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Fix Motor Mount
Roll Cage Braces
Throttle Body 65mm
Wheel Alignment
Heat Riser Replacement
Vent Window Repair
Center Console Mount
Straighten Bumper
Ford 6R80 6 spd
More Tech...
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speedometer gear reducer
- Thread starter orange 73
- Start date
Broncobowsher
Total hack
- Joined
- Jun 4, 2002
- Messages
- 34,982
Viperwolf made a chart that shows the speedometer gearing options based on axle ratio and tire size, and the number of teeth on the driven gear inside the transfer case. Someone will paste that here by the end of the day.
Broncobowsher
Total hack
- Joined
- Jun 4, 2002
- Messages
- 34,982
Depends on which option box was checked when it was ordered.
69_Sport
Full Member
- Joined
- Nov 5, 2014
- Messages
- 258
So if you have the truck assembled and can drive it, use a GPS or your phone to determine the actual speed you're driving and also record the speed your speedo is showing. Divide the difference into the indicated speed on your speedo to get a percentage it is fast or slow.
To select the correct driven gear, use this percentage to select a gear with approximately the same difference in teeth.
For example if, your speedo says 60 mph, but your GPS says your actual speed is 66, then you're 10 percent slow.
66-60=6
60/6=10%
So you'd need a driven gear with 10% less teeth to speed it up. If the GPS actual speed is less than indicated on your speedo, then you'd need a gear with more teeth to slow the speedo down.
Speedo reads slow = driven gear needs less teeth.
Speedo reads fast = driven gear needs more teeth.
Now remove your speedo cable at the tranny end and count the number of teeth on the gear. Let's say it has 16.
10% of 16 is 1.6.
16-1.6=14.4
You'll have to round it to the nearest whole number as the gears don't have partial teeth. So in this example you'd use 14.
A 14 tooth gear would be the correct one for your combo in this hypothetical example.
When buying a replacement gear, pay attention to the direction of the teeth. They are made with a curve in the teeth, either right or left hand. Automatic trannys use the opposite of manual ones. I forget which is which, but if you buy one for an auto you should be good. Always compare the new gear to the old one and be sure they are both either RH or LH.
Doug
To select the correct driven gear, use this percentage to select a gear with approximately the same difference in teeth.
For example if, your speedo says 60 mph, but your GPS says your actual speed is 66, then you're 10 percent slow.
66-60=6
60/6=10%
So you'd need a driven gear with 10% less teeth to speed it up. If the GPS actual speed is less than indicated on your speedo, then you'd need a gear with more teeth to slow the speedo down.
Speedo reads slow = driven gear needs less teeth.
Speedo reads fast = driven gear needs more teeth.
Now remove your speedo cable at the tranny end and count the number of teeth on the gear. Let's say it has 16.
10% of 16 is 1.6.
16-1.6=14.4
You'll have to round it to the nearest whole number as the gears don't have partial teeth. So in this example you'd use 14.
A 14 tooth gear would be the correct one for your combo in this hypothetical example.
When buying a replacement gear, pay attention to the direction of the teeth. They are made with a curve in the teeth, either right or left hand. Automatic trannys use the opposite of manual ones. I forget which is which, but if you buy one for an auto you should be good. Always compare the new gear to the old one and be sure they are both either RH or LH.
Doug
Last edited:
