Speedo Reads Off-Scale High

[galen1970b]

I assumed my gearing was stock 3.50s....but now that i have my speedo hooked up....its reading high 15 miles over at 45mph (indicated(60 mph))...I'm looking at new tires(and with new tires gears..) and usually when larger tires go on indicated vs actual speed is slower right? Granted I know jacking the back end of the truck up and counting would be easiest(and who knows what tooth speedo gear i have...) but just getting the theory down pat..

-Thanks!

-G

 

[Viperwolf1]

You need a gear with 33% more teeth. 60/45=1.33

 

[galen1970b]

Thanks for the reply Viper! I've got a large road trip ahead for work and figured new tires were in order. I don't think the ancient buckshots mudders i have on these are gonna make the 8 hour drive down to keesler. The old set of tires were around 31-32 inches. I will be getting a set of 35's and since my odometer is reading high figured it was a good indication i have lower gears than stock installed.

 

[.94 OR]

The end of the speedo cable is held in by a small bolt. Take the retaining strap off and slide the cable end out and see what color gear you have (and/or count the teeth). The vendors usually have a chart to show you what gear you should be using.

Mine used to run slow, and with my big foot, I'm surprised I never got a ticket since I didn't exactly know how fast I was going.

 

[galen1970b]

i probably should have specified....with the speedo showing offscale high i thought it was an indication that the diff gears were lower than stock.

 

[jckkys]

You may want to ascertain your actual front and rear gear ratios before changing anything. If the tags are still on center sections it's a good bet the ratio stamped on them is correct. Going to larger tires will increase your actual speed relative to the indicated speed.
I was an air-evac medic and loved riding in the C130 crew chief's seat whenever I could. Best view in the world.

 

[Viperwolf1]

i probably should have specified....with the speedo showing offscale high i thought it was an indication that the diff gears were lower than stock.

Maybe but there is also another possibility. The speedo drive gear on the tcase rear output shaft (not the driven gear on the cable) can have either 6 or 7 teeth. If you have the wrong one it can contribute to a 17% error by itself.

 

[ENDLIFE]

These are all great ways to find the correct speedo gear, after re-gearing, and 36" Interco tires I was lost on a new speedo gear. Get a GPS that tells you how fast your actually going, drive your rig until your transmission is at a 1:1 ratio, and record both speeds (Bronco, and GPS). Use Vipers suggestion of finding the percentage of error Ov-Fv/Ov multiplied by 100%, use this % of error by multiplying it by the number of teeth on your speedo, then add or subtract that number to the original number of teeth on your speedo, and it will give you the exact number of teeth you need. This method worked for me, or you could use the GPS and work your way through the gears like my buddy did (It worked for him).
Good luck on your endeavor.
Ov= speedometer MPH
Fv= GPS MPH

 

[galen1970b]

You may want to ascertain your actual front and rear gear ratios before changing anything. If the tags are still on center sections it's a good bet the ratio stamped on them is correct. Going to larger tires will increase your actual speed relative to the indicated speed.
I was an air-evac medic and loved riding in the C130 crew chief's seat whenever I could. Best view in the world.

It is pretty awesome, you lucked out getting to sit up front!!! I've seen full birds get denied and thrown in the back!! The new J-Models are badass the Rolls Royce AE 2100s smoke those old t-56's....its really like yanking a straitsix out of an EB and dropping a coyote motor in it...

I've searched all over for the stock gear stamps and can't find them tomorrow i'm gonna jack up the ends and see what gears I've got. Need to do that start fabing up some bumper mounts and finds a cargo net to replace my tailgate.