danzcool

5th gear on a stock 2005 Mustang GT is a 0.68 overdrive gear, 4th gear is 1:1

Pretty much right on... and yes, the stock tune de-fuels at about 145, and so does the stock whipple supercharger tune.

Yeah some sort of forced induction, oh, and swapping out for a 6-speed that adds a gear between 4 & 5. And eliminating that speed limiter at 145.


but really, figure out what you want your top speed racing to be, and what kind of racing you plan to do, and then choose which gear you want to be in to get there... then calculate what tire size and rear gear you will need to get there, after that, start adding power to get there quicker.
For autocross, I think a lot of people design around 3rd gear being the highest gear they will use, for drag racing if not using an automatic, 3rd or 4th are used depending on power and rear gears, there's something to be said for fewer shifts. For road course you may hit 5th on some of the longer straights. For 1/2-mile and 1-mile shootouts, it's mostly going to be about your own trial and error since you'll never be happy with you trap speed.

SkizZy

Any ideas on how to fix the tach and an estimate of how much it would cost?

Stkjock

so a 6 speed goes 1-2-3-4-4.5-5???

danzcool

WHATEVER
For all practical purposes, when comparing the gear ratios to the 3650... it's like getting a gear between 4th and 5th. Is that better for you?
3650 gear ratios:
4th=1.0
5th =0.68

T56 magnum XL close gear ratios (or TR6060 Ford spec):
4th=1.0
5th=0.8
6th=0.63

so 4th is the same on both, 3650 (5th) & T56 (6th) are nearly the same at 0.68 & 0.63, with the T56 (5th) at 0.8 which is pretty much in between the ratios for the 3650's 4th and 5th.

ticopowell

My first guess would be that it's a bad stepper motor... but it could also be a bad sensor in the transmission or bad wiring between the sensor and the pcm.

If everything else works on the dash I would plug a tuner in and see if that gets a good mph reading on a datalog, if it doesn't then you know it is the sensor/wiring. If it does then you know it is in the gauge itself.
The sensor is at the back of the tranny near the driveshaft, the stepper motor is soldered in so you will need someone with knowledge and experience with that to reinstall one, or look up on youtube.

If you don't have a tuner find a friend that has one that works for your car, it should be able to read the mph(while moving of course) to let you know what's broken...

I'm not a mechanic, but this makes sense to me on what could be wrong

Simon1

5th for the FI guys is very useful on the freeway. But I try not to go too crazy in 5th as speeds get pretty high really fast. A long stretch of highway, with little to no traffic is a must.

I would imagine that guys who do things like the standing mile and such get their cars tuned in 5th as well.

And 4th is a little more usable for me also. My valve train is all comp, manley and another aftermarket for the lifters. I forget which one, but I can spin it up to 7500 without issue, even though I keep the limiter set at 7k I think.

It would be fun to know what my car tops out at. I went 160 several years ago when I had 3.55s, seemed like there was quite a bit more left. But now I have 4.10s and ~29" tires. High speeds are fun, but really something most shouldn't mess with unless you are trained and experienced in performance driving.

PNYXPRESS

Is this whole thread a joke?

Stkjock

unless of course you get the TUKT12019, then the ratios are: 2.66 1.78 1.30 1.00 .74 .50

Simon1

Absolutely!

Insert punch line when you are ready.