DWSson1
06-04-2005, 12:49
I have an '03 GMC 3500 dually, Duramax, Allison. The truck came from the factory with Goodyear LT 215/85R 16 E M+S Wrangler tires. I am replacing them (47,000 miles) with commercial grade tires. The new tires have an additional steel belt on the sidewall to help with puncture resistance (and weigh almost twice as much). The problem is, I can only get them locally in a different size. They are Michelin LT 235/85R 16 E XPS, also M+S. The old tires had a load rating of 2680 (single), and the new ones are rated at 3042 (single).
On the way home I noticed that the fuel computer was showing substantially lower mpg, and the speed seemed wrong too. I was under the impression that the 215 v 235 was the tire width, and that the 85 was the height, so that there should be no speedometer difference. Apparently, this must be wrong.
Several questions. How much indicated speedometer difference should I see with this change? I followed my wife at a set speed, and am guessing that I am off 3-4 mph at 70, and 3 or so at 60mph (reading low). Is this about right? Is this a linear relationship? If the new tires are taller, then my truck will move farther with each rotation. This will effectively reduce my gearing ratio where the tire meets the road. Should I also expect this to be relatively minor (ie 4.11 to 4.04 or something like this?). Also, will my actual millage go up (more distance per engine revolution) or go down (more distance per rev, yes, but that requires more horsepower)or stay about the same (more distance per rev, takes more hp, but the two offset each other)? Is there any place I can get my speedometer recalibrated for anything short of highway robbery? Any recommendations? And I assume the larger tire will also affect the ABS system. Can this be recalibrated as well? Is it one calibration for both speedometer and ABS?
Thanks for any and all help.
DWSson1
On the way home I noticed that the fuel computer was showing substantially lower mpg, and the speed seemed wrong too. I was under the impression that the 215 v 235 was the tire width, and that the 85 was the height, so that there should be no speedometer difference. Apparently, this must be wrong.
Several questions. How much indicated speedometer difference should I see with this change? I followed my wife at a set speed, and am guessing that I am off 3-4 mph at 70, and 3 or so at 60mph (reading low). Is this about right? Is this a linear relationship? If the new tires are taller, then my truck will move farther with each rotation. This will effectively reduce my gearing ratio where the tire meets the road. Should I also expect this to be relatively minor (ie 4.11 to 4.04 or something like this?). Also, will my actual millage go up (more distance per engine revolution) or go down (more distance per rev, yes, but that requires more horsepower)or stay about the same (more distance per rev, takes more hp, but the two offset each other)? Is there any place I can get my speedometer recalibrated for anything short of highway robbery? Any recommendations? And I assume the larger tire will also affect the ABS system. Can this be recalibrated as well? Is it one calibration for both speedometer and ABS?
Thanks for any and all help.
DWSson1