Speedo off after tires/recal.
08-30-2016, 07:36 AM
#1
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Speedo off after tires/recal.
After putting my new wheels on last night and using the Flashpaq to recalibrate for the size change (255/75/17s to 33x12.5r15) I noticed it still reads slightly off - The dash at 50mph while the GPS reads closer to 47/48mph. I know all tires aren't true to size, but what adjustment can I make?
08-30-2016, 07:49 AM
#2
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usually the setting is about an inch less then the stated tire size. I run 285/75/17 Toyos AT2s, they are stated to be 33.9 diameter i have my procal set at 33 inches and its right on....HTH
08-30-2016, 08:34 AM
#3
Super Moderator
You can't use the size printed on the tire. You need to measure the true diameter after it's mounted and the vehicle is sitting on it. (choose your favorite of the three methods). Use that diameter with your programmer and you will be much closer.
08-30-2016, 08:40 AM
#5
JK Enthusiast
I measure from level ground to center of the hub then x 2. For my 40x15.50 on 10inch wide rim 24psi, I measured at 19inches. 19 x 2 = 38. Mine is never more than 1mph off my gps through 70mph
08-30-2016, 08:40 AM
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08-30-2016, 10:35 AM
#8
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That is where I started too, I did have to make a few 1/4" adjustments to get it right on. Don't forget your tires will wear down too, so if you end up 1mph the other way (GPS is 1 mph higher) It will come back to even pretty quickly.
08-30-2016, 11:28 AM
#9
Former Vendor
Just some tire knowledge to throw out there. Tire companies list the Overall Diameter as the inflated tire on the specified rim width and unloaded. Basically the tire on the wheel with air in it. Some manufacturers will also list a Static Loaded Radius. That gives you a more accurate rolling height to work with. That is their measurement for the tire mounted, aired up and loaded by the weight of a vehicle.
So you can obviously do what the others have said and measure from the ground up as that would be the combined (loaded and unloaded radius measurements)
So you can obviously do what the others have said and measure from the ground up as that would be the combined (loaded and unloaded radius measurements)