How fast am I going?
Post Fiend
Joined: Jul 2005
Posts: 8,462
Likes: 4
From: Durant, OK (SOSU)
Depends on what rear end gearing it was originally set for, and if the gearing was changed with the tire change.
Look on your driver's door and decode the original axle ratio from this:
https://www.ford-trucks.com/article/...r_Decoder.html
If the 3.27 was stock and is still in there (and I'd HIGHLY suggest regearing, your effective ratio is 2.74):
Actual / Indicated
20 / 16.8
30 / 25.1
40 / 33.5
50 / 41.2
60 / 50.3
70 / 58.7
Look on your driver's door and decode the original axle ratio from this:
https://www.ford-trucks.com/article/...r_Decoder.html
If the 3.27 was stock and is still in there (and I'd HIGHLY suggest regearing, your effective ratio is 2.74):
Actual / Indicated
20 / 16.8
30 / 25.1
40 / 33.5
50 / 41.2
60 / 50.3
70 / 58.7
Senior User
Joined: Nov 2003
Posts: 478
Likes: 0
From: ut
Drag out your (or your buddy's) GPS and compare it's speed to your speedometer. I bought a 94 last summer and found the indicated speed was faster than the speed shown on the GPS. At the time it had 30X9.50R-15's. I put new 31X10.5R-15's on and the GPS speed and speedometer speed now match exactly. The nylon gear where the speedo cable goes into the tranny was color-coded yellow, which I think is stock, but I'm not sure.
Cargo Master
Joined: Feb 2002
Posts: 2,414
Likes: 22
From: Tucson USA
find a straight road for about 5-10 miles where you can run a constant 60 mph indicated, use a watch/stop watch to time how long it takes ya to go whatever distance you can and then calibrate your vehicle yourself. Do search for conversions MPH and it will have the ability to tell you your speed based on time and distance.
I changed the speedo gear in mine, made everything worse so went back to the stock one found I was 5mph slow indicating and off about 4 miles per 100 in distance. So 65 = 70, 100 miles = 104.
I later verified with a borrowed gps unit on the speedo.
I changed the speedo gear in mine, made everything worse so went back to the stock one found I was 5mph slow indicating and off about 4 miles per 100 in distance. So 65 = 70, 100 miles = 104.
I later verified with a borrowed gps unit on the speedo.
Cargo Master
Joined: Feb 2002
Posts: 2,414
Likes: 22
From: Tucson USA
Elder User
Joined: Jan 2004
Posts: 537
Likes: 0
From: Yes
Doesn't FTE have a tire change calculator? If not, TRS does, you can use theirs. You enter the SAE measurements of your stock tires, then your new ones, and a given indicated speed, and it will calculate the actual speed you are travelling. It's useless if you've changed your gearing as well though.
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