I need a math genius out there that can do some math forme. I am planning on purchasing some replacement tires for my near bald comp t/a's that is now non-existent.

I plan on buying a set of michelin cross terrain tires for long life quiet ride. I like to get a wider tire size by downsizing from a 60 series tire to a 55 series tire. Here is the tire sizes:

Original size: 275/60/17

Proposed new tire size: 275/55/17

The tire will be slightly smaller, Iassume about a .5 inch to a inch smaller in overall diameter between the two tire sizes.

I want to know much many MPH's will be off from my speedometer needle. Assuming my needle is accurate. I do know based on ratio, the needle will end up being a little faster than actual speed because of the slightly smaller diameter of the tire. I need to know much much of that difference so I can recalibrate the needle or just subtract it in my head when I glance at the needle to know my true speed.

I know the Dakota R/T'sran 255/55/17's and the Daks pretty much have the same tranny/gear ratio setup as the durangos.

 

Try this site: Tire Calculator

 

Thanks Daddy Bear. That is exactly what I needed!

Now here is another problem. I wonder who's speedo from factory is correct based on the tire sizes.

Datoka R/T - 255/55/17

Durango R/T - 275/60/17

I hooked up my GPS unit to see how accurate my speedo is and currently showing right about 3 percent off target. I am seeing about a 2 mph difference from what my GPS says vs my speedo. Being my speedo is actually slower than what the GPS is saying. So if I decide to go with the 55 series profile tire that is the original profile size for the Datoka R/T's, my speedo would pretty much be near perfect accurate.