Speedo and Tach help with 4.10s
 

I use to have 3.73s but have recently changed to 4.10s. I had the correct speedo gear installed for the 3.73s and it was just right. Now that I have the 4.10s in, it seems to be off about 10mph. Is there even a correct tooth gear for the 4.10s???

Could this problem also make my tach way off too???

Thanks,

Tony

Re: Speedo and Tach help with 4.10s
 

I'm sure they have a gear for most any combo.
I don't see any way that could affect your tach.

Yeah...maybe it's just because it's the stock tach which I guess could be off by a couple hundred rpms...

speedo gears
 

i have 4.10's and was read on another forum that i need a black 6 tooth drive gear for a t-5 and a black 22 tooth speedo gear for t-5.i think you have to pull the trans to replace the gear drive.that is suposed to make it dead on m.p.h. ihave not done this so i cant say if it works that my FYI.

Best way to do it is to go out on the highway and put your speedo on 60 and time how long it takes to go a mile (use the highway markers). Take the number of seconds it took to go a mile and divide 3600 by that number of seconds (so if it took 63 seconds, you'd have (3600/63) = 57.14. That's how fast you were actually going. so then, take 60 divided by your actual speed and multiply by the number of teeth you actually have on your speedo gear, and that will give you the number of teeth you need. then you can go to ford and ask for a gear with that many teeth. So say your actual speed when your speedo says 60 is 52, and your speedo gear has 21 teeth. you take (60/52) X 21 = 22.15 so the closest you can get to being correct is a 22 tooth speedo gear. Unless you change the drive gear in the tranny like someone said. But, actually, you should be able to take your new rear end gear ratio, divide by your old rear end ratio, and multiply by the number of teeth your speedo gear has, so like (4.10/3.73) X 21 = 23.08 so 23 teeth. See what I mean? Oh, and there's no way that your tach is off because of a rear end gear change. As a matter of fact, when you don't change your speedo gear, but you change your rear end gear, if you just look at the gauges, you won't be able to tell there was a change, because if you used to cruise at 70 in 5th gear at 2500 rpm, it will still say 70mph in 5th gear at that RPM. Untill you change your speedo gear. If you have any more questions, just ask me.

You can get a 23 tooth gear for the 4.10's from livernois motorsports. It is like $20.

yea the

I already have the 23 tooth speedo gear and it's still off. I'm changing my output shaft drive gear today and hopefully that will resolve my speedo problem...

I'm also hoping that this will solve my rev limiter problem too. The chip I have was tuned for 3.73s. Since I put 4.10s in, the car constantly hits the rev limiter at about 5400 rpms. I'm hoping the speedo caused the VSS to not work right and thus made me hit the rev limiter. If not, then I guess I'll have to have the tuner re-check the rev limiter setting on my chip.

I know this is an OLD thread but...
 

but maybe it can help someone else...

The correct way to do this is go to Hanlon Motorsports and use their utility to figure out the correct drive AND driven gear, not just putting in a 23-tooth gear.

I had mine done and it's within 1mph or so at 60.