At one one time, before my little HD crash, I had a website marked that would show the difference between your current tire size and a new size. It showed front and side wiews and how much the speedo would be off.

Anyone know what that site might be?

 

I believe this is what you are looking for.

http://discounttire.com/dtc/brochure/info/tireMath.jsp

 

Thanks, that's what I was looking for.

 

I like that one, too.

Here is another one that includes gear ratio changes based from tire size. Can be a little confusing, until you realize what the intended answer is for. I have been able to fake out the formula, comparing tires/gears instead of using it to determine what gear I need to change to for the same performance.

http://www.4lo.com/4LoCalc.htm

Hope it is useful to you guys too.

 

OK, I'm confused. I thought if you put bigger tires on you lowered your effective gear ration (3.73 becomes closer to 3.5 going from 265s to 305s) but the gear calculator shows a ration of 3.99 when you do that.... And a 315 goes to 4.05. Of course, that's using their tire diameter calculator, but that does look right.

Do I have this all backwards??

 

Try this one!
miata.net/garage/tirecalc.html

BIG MIKE

 

What the calculator is doing is showing you that you need to go with a 3.99:1 to keep truck performance when increasing tire size. This site was designed for the off roader who wants bigger flotation/mud tires, but doesn't want to raise the gear ratio (killing the crawling ratio of their rig) in the process. This means you have to reverse what you want to do. Put in the new tire and old tire in reversed, and you will see what I mean.

 

Ya thats showing the gears you need to keep the performance that you have now. At first I thought it was backwards so I put the numbers in the diffenent spots and it showed me what my gear ratio is with 33's.

 

OK... I did reverse the tire data and got what looked about right. But what you're saying makes perfect sense.

 

This is my program... a little different for those who want better precision...

http://xj.cdevco.net/rpmchart

 

That program is awesome! I have added it to my favorites!