Does anyone know the conversion formula for going from my stock 245's to the 285 Toyo open Countrys that I have on now? How far off is my speedometer?
thanks
Joe

 

Your best bet is to use a GPS because the stock speedo is typically off, but ASSUMING it is correct then your speed will read about 7.4% slower than you are actually going, and of course the odometer will also tick up 7.4% slower.

I know when I went from 245s to 315s my speed and distance was almost exactly 13% off, according to my GPS.

 

I had the 245's & went to 285's. Should be around 7 MPH faster than the speedo says, but like the previous post says get a GPS to double check it.

 

As far as I know, the best thing to do is....make a mark on a tire and one that matches on the pavement. Drive until the tire has made at least three revolutions. Measure the distance between the first mark on the pavement and the one you make at the end of the drive and divide by the number of times you let the tire turn. This will give you a pretty accurate circumference of your tires. Then, figure out how many revolutions it takes to cover a mile and have your dodge dealer enter that number in your computer. I talked my dealer down to $37.50 to do it. I told them I wasn't paying $75 (1 hour labor) for 10 minutes worth of work. Be sure to check the air pressure in your tires and set them to the pressure you are going to run them.

 

Less physical labor a sweat involved in this method. I dont know the aspect ratio of the tire you have in question but for the example I'm assuming 70 series. EXAMPLE: 285/70-17: ((285/25.4*.7)*2)+17= 32.71" (tire height) * 3.14 (pi)= 102.71" circumference /12"= 8.56' circumference. 5280/8.56= 616.8 rev/mile.

Same here. I did not know these trucks came with such a small tire. I will assume at least a 75 series in this case. 245/75-17: ((245/25.4*.75)*2)+17= 31.46" (tire height) * 3.14 (pi)= 98.81" circumference /12"= 8.23' circumference. 5280/8.23= 641.5 rev/mile.

I get about a 4% difference in the tires as far as your speedo is concerned. But assumptions were made.

 

The speedo can be off even with the stock tires. I checked mine with a GPS and the speedo is off by 2 mph at 70 mph. 70 indicated, 68 actual.

MikeyB

 

Thanks guys. I stopped by the boat and got the gps. According to the gps unit I picked up almost 4.5% 70 mph read is actually 73.15 mph. I have the st with the snow plow pkg. The dealer said it was to alow extra clearance for tire chains. They were the stupidest looking tires I had ever seen. They looked like 4 temporary spares. I didn't even wait to wear them out.
Joe