Speedometer vs. odometer accuracy
Fleet Mechanic
Joined: Dec 2007
Posts: 1,512
Likes: 340
From: Wildwood Pa
Not necessarily,
The odometer is directly driven off of the speedo cable, which is directly driven off of the transmission.
The speedo is not directly driven thru the cable. The speedo relies on a magnet that is driven by the speedo cable. The magnet revolves around a disc, the disc is not directly attached to the cable or magnet. As the magnet revolves around the disc, it moves the disc, & the needle. The faster the magnet moves, the further the disc moves. But, if the magnet is weak, or if the bearings for the disc & needle are worn or lacking lube, it is possible for the needle to not move as far as it should. And in that case the speedo would read slow, or may not work at all. So, the odometer would read one way, the speedo would read another. I hope this makes sense.
I've tested dozens of speedo & odometers, I use a 1000 RPM drill to spin them. Most speedo's are designed to read 60 MPH at 1000 RPM, most do, but some are + or - 5 MPH. Usually 5 MPH or so slow.
The odometer is directly driven off of the speedo cable, which is directly driven off of the transmission.
The speedo is not directly driven thru the cable. The speedo relies on a magnet that is driven by the speedo cable. The magnet revolves around a disc, the disc is not directly attached to the cable or magnet. As the magnet revolves around the disc, it moves the disc, & the needle. The faster the magnet moves, the further the disc moves. But, if the magnet is weak, or if the bearings for the disc & needle are worn or lacking lube, it is possible for the needle to not move as far as it should. And in that case the speedo would read slow, or may not work at all. So, the odometer would read one way, the speedo would read another. I hope this makes sense.
I've tested dozens of speedo & odometers, I use a 1000 RPM drill to spin them. Most speedo's are designed to read 60 MPH at 1000 RPM, most do, but some are + or - 5 MPH. Usually 5 MPH or so slow.
Thread Starter
|
More Turbo
Joined: Oct 2020
Posts: 529
Likes: 54
From: Houston
Thanks Scott. I calibrated the odometer over a 10 and 25 mile stretch of highway. It is 18-19% low (10 on the meter is 11.8+ on the mile posts). Recalculating the old mileage and today's calculation yields about 14 mixed driving.
Now to fix rotten bed floor so the tailgate stops acting like a drogue chute.
Now to fix rotten bed floor so the tailgate stops acting like a drogue chute.
Fleet Mechanic
Joined: Dec 2007
Posts: 1,512
Likes: 340
From: Wildwood Pa
19% is a big error. Pull the driven gear at the end of the speedo cable & count the teeth. To correct the reading, you need a gear with 19% few teeth. That number will probably be 3 or 4 fewer teeth. If that tooth count gear isn't available, the driven gear in the transmission would have to be changed.
If the tire size is different from stock the speedo & odo will be off too.
If the tire size is different from stock the speedo & odo will be off too.
Thread Starter
|
More Turbo
Joined: Oct 2020
Posts: 529
Likes: 54
From: Houston
I'm running 21575R15 (27 or 28" diameter) tires which I believe are stock size. I have not counted the teeth on the speedo. The gears I have found range from 16 to 21 teeth. If the current gear is 20 or 21 teeth, a 16 or 17 could solve the error.
Trending Topics
Tuned
Joined: May 2013
Posts: 396
Likes: 5
From: Ottawa, Canada
To check the accuracy of the speedometer just download a speedometer app for your phone. They run off GPS and are something like within 0.1mph accurate when driving at a steady speed. I assume the odometer in app are pretty accurate as well.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
Lead Driver
Joined: Nov 2013
Posts: 7,178
Likes: 1,174
Find a local speedtrap. Here’s a database you can search:
https://www.speedtrap.org/
Drive through the speedtrap at a steady velocity, maybe 20MPH or so over the posted limit. Compare your speedometer reading to the number printed on your ticket. Repeat as often as desired.
https://www.speedtrap.org/
Drive through the speedtrap at a steady velocity, maybe 20MPH or so over the posted limit. Compare your speedometer reading to the number printed on your ticket. Repeat as often as desired.
Thread
Thread Starter
Forum
Replies
Last Post
52 F1 Stocker
1948 - 1956 F1, F100 & Larger F-Series Trucks
9
Oct 21, 2015 08:20 AM
bankert8806
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
4
May 14, 2013 07:25 PM
Bogaski
1973 - 1979 F-100 & Larger F-Series Trucks
16
Dec 12, 2001 02:00 PM