1996 Dodge Grand Caravan LE-Tires

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"RWM" wrote: "That is understood, but if all tires are the same size -- any size within reason -- rotations will be the same."
Yes and no. Yes all four tires are the same then revolutions are the same
per tire. The ABS is programmed by the manufacture for a specific size tire (OEM). That tire has a known rotation thus the wheel sensor turns some many rotations with the tire. Different size tires have different size rotations due to the circumference of the tire. If you go to a larger tire size even on all four wheels the tires will make a longer revolution thus throwing off the speed. The wheel sensor being programmed for a certain size tire looks at the revolutions to determine the speed it needs to kick in or kick out while braking.
If your speed is off due to larger or smaller tires, even if all four wheels or the same, the ABS system computer will function as if the OEM tire is on the vehicle. If it will not function properly due to more or less revolutions then programmed for. In reality the system still works just not as it is programmed to due. I not sure if the ABS computer can be reprogrammed. I would believe it could since it is part of the cars computer and tire sizes can be changed to correct the speed odometer.
If you are going to change tire sizes look at what is called OEM plus zero or plus one sizing. Tire dealerships can explain what tire sizes are available for your vehicle.
I once had a small pickup that was manufactured by one company but sold under to company names. The only difference was the grill, name of manufacture on the tailgate and OEM tire sizes. The sticker in the door even stated made by XXX manufacture for XXX of the US. The owners manual and the sticker in the door listed all the tire sizes that could be used for both manufactures. I started having problems with my ABS system when I switch to the largest size tire that was listed in the owners manual. The system still worked but the truck was harder to stop since the ABS system was not functioning as it should. My speed odometer was off by 11 miles per hour.
The next time I changed tires, I purchased a larger rim and reduce the diameter of the tire and it braked a lot easier but still not the same as the OEM size tires. My speed odometer was now off by 7 miles per hour.
Sarge
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Sarge wrote:

Good explanation. Thanks. Linkage to BCM based on presumed tire diameter was unanticipated.
That said, the differences in delivered tire sizes are diameters that nominally range from 27.0 to 27.1 inches, resulting in a speed of 65.2 mph at 65.0 indicated. Since this is well within the new versus run-out tolerances of a tire with 10/32" tread when new, I can't imagine that makes any difference to the BCM.
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"RWM" wrote: "Good explanation. Thanks. Linkage to BCM based on presumed tire diameter was unanticipated. That said, the differences in delivered tire sizes are diameters that nominally range from 27.0 to 27.1 inches, resulting in a speed of 65.2 mph at 65.0 indicated. Since this is well within the new versus run-out tolerances of a tire with 10/32" tread when new, I can't imagine that makes any difference to the BCM."
That small of an increase will not have the big of effect but look at something like going from P235R70/16 to P265265R70/16 and you are looking at .73. This link will help you see the difference in sizes http://www.net-comber.com/tirecalc.html
Sarge
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