I installed a T-5 in my stang some time ago, but now the spedometer says I'm going like 90 when I'm really crusing down the highway at 65 or 70. Does anyone know of a chart that will tell me what spedometer gear I need? I'm running a 3.25 rear end with 225/60-15 tires in the rear.

 

You don't need no stinkin' chart, just basic algebra...remember in high school (snore)
x/y=70/90, so x=70y/90, assuming 70 instead of 65 mph is actual and 90 is reading
where x = number of teeth on speedo gear that you need
and y = number of teeth on speedo gear that you have
You don't even have to care about tire size or rear gear...

 

http://www.finer.com/Internet/pages/speedo.htm

 

Thank you for the link Magiarn71. Now I can avoid performing a science experiment by timing my car over a known distance while taking note of the spedometer reading and then using "basic algebra" to solve for the number of spedometer teeth.

The web link you sent me will tell me what I need to know in about 30 seconds. Much easier than Mr. Mach 1's method.

 

NP. I think all I did was type SPEEDO GEAR T5 in Yahoo or something.

 

I suspected I would be wasting my time...

 

Not wasting your time at all. That formula would work perfectly; I'm heavy into math and didn't even think of doing that. Only one problem. You're assuming that he KNOWS he's going EXACTLY 70 and 90. A few miles off here and there could put it off by even 1 tooth. (Using an initial gear of 23 teeth as an ex., using 70 mph gets you 17.9 or an 18 tooth gear. Using 65 mph gives you 16.6 or a 17 tooth gear.)

 

Oh well, I really assumed nothing...just what I was given. In any case, you will never end up with an exact solution, using basic math or a lookup table. That's because there are too many variables along the way that will not lead to the whole number solution that a speedo gear requires...if you get within a few mph without choosing an exact tire diameter, you are doing okay...
Having been a degreed M.E. for close to 20 years reminds me that all the precision in the world can only tell you just how much you off by. To get things done quickly, KISS is a good method and you must live with what you end up with.
My previous post was really directed at the original poster, who does not seem to want to put much effort at all into his own problem. The table you showed was pretty neat...either the result of many 'science experiments' aka empirical data, or math...both of which required some effort on someones part.

 

Yeah, faster is sometimes better, but it's good to know what you're doing either way. It pays to put in the effort to learn.

 

Mustangs plus sells a 23 tooth gear to put on your speedometer cable for $15.95. I put one on my T 5 and it works fine.