Final drive higher or lower for auto turbo?
 

Hi everyone. I'm thinking of swapping my final drive to acheive a higher top speed in my 86. I am boosted 300whp in my auto 86 but 5th gear is still useless. My question is will going for a higher final drive (4.556) be more beneficial than a 3.9 fd if the only goal is to make 5th gear usable.

what you mean with useless.? you find it too short now? with 4.56 will become even shorter, with 3.9 longer

4.56 fd will be slower top speed. If you want more top speed you need to go a lower number than 3.9 fd. ie 2.9 direction.

Good website to give you an idea of max speed changes per gear. Google the ratios and tire diameter, etc. Diff ratio will be what you adjust based on your FD choice.
https://www.blocklayer.com/rpm-gear

On a stock car, here's following top speeds on 5th gear based on your FD.
4.6 FD: 118
4.3 FD: 126
4.1 FD: 132
3.9 FD: 139
3.7 FD: 146

Technically, your 5th gear should be the least of your worry. How often are you in the 100-146 mph range that your 5th gear will mean that much to you? Usually people complain more about the lower gears, as with more power, you are going through the gears quicker, making 1st and 2nd gears more "useless" as you say. So lowering your FD to something like 3.7 will give you more time in each gear and help reduce tire spin as well.

I haven't seen this website before. Thanks!

The automatic gear ratios make 5th gear useless because it is almost like 6th gear in the manual. My question was if the short final drive can make 5th gear useable or should I just go with 3.9 final drive so my 4th will reach a better speed.

https://scontent.fmnl17-5.fna.fbcdn....yg&oe=638DB9C3

for reference this is what i'm talking about. 5th Gear in automatic is practically useless at 4.1 fd

Thanks for this it seems 3.9 is a little bit better in this case.

I don't think that is what he is asking. Useless as in what context? What are you doing with the car where it is an issue?

Example: I find 1st and 6th useless on track though the gear spacing is fine for all the other gears. All gears I find useful on the street.

I totally missed over the fact that you're referring to AT. With different gear ratios, the data I listed is incorrect, as I was referencing MT ratios. Often, with AT on this platform, for better acceleration, you are better off going with a higher FD ratio - such as 4.56 and up. This is up to you of course, as it will cost you mpg and comfort of drivability with higher RPM in 6th gear on highway cruising. If this is something you don't care about, you will certainly benefit from going with a higher FD. But then again, you need to consider your current grip levels, particularly in the early gears - which I would think aren't as heavily affected in an AT compared to an MT.

So if I understand your question correctly, a higher FD like I mentioned in your AT will make 5th gear more "useable" as it will have more torque.

EDIT: Disregard my conclusion. You want a higher top speed in 5th based on your OP, then you want a lower FD.

Hmmm if you drive mt try to imagine going from 4th straight to sixth gear, that what I meant by 5th gear being sluggish and almost unusable even if i'm boosted. It's like for example I can reach 130mph in 4th gear that probably as high as i'm going to get as the moment I shift into 5th it just bogs down due to the long gears. So I just thought maybe a higher final drive would act like a shorter 5th gear or it would be better to go lower so I can make the most out of 4th gear totally disregarding 5th.

I know most people here don't really care about top speed but I was just genuinely curious if you could adjust that as I live in an area where such speeds are achievable legally (for the most part) Hope my post doesn't confuse others sorry.

it's useless as in when you shift in to 5th it's just for mpg and cruising. The Mt and AT gear ratios are different. Just genuinely curious if something could e done about that as changing gear ratios in an AT is not worth the effort.

then would be stupid to put like 3.9 finaldrive in your car. You 'd want 4.56 or sth higher in number, as you wouldn t have the torque necessary for enjoying the car

I think 300 hp is nice Regardless of FDR