I have a Tremec T5 with a 9 inch rear end that has 3.50 gears. The rear tires are 235 60 15. The tires are 26.1 inches tall.

I have been to a couple of sites like this one for speedo gear calculators.

http://www.tciauto.com/Products/Tech...eedo_gears.asp

According to this site, I should need a 18.9 tooth speedo gear. So, I got a pink 19 tooth gear.

I haven't had my Mustang up to any moderately higher speeds yet to see how off it is, but I do know that it reads low. I am getting the alignment done tomarrow and hopefully the intermediate pipes for the exhaust tomarrow or the next day.

Does anyone have anythoughts on if this is the right gear for my setup?

 

I would figure out how far out the speedo is, at a few speeds, convert it to a percentage, and apply this percentage to the speedo gear ratio.

say your speedo is 10% slow, decrease the number of teeth on the driven gear by 10% (two teeth in your case) to 17 teeth.... as an example of course.

Of course Murphy's Law says if you need to change the driven gear 2 teeth, you will need to change the drive gear, if this is the case, the same formula will apply, you will need to figure some ratios though... and I am too lazy to go further right now..

I am not sure about Ford, but for GM in order to use a certain color driven gear you need to use a certain color drive gear, or at least know the color of the drive gear in order to select the right driven gear to provide the desired ratio.

 

You need to get the right gear to match the drive gear on the tranny output shaft. On Ford manuals some are 6 tooth and some are 7, and I think some are even 8.

 

I used the NPD catalog to figure it out. On page 141 of their catalog read the RED text and work out the calculations. It tells you everything you need to know.

 

67Mustang302, My T5 has the 7 tooth drive gear.

Thanks for the additinal info Adrenolin

According to everything I have read, the pink 19 tooth speedo gear is what I need. I did some breif experimenting earlier today after the alignment was done. It appears that my speedometer may be reading right after all. I got up to about 55 where the speed limit was 55. It seems to be closer to accurate than I thought. That was the first time driving the car over 30 mph. Tomarrow the exhaust gets finished. Then I will have inspection and I can drive it some more and see how the speedo is on the interstate at 70.

 

Good luck getting it totally accurate. Unless you run a precise tire size it's gonna end up being off a bit. Factory speedo calibration can be way off sometimes, but if you can get within a couple mph on the highway you're doing really good. You're just limited with a gear driven speedo. I managed to get mine at 70mph accurate to within about 1mph, maybe 1.5 at most. If I change to a different tire size then it'll be off.