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changing diff ratios

2K views 8 replies 4 participants last post by  WBT56 
#1 ·
For anyone who has put in 3.73 or lower on thier falcon, was it worth it? I am thnking of 3.7 or 3.9 gears for my XR6 (EL), just wanted to know if it worked better on the quarter, and how much more it revs to on the hgihway. Mine sits at 2100rpm in 5th. I don't do many highways kays, only up to melbourne then and when. Although we would all love better fuel economy, it's not that much of an issue to me. I would use more fuel to go faster anyday!
 
#3 ·
the revs on the freeway also depend a lot on your gearbox ratios.

Cobra as you can see by the other thread in this section, im now trying to orgainse a 3.9 diff for my car, im getting a housing+ axles etc next thursday, it will depend on how long the shop takes to build it as to how long it will take to get in the car.

currently it sits on 2000rpm in 5th gear at 100km/h with 3.45:1 diff (which you also have as your car is an XR) and i figure it will sit on 2255rpm at the same speed with the 3.9s.

formula is:

rpm (at whatever speed you choose) divide by current diff ratio, then multiply by new diff ratio -

2000/3.45*3.89 = 2255

so if your 5th gear in your gearbox is the same as mine, you should have no dramas.

the only thing im wondering about is 1st - its already pretty short now, with the 3.9s it might be too low.

if you are willing to wait about 2 weeks i can give you a definitive answer.
 
#4 ·
Hey thanks Rollin. Yeah I anticipate your results. It's not like I need to know now or anything. Just trying to get info for a mod that i will eventually do down the track.
 
#5 ·
Rollin said:
2000rpm in 5th gear at 100km/h with 3.45:1 diff (which you also have as your car is an XR) and i figure it will sit on 2255rpm at the same speed with 3.9s

the only thing im wondering about is 1st - its already pretty short now, with the 3.9s it might be too low.
For those interested, there is a way to fix this. You can build a direct bolt in replacement V8 spec T5 for an I6 with a 2.95 first instead of the 3.28 or 3.5 version. You can run an ultra high fifth gear also, so it will do 2040rpm @ 100km/h with 3.9 diff. This can be done with XR6T spec internals.
 
#6 · (Edited)
WBT56 said:
For those interested, there is a way to fix this. You can build a direct bolt in replacement V8 spec T5 for an I6 with a 2.95 first instead of the 3.28 or 3.5 version. You can run an ultra high fifth gear also, so it will do 2040rpm @ 100km/h with 3.9 diff. This can be done with XR6T spec internals.
Yeah but how much is that going to cost? The price of doing that would probably excessivly override the bit of extra fuel that you will save.
 
#7 ·
Cobra said:
Yeah but how much is that going to cost? The price of doing that would probably excessivly override the bit of extra fuel that you will save.
The cost would be no more than buying a rebuilt T5 anyway. The point was that you can go the low diff gears for performance in the first four, and still have the economy in fifth.
 
#8 ·
WBT56 said:
The cost would be no more than buying a rebuilt T5 anyway. The point was that you can go the low diff gears for performance in the first four, and still have the economy in fifth.
While having a more durable T5 aswell? XR6T runs the V8 gearset, but it's supposed to be able to take more torque too. How much better would this gearset be combined with the 3.9 diff for performance? And how much is buying a rebuilt T5?
 
#9 ·
Cobra said:
While having a more durable T5 aswell? XR6T runs the V8 gearset, but it's supposed to be able to take more torque too. How much better would this gearset be combined with the 3.9 diff for performance? And how much is buying a rebuilt T5?
This is as good as the T5 gets. This is T5Z specs. This box with 3.9 diff is the same O/A first gear as an XR6 box with a 3.5 diff, so you don't ruin first gear. Performance increase will depend on state of tune and engine mods. A fully rebuilt V8 spec T5, with all new parts except for cases and mainshaft, will normally cost about $2200- $2400 outright. (no exchange)
 
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