Just got a quick question about the speedometer. Im thinking of putting 305/70/17 tires on my 07 Power Wagon. I know if i do 315's I have to adjust the speedometer for the bigger wheels. The stock tires are 285/70/17.....I guess my question is at what point do you need to change the speedometer when you increase the tires sizes??? Going from a 285 to a 305 is a 2 tires size jump how much is that going to throw my speedometer off??? I live in Va Beach so I def don't need any speeding tickets LOL

Any advise???

You can find a calculator pretty quick on the internet that will run the tire size math for you, The usual tire specs are not the most important. You need to know the tire's diameter because that is what the calculator will use to run the math. I think any change that could get you a ticket, is worth making sure the speedometer is accurate. My race car had tires that were 28" tall. The speedometer would show 40mph or so at a given rpm. I put on a set of 26" tall tires for giggles and at about the same rpm, the speedometer showed 80mph. I think less than a half inch of height is debatable, but more than that is a pending ticket. I don't know tires well enough to suggest whether or not a "step" in size has any correlation to real math in regards to speedometer accuracy. It seems that some of the posts, mentioned programmers having an option to account for tire size. Again, if it does, then it will need to know diameter.

Found this calculator that one of the members posted. Claims it is good.

http://www.net-comber.com/tirecalc.html

If i am correct. 305 is wider than the 285's. So wider should not change mph. 70's is the hight. I changed my 20S to 305 50's from 275 60's and i was off by 3 mph.

quote:

Originally posted by leadfoot:
If i am correct. 305 is wider than the 285's. So wider should not change mph. 70's is the hight. I changed my 20S to 305 50's from 275 60's and i was off by 3 mph.

You're right to a point. The 70 is the aspect ratio which is usually a percent of the width(315). So if you have a 275/40/17 its exact replacement is a 315/35/17. the aspect ration usually has to drop if you go up in the biggest number.

between the 285/70 and the 305/70 will be about 2mph more at 65mph. So if the speedo says 65 you'll be doing about 67mph instead.

NOW, If you use a 305/65/17 its almost dead perfect on tire diameter and a 315/65/17 will be less than 1MPH difference.

Superchips says to actually measure the tire height with the tires on the truck and properly inflated and use that number.