speedo off

[Ranger8867]

hello got an 88 ranger and the speedo reads about 6 mph slower then what my gps says I know my tires are bigger being 235/75/15s what speedo gear size would correct the speedo ive tried to use the calculator and it gave me to use a 18 witch probably was wrong because still reads all most the same just need some input what gear some other people have used to correct this

 

[sgtsandman]

@RonD will know the gear you need to correct that.

 

[RonD]

Assuming its 6MPH off at say 60MPH
Not at 30MPH
Because its a percentage, so off by 6mph at 60mph is 10% error, so at 30mph it would be off 3mph
Most speedometer cable systems allow for 3% error, but 5% is not unheard of, lol
10% needs correction


235/75/15 tire is 29" diameter(28.88")
Speedo DRIVE gear is 7 tooth on 1988 manual
You need your rear axle Ratio now

Look on driver's door back edge for the Build Label to get the AXLE code
Look here for the ratio: https://www.therangerstation.com/tech/ford-7-5-8-8-inch-axle-tag-door-codes/

Then go here when you have the ratio: https://www.tciauto.com/speedometer-gear-calculator

Scroll down to "Speedometer Gear Calculator"
Enter your data
7
"ratio"
29
Calculate

Ratio of 3.73 would be 18 tooth DRIVEN gear
4.10 would be 20 tooth
3.45 or 3.55 would be 17 tooth
3.27 would be 16 tooth

So ratio matters a lot

 

[Josh B]

You also should look at the tire size stated on the door to see if it has tires that match the truck
Mine came with 265/15 and when time to get new ones came I decided to downsize to 235/15 for the much smaller price tag.
I found on michelin or some tire site a calculator that tells the percentage change in size, revs per mile, etc.
In my case the 235 figured like 7% smaller and it gave me all the different calculation numbers.
I took it out on the turnpike and gauged by their mile markers, everything came together as it should.
Now all I do is glance and know , even tho all the numbers have changed, how fast or slow I am going.
You can also use the same number when monitoring miles per gallon etc
I should also try to find the gear I need for the speedometer

 

[Ranger8867]

It’s got an 18 in it and the rear gear is 3.73 so should be the right gear buts it’s not sorry for the very late reply busy

 

[sgtsandman]

My guess is a 17 will correct it but that is only a guess. If nothing else, it should get you closer, if my thinking is correct. Since the speedometer is reading slow, you should need the signal to spin faster. So, a smaller gear.