I'm assuming here we're talking about the fully-adjustable "Racing Transmission" option.
Each numbered gear ratio has a decimal value shown beside it. There is a slider which allows you to adjust that between values of about 0.500 and 4.400, with additional constraints for each gear, varying by car.
And there is a similar number for the final drive, which can be varied from something like 2.700 to 5.000. (Those now appear to be the precise values).
The value of the gear ratio is the number of times the engine crankshaft must turn in order to turn the output shaft of the gear box, that is, the driveshaft, once. The value of the final drive is the number of times the driveshaft must turn in order to turn the driven wheels once. (FF and MR may not have a long drive shaft, but there will always be some equivalent input to the differential which connects the driven wheels).
If you multiply those two together, you get the number of times the engine crankshaft must turn in order to turn the wheels once. Larger numbers are called "lower gears" because the wheels turn less often (or less far) for each engine revolution. This allows better acceleration, but limits your top speed. (In fact, we use the number as a denominator in later calculations, which also means that smaller is higher and bigger is lower).
Let's call the product of those two numbers the "effective gear ratio". We can see that if you pick any particular "effective gear ratio", then, if you select different final drives, there will be for each a corresponding different transmission "gear value" which will give the same "effective gear ratio". Actually that's not quite true, since you may run into the constraints which limit the minimum and maximum values for a particular gear.
We have already said that "effective gear ratio" is the number of times the engine will turn to turn the wheels once. If you knew the diameter of the wheels, you could calculate the speed for a given RPM in a given gear.
Let
RPM = revolutions per minute (engine speed)
ER = effective ratio (no units) (that is, gear value * final drive value)
pi = ratio of wheel circumference to diameter (constant, no units)
WD = wheel diameter (in some units)
speed = (RPM / ER) * pi * WD
That will give you speed in units per minute, where "units" is the unit you use for wheel diameter.
Assume WD is in inches.
speed in mph = (RPM / ER) * pi * WD * 60 / ( 5280 * 12)
to convert from inches-per-minute to miles-per-hour.
Also involved in a precise calculation would be wheel slippage, but that's generally negligible.
Now, in [size=+1]
GT1[/size], one approach would be to infer a working wheel diameter by taking observations of speed at rpm, and working backwards through the equation. (Solving for WD).
I.e.
WD in inches = (speed in mph) * (5280 * 12) * ER / ( RPM * 60 * pi )
(It might be difficult to make precise enough observations to solve this accurately enough. Wheel slippage does enter into things here, but, except where slippage varies greatly in the same setup, you can actually work with an "effective diameter" which includes the effect of wheel slippage. I.e. the diameter you might get from your calculations might be smaller than the true wheel diameter for the particular car, but that's probably the value you want to use to calculate speeds at differing RPM and ER).
Edit: Actually, I was forgetting the wonderful calibrated gear charts that [size=+1]GT1[/size] gives. You should be able to read RPM and speed off that chart, and would know the ER for a particular gear. You should be able to use such a point from the graph to solve the above accurately. And, in practice, what you do is move the graph lines so that the attainable speeds in each gear are appropriate for the conditions--bypassing the need for much calculation.
In practice, an educated guess often works well to give adequately precise calculations.
In any case, each effective ratio ER will have a speed which corresponds to the redline of the particular car. To increase the speed at which the redline is reached, you must reduce ER (since it is a denominator). You can do that by reducing the value for the final drive (which is termed "using a higher final drive", since it is a denominator), or by decreasing the value for the gear being used. (Or both). (If you want to prevent hitting the redline in top gear, then use the slider to reduce the value of the top gear).
Summary: If you slide the final drive all the way to the left, and the top gear all the way to left, I guarantee that you will not hit the redline in top gear. In fact, you may not even be able to get in to top gear.
Someone please proof-read this, i.e. especially the equations. Someone who relates to starvation, perhaps. 
