Last few long trips I made with the FD were with the JDM trans with .806 5th and 4.10 gears. And 265/40/17 tires on the back.

MPG seemed comparable, low '20s, which is what I normally get. The difference in RPM isn't a lot just going to the JDM trans - about 300 RPM.

Dale

I'm running the same tires, so I should have the same results as you. Thanks for the info, that does put my mind at ease. Can't wait to drive My FD with the new trans.
Thanks again, Dale

After a gearing change as is being discussed here, can anything be done to the stock cluster to adjust for the speedometer error that will result?

Maybe there is a market for new speedo faces that have the numbers slightly re-spaced for various common gearing combinations?

Adjust the digital signal going in to speedo

Considered it but it is a street car that I autox rather than vice versa. I drove Alan Densmore 368rwhp with 4.77 gears, nice bit not on the street.

i'd have to look, but if its a factory option, it'll either be right, or the correct gear(s)will be available.

There are 5.125 gears that fit the TII/FD/RX-8 now........

I'm not sure if this is common knowledge, but the stock speed o gauge has a potentiometer in it and can be adjusted for speed. I figured this out when I had removed my cluster to change the bulbs. Being the tinkerer I am, I took everything apart and unfortunately ended up turning the screw on the side of the speed o circuit board. As soon as I turned it I realized what I had done. I turned it back to what I thought was the original position, but when I test drove it was off by an easy 20 percent. It took two more attempts to get it set to my GPS. So if you have a GPS and patients I'm sure it can be adjusted for most gearing(This makes sense to have a potentiometer because you can use the same cluster for MPH and KPH and the several available factory gear ratios)

my speedo is off by atleast 5-6mph.. kinda hate that tho.

Same here. I have 275/40/17 in the rear.

Yep, I've seen that pot on the speedo. That makes sense, they calibrate them at the factory that way. Would take a GPS and a few runs to get it dialed in.

BTW, if anyone has a bad US transmission they're willing to take apart and sell me the 5th gear, I'm lookin' :).

Dale

235/45/17... how wide are your tires?

275/40/17 on a 9.5" wheel.

I dont remember it being this off when i was running 265/35/18 on a 9" wheel back in the day.

My speedo reads high by by about 5 mph at 70 mph, and everything is stock including tire size.

could the speedo sensor just be going bad? i mean that is something we could be over looking.. dont know if a new one would fix this, but would be interesting to find out.

so i was looking into this since my 5th just started to feel a lil grinddy =( and found that rhdjapan sells the OEM 5th gear synchro ring at a pretty good price ($35.33) but was wondering what ratio it was ??? the part # is R503-17-265B but thats all it tells me. heres the link

http://www.rhdjapan.com/mazda-oem-5t...zer-fd3s-53244

I may very seriously consider going with 4.4 or 4.7 gears if RZ ever comes back with the info on a t56 swap kit. I would love to get some lower gears, but I'd need a few more overdrive options to bite the bullet. A t56 with a 0.8 5th gear and a 0.6 or 0.5 6th gear would be excellent.

Just an update on my end...

First off, the 4.44's are GREAT. Really made a seat of the pants difference.

With the JDM 5th gear, it's REALLY obnoxious on the interstate. You easily get up to 4000 RPM cruising. After I get some other projects out of the way I'll be putting in a US 5th gear, I have one ready to go in now.

Also, the speedo is now WAY off, like nearly 10 MPH on the interstate vs. my GPS. I'm going to wait until I swap the 5th gear out, but I'd like to see if the pot on the speedometer has enough range of motion to fix it.

Dale

From where can you get this?

JT

Dale, how much power are you making? What turbo(s) and spool speed?

Keep us posted on the fifth gear. I have my old tranny that needs a new main bearing. Going to open it up and completely go through it.

I also have a .806 JDM 5th gear, but with a 3.909 diff.
The stock engine was only putting out about 225 whp while mine and many others are in the 400+ range.
The lower gears are too short and hard to get traction in. The 3.909 diff helps with this and also helps to get a little better mileage.

With 255/40 - 17 tires, my speedo is 2 mph low at 80 mph.

Back in the day I bounced around the idea of installing the 4.33 rear end, but later after going from 15 PSI and 400+ rwhp to 25 psi and 500+ rwhp I quickly realized that any shorter gearing would not be a good combination with the amount of power I lay down. Anyone that plans on major power should not go with lower gearing unless you just want to see a lot of smoke.

Sequential '99 twins. Making around 300-350, been ages since I dynoed.

Dale

I'll consider changing gears lastly then. After I get up to the 450+whp range, I may be fighting for traction with anything lower than 4.10s. Plus, I want to see what RPMs I cross the traps at in the 1/4 mile so I can adjust accordingly.

When I drag raced my 03 Cobra, I switched from stock 3.55s to 3.73s because it put me right at my shift point in 4th gear as I crossed the line. I picked up a solid 2 tenths with no other changes even though it's a seemingly worthless drop in gear ratio. If I had gone any lower, I'd have had to shift again or ride through outside of my powerband for the last bit, so 3.73 was perfect with my tire combo and RPM range. The restoration part of this project is getting old, I want to be back at a track!