First post! First F250! Picked up a 2015 with 23k miles on it yesterday. Today I was curious why a line was starting to build behind me so I busted out my phone and used a gps-based speedo app and found that my speedo is marking 2 MPH faster than what I'm actually doing.
Truck has stock 275/65r20s on. Truck was set to that tire size (forscan). I had to drop it to the 275/70r18 size and the speedo is spot on but the trucks speed sensor is showing 2 mph slower.
I'm thinking I should go back to the original size so that my odometer is accurate as I believe the speedo and odometer do not have a symbiotic relationship.
Any comments or answers as to why the speedo might be off?
GPS and speedo were 2MPH off. Do you know the expected error for GPS? For the speedo? There is a % of error for both so if you really want to find IF there is an error you need a local speedo calibration service. In VA I found a place that will certify your speedo for 6 months for $89.
GPS is 100% dead accurate. there is an error (like mentioned above) because of the time delay between your unit (where its mounted, too) and it going to a satellite and back to your unit.
GPS needs more than one sat, up to three or more to obtain the "best" position/speed data. Any overhead/side area "blocking" of the signal can cause errors, at times more than expected. 2MPH between the two devices isn't anything to loose sleep over.
I've verified this app with road side radar. And 2 mph is something to lose sleep over when you live in a county with a population of 60k but the Texas DPS office holds 80 permanent troopers plus more temporary/rotational ones that are itching for a reason to pull everyone over. Thanks Border Surge!
Actually I'm just weird and want the VSS and speedo to match.
60k...must be a small town. Sorry to hear 2mph will get you a ticket, the courts must be filled daily. I doubt you would understand not every tire and brand of the same advertised size is EXACTLY the same dimensional. I have sort of the same issue with my stock sized Nitto EXOs they are different than the stock OEMs. The best "number"/size to use is tire revolutions per mile. Go out and roll out the truck for one tire revolution, measure the distance in inches, use 63,360 divided by (number rolled in inches)= Tire revs per mile. That number is what your truck should use. Not the "generic" setting for the stock 20"s. Trust me the numbers are going to be different AND make a difference between tire brands/styles. Good luck chasing that dragon.
The county is 60k, town population is way smaller. In a one mile stretch you'll probably see about 15 to 20 troopers. Not always a citation, just a hassle and a chance for them to check every little detail. I DO understand that every tire measures differently. Just because it says "35 inch" does not mean it's a true 35". I said this was my first diesel truck, not my first vehicle. My last 3 vehicle have been modified, including tire size changes, so I'm fully aware that they don't all measure up to what they claim to be.
My post is about the mismatch between what speed the PCM sees vs what is displayed on the speedometer. Since I prefer a more accurate odometer, I'll set it back to the original tire size and just deal with the speedometer reading higher than what I'm actually doing.
Thanks for your help though. I can't post a link because my account is too new but the Houston chronicle did a piece on the border surge. Check it out if you feel like I'm exaggerations about the amount of troopers down here.
Im curious to know if the tire is actually as big as the sidewall says it is.
Now a days I use my phone (Waze) for speedo, especially in any place that has a ticket policy for 2 mph over
Nope, but if you do the math you will see where your problem is....It's not the hardware. It's your values and variables used in the algorithm the PCM uses for the math. The ABS module last I tried can only go to like 601 for tire revolutions. If your tires require 598 it may "add up" to 2mph. GPS errors(up to 10 meter area) and radar (up to 4% error) errors do exist. Add all those errors up and you start chasing that dragon. Good luck or just do the math, find the corrective factor (tire revs) for your setup and use it.
In any court or legal proceeding in which any question arises about the calibration or accuracy of any laser speed determination device, radar, or microcomputer device as described in this section used to determine the speed of any motor vehicle, a certificate, or a true copy thereof, showing the calibration or accuracy of (i) the speedometer of any vehicle, (ii) any tuning fork employed in calibrating or testing the radar or other speed determination device or (iii) any other method employed in calibrating or testing any laser speed determination device, and when and by whom the calibration was made, shall be admissible as evidence of the facts therein stated. No calibration or testing of such device shall be valid for longer than six months.
Yes...we all have...LOL Anyone wanting to use GPS as their argument will get laughed at in court....BUT....show them your certificate....They have to accept it. That's why a GPS device that is recording AND the certificate would outweigh the officer all day long. Hence I use a standalone GPS that logs that data; position, speed, location, date.....
It's called speedometer bias as it accounts for tire wear and variation. Better to error on the side of being 2mph slower. My VSS and needle are also about 1mph off at highway speeds. GPS matchs the VSS almost spot on.
I'm trying to investigate if there a is way to correct it using Forscan in the IPC module (cluster).
Interesting! Keep me posted. My previous vehicle was an F150 that I modded with forscan. I'd love to get them closer together as my VSS and speedo seem to be about 2 mph off. Oh well.
I'll agree that 2 mph isn't business critical, but I've noted the same thing on my '16 XL. I do get passed a lot and I am sometimes late for work.
Such an error can be significant depending on timing since it would effectively rob you of 3% of your warranty by mileage.
I owned a Honda a few years back and was a party to a class action suit against Honda because of this. The average variation was somewhere in the 15% range. Honda extended the warranty of all effected vehicles to compensate.
I agree with the whole being robbed of warranty time and all, but if 2MPH is the difference between you being late for work and on time, may I suggest some different habits?
I used a Hypertech Speedo Calibrator on my 15' F150 before Forscan was popular on the F150 site I frequented. I had 34.1" tires but I had to set it to 32.75" to get my speedo and app-based GPS to match.
Well for anyone following this ridiculous post of mine: I set the tire size back to where the VSS is accurate and the speedo is 2mph faster as my brain prefers a more accurate mileage estimate over an accurate speedometer reading.
I was able to set one tire size smaller and same results, speedo reflects actual speed. It was I thinking about 1" smaller tire; I think this is significantly more than tire wear. I went back as the previous poster did for the same reasons. I do have vss speed showing on torque pro, and use that if I need an accurate speed. It would be nice to remove the bias and have the speedo reflect actual speed. I guess Ford (and probably other companies) don't? think I am a good enough driver to maintain a speed limit so they feel the need to make me safer. The European bias is 4-5mph higher, they must really bee reckless and need even more controlling.
I realize, but still don't like it. Sort of similar to the sheepeople thing. I would rather things be accurate and truthful. Maybe a little OCD.
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