It depends on what 315s you chose to go with. All tires are not the same in terms of height and width. A 315 can be as tall as 35", or as short as 33.5. The most accurate way to determine speed is to use a GPS unit. Your trans will also shift a bit different due to the speed differential from your old tires to the new ones. LE

 

This little calculator will let you compare the two tire sizes and the associated errors in speed and odometer readings, plus give your new effective rear end ratio:

http://www.dakota-truck.net/TIRECALC/tirecalc.html

And LE is right, they're all potentially a little bit different, but this should get you pretty close. And yes, it probably will throw off the shift points a bit, since the speed signal will be wrong.

 

Ya probably already know this but, You could get a Superchips programmer ($300) or other electronic speed corrector (not sure on price, I think around $100) to fool your ECM into thinking that you still have the stock tires on......

 

Mine is off by exactly 10%, so at 65 mp by speedo you're actually going 71.5

 

My superlift true speed says the difference between stock is about 200 rpms checked by gps. At 70 with the stock tires I would be at 2000r's. Now with the 315's, I go 78 at 2000r's.

 

Yes the 3.77 effective ratio will give you a slower take-off, but might give you a slightly higher top end.

 

[quote author=LE link=board=7;threadid=20486;start=0#msg192162 date=1065151733]
The most accurate way to determine speed is to use a GPS unit. LE
[/quote]

I believe this is the most accurate answer......beg, steal, rent, or borrow a GPS and check it.