So question for those of you who have done this. I used forscan to try to adjust my speedometer and was able to get it changed-was a lot easier than I thought it would be once had the correct ELM adapter and could actually pull some data. However my question is how are you guys calculating the correct revolutions per mile to input into forscan? I have a 315/70/17 tire now that the manufacturer specs at 34.33" . They do not list a revolution per mile on their website, but tirerack.com does for that tire and puts it at 604. However when I use other online calculators to double check it I get anywhere from 586 to 611. I did change it to 604 and at 20 mpg I was still roughly 2-3 mph slower than the speedometer but at 55 I was 1 mph slower compared to the speedometer when using a GPS speedometer app on my phone. Before I made the change with forscan I was running about 3 mph faster than what the speedometer said. I hate math but hand calculating it might be easier.

 

Use 620, dead on for me with 34.4" Dia. tires.

 

I set them to be exact at 65 mph which is my typical highway cruise speed so that is where i want the accuracy.

i have found a few things to be generally be true-ish

first off you have to understand the speedometer is just a stepper motor that is controlled by varying the voltage,

1. Sensor are often replaced with a non motorcraft part, Maybe Napa or ebay who knows but as a general rule always use Motorcraft sendors. Many ( especially engine related on 7.3 and 6.0 ) Ford uses proprietary coding on the chip sets logic. Off brand sensors will use a generic code so They fo not have to pay ford royalties. They can be similar but not the same

2. Like many gauges in general they are not accurate at their full range, if you are ever selecting a gage for use in something say a psi gauge you buy one that your normal operating parameter is in the middle of the gauges range, this is where it is most accurate. So if you are wanting to keep tabs on a thing that should be 50psi you would want a 0-100 psi gauge and at about 50 psi is where it will be most accurate.

3. 20 year old wiring harness and stepper motors is not likely to be in the same condition as when new, Oxidation has likely changed the resistance and voltage drop resulting in anomalies that may not have existed when new, then again i would not say Ford had ever been know for having highly accurate gauges. Other then the crappy temp gauges the rest are typically quality.

 

For the sake of anyone else stumbling on this thread, our trucks' computer doesn't comprehend any value lower than 600. My truck straight up turned down any input lower, and "accepted" 600, but it reverted to the original number immediately. I tried 605 next and it worked so I stuck with it.

FWIW, I have the "37-inch" BFGs that we all know are undersized a bit more than most. If I had to guess they're probably a 35.5" or 36" Regardless, 605 is not enough to get it dead nuts accurate, but it's within 3 mph around 60, and that's good enough for me. The fuzz don't care about a little speeding out here in the country.

 

i run 40’s and have programed one for 42’s. Forscan wrote us a cleaver work around by asking the number of teeth on your tone ring.

rear is 120 and fronts are 60 ( or 1/2 the rear )

do lets say you install a 40” larger tire.
that is 26% larger then the stock 31.4” tire snd since the revs per mile function only allows for about s 10% yore variance the solution is :

change the the tone ring by 20% or 12 teeth. Instead of 120 try 108 on the tone ring count. Your reduced number must be divisible by 2 since the fronts are 1/2 the number of teeth then the rear you want the computer to still divide the rear by 2 to calculate the front

now the computer is only 6% off snd you can dial that in by the revs per mile.

technically your ABS is going to be react miliseconds slower but i will wager you nor any humans. can actually detect a difference.

i asked mark a partial question eithout hiving him the whole picture ( because we didnt have the whole picture at the time) and based on the info i gave him he thought it may alter shift points since they are s function of mph, however that was without telling him the tire size change. My thoughts id the shift points should be the same since when you add the tires, rev per mile the speedo indicates the Accurate speed being traveled so unless the trans shift point gets its speed by means of tone ring speed only then i think it shifts correctly. I certainly have not noticed otherwise but my trans is also heavily modified so maybe i would not notice Either wsy.

 

Can you do this in Forscan lite or is the full version required?

 

The windows version is the only one that can change this information. You can buy an extended license to do it or you can sign up for a 2 month trial to be able to do it.

So at 55 I was 1 mph slow and 70 was 2 mph slow. Definitely variable throughout the speed like pirate said . I might try to go out and tweak it to get it dead on at 55.