[diss7]

Sigh.
I get so sick of reading about these final drives.
They DONT give the car more power. All they do is shorten the gearing. Remember the mountain bike you had as a kid. Lets say stock car 6 speed is like only using gears 3,5,7,9,11,13. Changing the final drive in the car is like those 6 gears now being 2,4,6,8,10,12.

Get it?

Its the same reason the car feels faster in 2nd than 3rd, and 3rd to 4th.

You are making 3rd shorter, say like a 2.75, so its going to FEEL faster than 3rd was, but not quite where second was. BUT it won't go AS fast, because the gear is shorter.

Here is the easiest way to look at it.

Car has 4.1 now

If you are looking at getting 4.5, thats roughly 10% shorter. So go drive your car. Which ever gear you are in, look at the tacho, if the tach is reading 3000rpm, with a 4.5FD it will be 10% more, so 3300rpm, at that speed, in that gear.

a 4.8 would be roughly 20%.

IMO a 4.8 is too short, both for track and daily. On the track you DONT want to use 6th, as the drop from 5th to 6th is a big rpm drop. A 4.8FD will mean you'll top out 5th on the track, and have to chop into 6th.

For the street, a 4.8 means it'll suck on the open road. In 6th where it was revving 3000 - 3200 (depending on where you cruise) it'll now be 3600 - 3800.

Going to something even further like a 5.1 is even crazier. It makes 1st useless, and puts all the other gears close to where the previous gear was. 3rd will now be as short as 2nd etc. You'll just end up having a 1st gear like a van; and just use 2nd to take off.

Go on youtbue and search autofactory 86. They have a 5.1 FD in their car, on Fuji. Which was a stupid move. You see how soon down the back straight the car has to get into 6th, and you can hear the car just die in the revs.

IMO the need to make a shorter 6th gear available, that had much less RPM drop. Then you could use it as a track gear and really shorten the FD down. But this would also worsen the 6th gear cruise RPM. Which is why the 6th is where it is.

Sure you could argue its but the closer gears where you want them, and if thats what you want thats great. But you'd only do that if you only needed upto 160 - 170km/h on a track (which is slow) as thats all 5th will get to. Tyre dependant obviously.

Look at the track you are using it at. See what the max speed you're going to get is, and using ratio calcs, make sure 5th can reach that speed.

If its just for a street car, that you will use to cruise at 100 - 120km/h (60 - 70mph) get a 4.55. If you get a 4.8 it will ruin the car on the motorway. (Unless you like it singing at near 4000rpm) If you don't care about that get a 4.88.

I strees that you should do your OWN calcs. Its dependant on what you want, the compromises you are prepared to make. The wheel/tyre size you run. I run 205/40/17 tyres at the track, and I have a 4.55 sitting in my garage. However I wont put it in yet, as with that tyre and that FD I will outrun 5th at the track. So I'm waiting till I need to go to a wider tyre, which will mean taller, which will then mean using the 4.55FD, mainly to regain the gearing I have just using the smaller tyre.

Asking how a FD feels. It feels faster, in that gear. Because its shorter. But it doesnt go as fast in that gear now.

ALso, if you're getting a FD, consider underdrive pulleys. Because you are logically going to be revving the car more, so why give the ancillaries more drive than they need. Although there seems to be a massive fear/misconception of under drive pulleys on this forum, so its not even worth going there.

/rant.