles you measured is really 370 miles divide that by 30 and you get 12.3MPG. (280 x 1.32 = 369.6)
At 70mph a mile should take about 52 seconds. At 90mph a mile takes 40 seconds.
(The numbers used here have been rounded off but not by much.)
You are running on 42" diameter tires
Stock size being 235/85R16.
That makes stock diameter 31.7".
10" diameter difference.
42s have a circumference of 132".
32s have a circumference of 100".
Difference in circumference: 32" or 32%
So when your speedo reads 70mph, you're actually traveling at 92.4mph. (70 x 1.32 = 92.4)
That means that the 280 miles you measured is really 370 miles divide that by 30 and you get 12.3MPG. (280 x 1.32 = 369.6)
At 70mph a mile should take about 52 seconds. At 90mph a mile takes 40 seconds.
(The numbers used here have been rounded off but not by much.)
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
By following that formula you're making the assumption that your tires are actually 42" in diameter. Most oversize tires are actually smaller. Your 42" tires may be as small as 40". Measure them.