What you actually want to do is determine the ER you should use to achieve desired conditions.
speed in mph = (RPM / ER) * pi * WD * 60 / ( 5280 * 12)
MPH = (RPM / ER) * pi * WD * 60 / ( 5280 * 12)
ER = (RPM / MPH) * pi * WD * 60 / ( 5280 * 12)
With that you could plug in a desired RPM and MPH, and get back the required ER. Depending upon other constraints, you could adjust gear value, or final drive, or both, to give that ER. A case study would be good, but for that I need my PlayStation available.
And again, in [size=+1]
GT1[/size], the calibrated gear charts actually let you do precisely this without calculations. (Move the slider to achieve the desired speed at the desired RPM).
Case Study: Concept Car LM (USvsUK prize)
Take the Concept Car LM and restore its default settings. (Go to change parts, click the part, click the grey alternatives, then reclick the valid part).
You should get gears of
3.178
2.182
1.643
1.241
1.010
0.929
-----
4.292
There's a table showing
2nd 49
3rd 72
4th 95
5th 126
6th 155mph
8200rpm
Experimentation suggests that is the speed when the previous
gear reaches 8200rpm.
I.e. 3rd gear will achieve 95mph@8200rpm.
Eyeballing the chart suggests 3rd gear is 100mph@8200rpm, or maybe even 100mph@8000rpm.
3rd gear is ER 1.643*4.292
Plugging into
WD in inches = (speed in mph) * (5280 * 12) * ER / ( RPM * 60 * pi )
gives
95mph 8200rpm => 27.5"
100mph 8200rpm => 28.9"
100mph 8000rpm => 29.6"
A fairly substantial variation. But we can work with 27.5". (Checking an online wheel diameter calculator, all these sizes seem a bit large for reality).
For max speed tests, you always set the downforce figures to the minimum. But, even after doing that, the initial Max Speed test, using these settings, does not seem to go beyond 192 or 193mph.
In the Engine:NA Tuning "Change Parts" screen,
max torque is shown as 439.7lb-ft@6000rpm
max power is shown as 568hp@7000rpm
It seems like the car should be easily capable of 200mph. Let's try to achieve that in 5th gear, to leave room in 6th gear.
And let's try to achieve it at 6500rpm.
ER = (RPM / MPH) * pi * WD * 60 / ( 5280 * 12)
ER = (6500/200) * pi * 27.5 * 60 / ( 5280 * 12)
= 2.6589
Based on that, let's simply keep the default gearing, but slide the final drive all the way down to 2.700, changing nothing else. (Giving ER for 5th gear of 1.010 * 2.700 = 2.727).
Without changing anything else, the car now easily does 227mph, (that's if you have minimum downforce, remember) and that's before suspension tuning, which will help too.
Such a low final drive value is probably good for the Maximum Speed test. Perhaps raise the number for road racing, unless you see yourself running out of revs on the really long straights, like Grand Valley and SSR5.
Also note how close 6th gear is to 5th. Sliding it to a slightly lower number might be an easy way to get a bit more top speed, although you've got to watch the gap between 5th and 6th; at high speeds it's easy to get stuck in a bad spot on the power curve.
And, in general, if you tweak both gears and final drive, you can leave yourself more flexibility for future tuning.
Can i be a scientist? 
I be real good at it!
And after all, I meant it! 
I just meant in terms of being a great Gran Turismo 1 mechanic. After all, I've seen many ideas (and discussions) in some of Gran Turismo 1's threads about different problems each Gran Turismo 1 player is having with his/her car(s), or even about different viewpoints about the cures to each of the said problems. Or even just discussing some things about the players' cars in general in the game, that's all I meant. And when I said "Website's top leading scientist", I was trying to come up with a cooler sounding name than just plain old "Great Gran Turismo 1 Mechanic". I mean, doesn't my first name sound a WHOLE lot cooler than my second name? Plus, I tried to come up with that name for my new avatar of Vexen. How about this then; what if I called myself a "GT1 Mechanical expert"? That okay?
And by the way, what does "Senior Member" mean?
As far as i know, there are no technical newbs present who haven't got a clue about engine revving, so far as i know. But there could always be some folks out there lurking, so here goes. 
