Speed to Tire Size Ratio?

[Dagger_91Beast]

I'm new so this question may have been asked before (searched but couldn't find it). I have just started to enjoy my Bronco. Unfortuanaly my local police department is really tough on speeding - even under 10 miles per hour over. Is their a ratio for determining the +/- of the speed showing on a stock speedometer vs. the actual speed once you change out tires? I'm running 35's with 456 gears and am afraid I'm pretty far off the mark.

 

[Roadkill]

With the 35's and 4.56's you are probably pretty close on the speedometer since the 91 and older speedometers are gear driven and your tire/gear setup is only slightly lower than stock. Two quick ways to check:

1) Drive at steady 60 mph down the interstate and time how long it takes to go one mile; using the mile markers, not the odometer. It should take 60 seconds. less than 60 seconds speedometer is lower than actual speed. More than 60 seconds, speedo is reading faster than actual speed.

2) If you have a GPS, compare the speed function with your trucks speedometer. This method is very acuarate.

If you find that it is too far off for your needs, it is pretty simple and cheap to fix it by changing the drive gear for the speedo. (and you won't have to do math in your head to know how fast you're going)

Use this formula from Bronco Graveyard to figure out which gear you need:

(picture originaly posted by Seabronc)

You can order whichever gear you need from Bronco Graveyard for $10.

 

[Dagger_91Beast]

Thanks Roadkill, just what I needed.

 

[sbveenker]

K someone please explain this to me. I have a 94 6 in lift 35's, checked the speedo with a gps and its off of course. I put 4.10 gears front and back and checked with the gps again. ITS EXACTLY THE SAME! WHY?!!!!!

 

[Roadkill]

In the '92 and up trucks, the speed is detected by a tone wheel mounted directly to the rear differential carrier. The carrier spins the same speed as the fastest spinning wheel. Therefore, it does not matter which gears are in it because speed is essentially detected straight off the wheel. In older trucks, the speed sensor is in the transmission or transfer case so a new gear ratio would effect the speedometer.

The only thing that will cause your speedometer to be off is a change in tire diameter.

 

[Broncobeginner123]

im running procomp 35 12.5, which in actual size is 33 12.5, anyways.. imy speedometer reads 5 miles slower then what i am actually going, I've checked it with speed checker on the side of the road, the mile marker method, and someone along side me. its always 5 miles difference. i have a 95.. what should i do?

 

[Roadkill]

Here is an article on How to Recalibrate the Speedo on you truck.

 

[WPW Bronco]

I'm new so this question may have been asked before (searched but couldn't find it). I have just started to enjoy my Bronco. Unfortuanaly my local police department is really tough on speeding - even under 10 miles per hour over. Is their a ratio for determining the +/- of the speed showing on a stock speedometer vs. the actual speed once you change out tires? I'm running 35's with 456 gears and am afraid I'm pretty far off the mark.

The best source of info I have found for speed adjustment and chosing gear ratios based on engine and tire size is Randy's Ring & Pinion website. Go to the Calculators tab and it gives you 5 different choices of neat stuff. Just put in your known info and it will belch out the info you WANT to know. He's a link right to the calculators themselves

http://www.ringpinion.com/Calculators.aspx

It's bookmarked in my net favorites, I use it all the time, hope it helps you.