I normally recommend a Speed-cal but with just a 3.73 swap, a $5.00 part is better than the $100.00 Speed-cal, every day of the week. The only speedo gear that wears out is the 23 tooth which he doesn't need for this swap. Speed-cal's still use a speedo-gear...
What I am saying is that if a speedo gear wears out using a speed-cal, not likely anyway as long as it is not the 23 tooth one. He will still need to replace it. So that is a moot point about them wearing out.
A 96 TBird has the exact same 4R70W that a 96 Mustang has. I ran 3.73's in my TBird for quite awhile. I used the purple 21 tooth speedo-gear. When I put the 4.30's and Speed-cal in, that purple gear was like the day I put in, not worn at all.
I only put the stock speedo in because I was updating to a different 4R70W and getting the proper speed-cal value for the 21 tooth speedo gear, transmission and 4R70W was going to be tricky.
I know of many, many other 3.73 swaps that just used the speedo-gear, not a single one wore out.
TJG, thanks that's good news for me... I was considering a Speed-cal but didn't want to spend the $100 if it wasn't necessary. Question for you though- if I was getting 4.10s, do I use a 23 tooth speedo gear? Or is that wrong? thanks
the 23 tooth (white) speedo gear (which IS NOT a Ford part) has the wrong pitch for a Gt transmission drive gear and will wear-out in just a few k.
if you need to-go the 23 tooth route on a Gt manual trans, you'll have to swap-out the drive gear for one off a Cobra T-45 as it will have the correct pitch.
btw the 21 tooth gear for an auto is (PURPLE) DOOZ-17271-B just incase your dealer can't look it up and a (RED) manual 21 tooth is C4OZ-17271-A
the 23 tooth (white) speedo gear (which IS NOT a Ford part) has the wrong pitch for a Gt transmission drive gear and will wear-out in just a few k.
if you need to-go the 23 tooth route on a Gt manual trans, you'll have to swap-out the drive gear for one off a Cobra T-45 as it will have the correct pitch.
btw the 21 tooth gear for an auto is (PURPLE) DOOZ-17271-B just incase your dealer can't look it up and a (RED) manual 21 tooth is C4OZ-17271-A
Ok this has me confused now... the number of teeth changes depending on the specific rear end gears installed, right? So for my particular case (installing 4.10s), I would need a Cobra-spec 23 tooth gear? Or am I completely wrong. I'm trying to figure out how I would go about ordering the correct gear so I don't end up with the wrong one.
hmmmm, let try again. there is a big gear(drive) on the out-put shaft of the trans, base part # "17285". Cobra has 7 teeth and a Gt has 8 teeth and the pitch between the 2 is different.
now the VSS(vechile speed sensor) also has a gear(small) on the end on it, base part # "17271" (this is the driven gear). Ford only makes the small gear upto 21 tooth. the small 23 tooth gear that the aftermarket sells IS NOT a Ford part, I believe it to be a Jeep part used way back when Jeep CJ's had Ford 3 speed manual trans in them wth 4.56 gears.
now to make that 23 tooth "aftermarket" gear work in a GT trans, you'll need the Cobra 7 tooth drive gear(F6ZZ-17285-BA).
Though you've heard to not get the DM speedcal i'd still recommend going that route. If you use the speedcal you're using all stock parts + an add on electrical part...this is one time in which i'd trust the stock parts over a gear change any day. Plus after we installed the speedcal we took the car out with my dads GPS and my speedometer was on EXACTLY to the mph, 25 was 25, 50 was 50, 60 was 60, 70 was 70, you get the point. I doubt a gear will give you that much accuracy.