Calibrating the Speedometer

[dogbert4pres]

I have a 74 EB and put on 35" tires. Now the speedometer is off about 50% of the registered speed... ie 40 mph on the dial = 60 mph on the road. I came across numerous articles on calibrating the speedometer, here is one http://www.bronco.com/pmwiki/pmwiki.php?n=...eterCalibration

I pulled the gear and it is wine colored, meaning that it hsd 26 teeth. Now according to the above article I get a 16 * .67 = 10.72 or that I need an 11 toothed gear. Since 16 is the least amount of teeth I can find AND this is what I have, any increase of tire size can not be compensated by a gear change. Now if I were to decrease the tire size from stock, I could get a gear with more teeth and fix the over-reporting of speed.

Can anyone assist?

Thanks!

 

[Seabronc]

What is the rear end gearing. If you can't get a gear to bring it in tolerance, you may need to change the differential gears. Usually if you increase from stock tire size, you need to upgrade the differential gears to keep the engine in the proper cruise power range. Increasing the size of tire can make it a power Dog if you don't increase the gear ratio in the differentials.

Good luck,

>-

 

[S_bolt19]

any increase of tire size can not be compensated by a gear change. Now if I were to decrease the tire size from stock, I could get a gear with more teeth and fix the over-reporting of speed.

Sure it can. You are misinformed about the gear/tire change. If you lower your gears then you will be able to make up at least some of that difference. I used to be running 33s with original 3.50 gears and until I changed out to 4.11 gears & I was only 10% off. That was checked by the police for me. Now, with my current 4.11s & 33s I am about +/-2%. Changing those gears will help.

As for going with a smaller tire than stock and getting a bigger toothed gear, yes, you are correct. You will make up for the over reporting.

But you can compensate with lower gears to get a more accurate speedo reading. If it is that big of a deal for you, get a digital speedometer from BC Broncos. I think those are programmable so you can keep the stock speedo gear, axle gears and change tire size and all you have to do is reprogram the speedo. They run about $500 so if you are worried about getting a ticket, it is a little bit of insurance.