88clone - how do you know the people giving you the info have accurate speedos and tachs? Garbage in = garbage out.
You can calculate the speeds pretty easily. But remember, you don't know if your tach is accurate -- and all of this is gonna be based on the rev count that's showing up on you un-calibrated tach AND on your rear tire size. So take it with a grain of salt.
Speed is in units of miles per hour. If you know how fast the engine is revving (revolutions per minute) - you can convert that to revolutons per hour. If you know the gear ratios in the car, you can figure out how many revolutions per hour the rear tires are turning. Lastly, if you know the diameter of the rear tire, you can calcultate the circumference -- and you'll know how much distance (in inches which can be converted to miles) you've covered in, voila, miles per hour.
Let's start with rear tire size. Let's assume you have one of the stock tire sizes on the car. 225/55/16. To calculate the height you convert the section width to inches -- 225 mm / 25.4 inches/mm = 8.86". Multiply that by the aspect ratio to get the sidewall height - 8.86" X 55% = 4.87". Now add the wheel diameter plus 2X sidewall height (top/bottom) to get tire diameter/height - (2 X 4.87") + 16 = 25.74". To calculate the circumference of the tire you simply multiply the diameter times the constant pi - 25.74" X 3.14159 = 80.88". So each time the rear tire turns 1 revolution, it covers 80.88 inches. To convert that to miles - 80.88" X (1 foot/12 inches) X (1 mile/5280 feet) = .001276 miles. So each time the rear tire rolls 1 revolution it covers .001276 miles.
Now, let's move to the engine/driveline to see how many revolutions that tire is making at a certain rpm in a certain gear in one hour - that'll give us mph.
We'll do our calculations at 5000 rpm - your engine is mostly stock; it's power peak is gonna occur around 4600 rpm. To get quickest acceleration times, there's no reason at all to even rev it above 5000 (especially with 2.73's) even though it will rev to the ecu cut off at 6250. You're just wasting fuel, making noise, putting wear/tear on the engine and accelerating more slowly.
So, at 5000 rpm in 1st gear, how many revoltions will the rear tire make in an hour? The first gear ratio in a stock T5 is 3.35:1. That means that for every 3.35 revolutions the engine makes, the driveshaft turns once. Similarly, with 2.73 gears in the back, for every 2.73 revolutions the driveshaft makes, the tire makes one revolution. So, in low gear, you multiply those ratios to get the overall effect -- 3.35X2.73=9.1455 --- for every 9.1455 revs the engine makes in low gear, the tire makes 1 revolution. At 5000 revs/minute -- the engine covers 5000 X 60 revolutions per hour = 300,000 revs/hour.
So, let's put it all together. At 5 grand in low gear ---
(300,000 revs/hour)/(9.1455 revs/tire rotation) X (.001276 miles/tire rotation) = 42mph.
Second gear ratio is 1.94 -- so 9.1455 in the previous equation becomes 5.296 -- 2nd gear @ 5000= 72 mph
Third gear ratio is 1.29 --- @ 5000 = 109 mph
4th gear ratio is 1.00 --- @5000 = 140 mph
5th gear ratio is .675 --- @ 5000 = 208 mph
At 6000 rpm those numbers become - 50, 86, 131, 168, 250 -- but your mostly stock car won't pull to 6000 in 4th; and don't forget -- the factory tachs are COMMONLY known to read as much as 1000 rpm high on the high end of the scale. A clue is the number of people who claim to rev to 7000 rpm without hitting the ecu's rev limiter which comes in at 6250 rpm.
Most magazine tests of the day reached top speeds of 137-142 for the 5.0 foxes -- and they did that in 4th gear, not 5th. Play with the math above and you'll see that 140 mph in 5th gear works out to about 3400 rpm -- at that rpm the engine isn't making enough HP to push the car through the air at 140. Conclusion? Shift to 5th at 140 mph (5000 rpm in 4th) and a stock (or mostly stock) 5.0 Stang will slow down, not speed up.
Put 3.73 gears in the car -- and here's what you have, all speeds at 5000 rpm:
1st - 31 mph
2nd - 53 mph
3rd - 80 mph
4th - 103 mph
5th - 152 mph
4.10's -- give you
1st - 28 mph
2nd - 48 mph
3rd - 72 mph
4th - 94 mph
5th - 138 mph
You can start to see that with mild HP numbers, and steep gears (4.10's), you might want to hold the car in 4th at the end of the 1/4 and let the revs climb, instead of shifting to 5th -- probably give you a quicker ET.
With the info above - you can play with different tire sizes, different revs, and different gear ratios. My T5Z has different ratios - instead of 3.35, 1.94, 1.29, 1, .675 -- it's 2.95, 1.94, 1.34, 1, .625. My car also has a shorter tire -- 245/45/16 is 24.7" tall - so the numbers work out slightly differently for mine w/3.73 gears
1st - 33 mph
2nd - 51 mph
3rd - 74 mph
4th - 98 mph
5th - 149 mph
My speedo and tach are fairly accurate -- those calculated numbers match up pretty closely to what my instruments are telling me.
Too much time on my hands this morning...
And please don't point me to one of those calculators cus they give me a headache. 
