I am looking at swapping the rear ratio in my 84 12a road race car. Does anyone know what top speed I might see doing the 4.78 Kia swap? I don't want to run out of gears on some of the longer straights.



Thanks ahead of time.

 

You will need to share your gear ratios and tyre sizes so someone can calculate your theoretical top speed. Obviously wind speed and resistance can't be calculated but it will give you a good idea.

 

Here a calculator you can enter all you details in. http://www.f-body.org/gears/

I did it based on a 254/50/16 and the car will do 196mph in 5th at 8500rpm

 

My question was pointed toward anyone else that may be running the same setup. I was hoping for a real world speed, so I can decide if I need to go up or down before I install the gear set.

Thanks for the website though.

 

it really depends on the track.

For example 4.7 gear on my home track is ideal and your top speeds are 120mph tops no matter what car you drive. My home track is a technical track with many tight corners and not a high speed one. I have a 4.7 in my race car with similar tire sizes and gear ratio's to the FB and you will suffer a lot of top end.

Realistically, the stock 4.11 is adequate for most long legged tracks.

As with Grant M's comments, your question without further information is extremely ambiguous.

 

We ran an ITS prepped 2nd gen for quite a while and the 5.12 rear gear with the GTUs OD gear in the trans was the hot setup. Most tracks we'd see about 130-ish top speed, so we had plenty of gear. The only place I can think of that we could of gone and run out of gear is Daytona.

The idea with the 5.12 is to have a rear gear that's short enough to allow you to run 3rd, 4th, and 5th instead of 2nd, 3rd, and 4th. The top three gears are spaced closer together so you don't have that big RPM drop from 2nd to 3rd. Overall it's a much quicker setup than running the 4.10 because of this.

I looked through a bunch of different trans ratios from the 1st and 2nd gens and Miatas back in the day and did come up with a combo for a good cheater box. If you're not limited by rules, the hot setup would be one of the 1st gen boxes fitted with the GTUs OD. Forget which box it is, but one of the 1st gen 5-speeds has a lower input ratio than the rest. That box, paired with a 4.88 final drive, created a box that had four useable speeds on track versus three. The OD ratio dropped to a .8xx versus the GTUs .756. It worked out that you'd see something like 135 at 8500 in the ITS car, so it would of been a perfect cheater setup. Sadly, I forget just which 1st gen box would of been the basis for the configuration.