Quote:
Originally Posted by JCL
You stated that different size tires wouldn't screw up the truck, and focused primarily on the differential wear issue. Yes, that is what a differential is designed for, and I agree that the wear on the differential is minor.
However, the transfer case wear from constant relative rotational speed difference in the drive shafts is more serious, since there are clutch plates and the heat buildup from the clutches constantly slipping leads to failiure. The tires aren't where the slip happens. The transfer case will see a difference, unless the rear wheels always follow in the same track as the front wheels, even more so if the tires are different front to rear.
And then there is the issue of the electronic traction and braking aids. They don't like different size tires. Lots of exampls of warning lights coming on from that problem, and it is the reason that BMW (and other manufacturers) publish maximum tire diameter differences.
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Exactly!!
The manufacturers all publish these specs for a reason. BMW (Porsche, VW, MB, GM etc) isn't in the business to sell tires so it's not a profit center rationale. I'ts because they've designed in these parameters for many different reasons. BMW's engineers are the tops in the business, why second guess their specs?
Couple of years ago when towing my trailer back from the east coast I got a nail in the sidewall of one of my front tires in the middle of WV- Monday of Labor Day weekend. Went to put the original full size spare on and found that it went flat moments after I lowered the truck down and put weight on it. Local WalMart was the only thing open that Monday on Labor Day weekend. Naturally they didn't have a 255/50-19 in stock. We inspected the spare- keep in mind this spare was OE and had never been out of the spare tire well other than to check air pressure twice a year- and when we pulled the tire off the rim, the inside of the sidewall was abraded, with lots of tire shavings on the inside. This tire had never been mounted from new, there was still virgin paint on the bolt seat of the steel wheel. But it was junk. And of course they didn't have a 235/65-17 tire in stock. But they had some inexpensive 245/60-17 BFG's.
Calculating the rolling diameter, it should be virtually identical to the 235/65, just 2% or so off. Mounted one up. Hit the road. Within 50 miles I had the ABS light flickering, followed shortly by the Trans Failsafe warning. Pulled off the road, shut down, waited a few minutes, restarted. Drove a while then trans failsafe again. So I found an exit with a couple hotels and quit for the day. Next morning found another tire dealer who had a 235/65 in stock. He even took the 100 mile old BFG in trade! Hit the road and never had a problem again. Went straight to my dealer on the way home and had the codes pulled, was told the rolling diameter difference threw off the ABS and confused the stability control, and that resulted in the Trans Failsafe light. And that was with just a 2% difference in rolling diameter.
BTW- I showed the dealer my spare tire. They replaced it for me without question. No excuses, just good service. And I now carry one of my 255/55-18 mounted snows in the trailer just in case.