baddceo

That is exactly where I educated myself in this process. I am trying to figure out if it's worth the effort to gain the .5mph. If the tire is truly 32.5 tall as mentioned then I've simply used the wrong input so I am going to take an actual measure inflated on the truck and see where I land. From there I will take another stab and see what happens. Worst outcome is I go back to what I have now which I can live with.

baddceo

Well my 33x12.5x20 setup is actually only 31.5 tall. How does that make any sense. I guess I'll plug in that conversion which would be 318 x 46 x 20 and see what happens. On the fence as what I am working with now seems pretty good. I have yet to get on highway and check things at 70.

what's confusing is that the current hexadecimal code loaded would appear to have my setup smaller than my OEM code as gps says 49 while speedo says 50. If I load the code associated with a 32.5 inch tire its gonna be worse as that would tell the ECU the opposite of what is needed. I think it's not worth any more effort1

jp360cj

I'm not surprised, especially if that's with the truck sitting on the ground. The weigh of the truck will compress the tire.

Sounds like you are thinking about it backwards. If your speedometer is reading slower than gps, then you need at input a bigger tire. If your speedometer is reading faster than gps, you need to input a smaller tire.

Think about it like this... All the speedometer knows is how fast the tire turning. A bigger tire is going more distance for each rotation, so the truck is actually going faster than the speedometer is reading. GPS speed will be greater than speedometer speed. Enter a larger tire size to slow down the speedometer.

baddceo

So i upped the Hexicdecimal to match that for a 318x53x20 which is just slightly taller than what I had before at 318x52x20. Now when my speedo says 50 my GPS say 48-49 as does Wayz and another GPS unit I checked. This would tell me that the programmed tire size is now too tall, correct? It wasn't even a full mph off on the 52, but as the truck gets faster going the more it doesn't match the speedo again this case Speedo says faster than GPS.

jp360cj

To recap (just to make sure I'm following correctly...

You had values for a 318/52/20 tire. Speedometer read 50, GPS read 49. Speedometer too fast.
Changed to 318/53/20 (1/4 inch taller). Now Speedomter 50, GPS reads 48-49. Speedometer too fast, but by more than before.

Sounds to me like when you got bigger, the speedometer got farther ahead of the GPS reading, so you need to go smaller. This would confirm what I was saying in my last post.

If there is an error in the speedometer, I would expect for it to be larger at higher speeds. The % error should remain fairly constant, so the higher the speed, the more MPH the error will be. Just out of curiosity, what do the hexadecimal values you enter correlate to? You keep mentioning metric tire sizes (i.e. 318/53/20)..is that what the hexadecimal actually represents, or are you converting that to diameter, circumference, or rotations/mile, or something else?

baddceo

that is a metric conversion from 33x12.5x20 which what is needed to convert to the 4 digit hexidecimal for forscan. The speedo and gps match until 25 mph and then its gets progressively worse the faster you go. So maybe I'll drop to a 51 and see what happens since 53 to tall, and 52 almost right

jp360cj

So you have to convert the metric size to hexadecimal? That really doesn't make any sense. I would expect the value entered to be based solely on the diameter/circumference/revs per mile. Circumference would be most logical as it would give the most exact figure without decimals. Anyway, if you can't get it close enough using the 12.5" tire and 20" wheel, you can change either the section width or wheel diameter in your calculation. A narrower tire would give smaller step changes for the same change in aspect ratio.

baddceo

This is the link from this website that allows you to load the tire size in metric measurements. https://docs.google.com/spreadsheets/d/1mWnuQZlDTUGjKjmJje1QJIR8Z2c7Kf8KBZxKnw_8EqU/edit#gid=0

If 318/52/20 was the closest but still showing too tall and 318/53/20 which is what I have now is even taller then you are right what I need to do is go in and try 317/52/20 or 316/52/20. Didn't even think about changing the profile size. Thank you for that tip.

Kingofwylietx

Which Dongle did you get? I'm still on the fence about doing this exact thing (tire size change)...but using FORScan is a bit intimidating. I'm usually pretty good at figuring things. The sheer size of that FORScan thread gives me a pause. Add that to having never done it and then throw in a little fear of melting the truck with a typo.

baddceo

Here is the one I ordered. - I can get the link to work. Just search on that number

I was on the fence about BT and decided I would rather have a hard wired connection just in case BT dropped or something in between. Just taking another variable out of the equation was good enough for me, plus it's only $29.

I had the same reservations you did but after watching the videos and doing exactly what you see it turned out to be much less intimidating once I got under way. The heated seat and steering wheel options on the home screen is a nice little touch and having auto window up and do from my key fob will come in hand in the warmer months.

Anyway the tire size change is painless, just save each module as original before changing them. I also took a screen shot snippet of each forscan change I made before I made it.