How Ford’s Electric Speedometer Works
As you may, or may not know, the speedometer accuracy is determined by a few factors with the biggest two being your differential ring gear ratio and the size of your tires.
When a Ford Truck leaves the factory, it is programmed to know that a certain tooth count equals a certain speed on your speed sensor located on your axle housing. This is calculated by the tire size that it left the factory with, ring and pinion ratio, and the count of the ABS Tone Ring on the axles ring gear.
So, you can begin to see why your tire size makes a difference in speedometer accuracy. You’re probably also wondering why the ECU isn’t able to adjust for it and that is because that calculation in the ABS Module that calculates speed is set and isn’t dynamic. Ford trucks will send the tone ring signal to the ABS first, then will send it to the speedometer. How this is exactly done does vary from generation to generation.
Now, I don’t know the exact tooth count that Ford uses, but this example will explain it a bit. The first thing your ABS Module needs to know is how far your truck travels in a full revolution of its rear tires. If you have a 32-inch tall tire, it will have a circumference of 100.5-inches.
If you drive a mile, that will equal to about 630 revolutions per mile. You can also get a rough number by dividing 20168 into your tire diameter as well (20168/32=630.25). Now, we get to the speed sensor. Let’s say a full rotation of the speed sensor has 20 high counts or 1 in binary speak. So, now, there will be 12600 high counts in that mile and when the ABS Module sees that many counts from the speed sensor, then it knows that the truck has traveled a mile and ticks off 1 mile on the odometer.
Now that we know that is the number seen in a mile, how do we get our mph from that? Still comes down to the calculation for velocity, just instead of feet it’s now counts on the speed sensor. 1-mile is 5280-feet; 12600/5280=2.39 counts per foot; travel that many counts in 1-second is 60-feet per minute (1-foot X 60-minutes) which is .68 mph or (60FPM X 60)/5280. To get 1 MPH, you need to count close to 3.50 counts per second. Again, this is just an example to show why a speedometer isn’t dynamic as your ABS Module needs to know these numbers to make the speedometer show the mph and these are not numbers that come from Ford. They are only examples.
So, what happens when we decide to join the 34-inch club with that 107-inch circumference? Well, with all other things remaining the same save for the tires, your revolutions per mile just went down.
So, now your tires are rotating 593 revolutions per mile instead of 630. Your ring gear is now spinning slower and your speedometer will read lower while your actual speed will be faster. Remember, in our example we know that 1 mph is 3.50 counts per second and 1-foot equals 2.39 counts at the speed sensor with 32-inch tall tires.
Now that you cover more ground in a single revolution of your 34-inch tall tires, that original calculation is thrown out the window. What’s even more troubling for a modern truck owner is that you can’t head over to Ford or your 4×4 stores and pick up a new speedometer gear like on trucks.
However, programmers will allow you to fix that issue. The programmer knows that each tire size is a difference of a certain percentage and re-flash the ECU to that difference. In our case, a 32-inch tall tire is 5-6-percent faster than the 34-inch tall tire and the count the ECU looks for must fall that same 5-6-percent. That now means that there will be 2.53 counts per foot or 3.71 counts per second at 1 mph, again citing our example and not Ford numbers.
One more note before we leave you today; your ABS system will work correctly. This is because the ABS is looking only for a difference in wheel speed between each front wheel and the rear axle and not the mph. So long as all of your tires are the same size and you begin to lock up the brakes, the ABS will see the difference in tooth counts between each tire and the rear axle and will release pressure from the wheel that is or when the rear axle is going into lock up.
All the ABS is looking to do is maintain the same tooth count between each, not maintain the same mph between each wheel. Now, if the left tire is taller than the right, then your ABS will act incorrectly and you’ll probably set a code for the wheel speed sensor before it does anything else.
So, don’t be afraid to get those taller tires, just know that you should know that your speedometer will be reading lower than your actual speed and when that cop gives you a ticket; he will be justified and you’ll lose your case because you were traveling at the speed he caught you at. So, keep the difference in your head, plan to buy a tuner, or get your truck flashed for the correct tire size.
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