Gun Jam

I also have the performer intake and I believe the cam as well.

a 3.80 will provide more response at low RPM than a 3.55.

Which rear end gear is in there now? a 3.00? Not much is going to work right with a t5 and a 3.00 its going to be a lot more drivable with a 3.80. If I knew the exact gear in there now calculating freeway RPM is easy.

-Gun

MBDiagMan

Assuming this is in a Mustang with near stock diameter tires, the car should be light enough for the 3.55's to work well with a T5.

_Remi

Yes, it works today with 3.55 but since the cam is more of a RPM type of cam, I was thinking that this should matter as well.

To answer Gun, it has a 3.55 today, it is ok but still a little too low in 5th gear and under 2k on any gear.

jspagna1

With a T5 you can go with the 3.80 gear. I wouldn't go any deeper though,4.11's are too low for the street IMO.

Gun Jam

there is no way you can turn 2000 @ 80 with 3.55s unless your total rear tire height is 30"

Is this indeed the case???!! If not its because your speedometer is reading wrong because you have the wrong gear at the end of the speedometer.

What is the rear tire height (measured down the center of the wheel from ground to the very top of the rubber)

-Gun

_Remi

The speedometer is probably off, here is a picture. The wheels are 225/60/15 by the way. I was just saying that the car is not super happy in 5th gear when below 2k RPM.

urban_cowboy

That is because you lose more mechanical advantage as your gearing decreases. Your engine is also not producing much torque in low rpms, so that is why it is a dog in 5th gear below 2000rpm. This is not an uncommon problem. With a lower gear (higher number) in the rearend, you will gain more torque through more mechanical advantage in all the gears. If you were running a torque monster, this may not be a good thing because you then have traction problems in 1st and 2nd, but with a 289 you do not have a torque monster. I would think something in the 3.80 range would help with more torque to the ground through all gears, but still not raise the rpms a bunch at highway speed because you have an OD. That is the beauty of an OD tranny...you can run more rearend gear giving you more torque to the ground throughout the gears while still keeping the rpm on the highway below 3000rpm at 70mph while in the OD gear.

I do not run an OD in my car, but I like it to sing when I drive it and I am not using it as a DD.

Gun Jam

based on that a rough actual tier height is about 25" even

so at 2000 rpm in 5th you're actually going 66 mph bet you wondered why that semi was going 85 hehehe.

SO if you switched to 3.80s at 2000 RPM you would be going 62
OR at 80 MPH 2600
OR 2400 RPM at 75 MPH

3.80s would give you a perfect freeway RPM 2400 is about perfect its not bogged down and its not over revved by any stretch of the imagination.

BUT

The above assumes a .63 OD IF you have the .67 OD which is more common
THEN:
Right now you would be going 62 MPH at 2000 RPM with your 3.55s
With the 3.80s at 80 MPH 2750 RPM
With 3.80s at 75 MPH 2570 RPM

SO despite which t5 you have the 3.80 is still an excellent choice especially if you feel the 3.55s are a bit doggy.

Now just for fun lets say you nut up and buy 4.11s and you have a .67 OD t5

2950 @ 80 mph
3350 @ 90 MPH
2800 @ 75 MPH

Depending on how much freeway driving you do 4.11s are entirely doable. 3000 RPM at 80 is no big deal although fuel mileage is going to drop off at 80 with 4.11s when compared to 3.80s I consider anything above 2600 to be above optimal engine RPM resulting in fuel mileage loss. But as you can see running 4.11s would not be murder.

-Gun

_Remi

Thanks, I think I am now convinced that 3.80 makes more sense with a Performer RPM application.

Gun Jam

Lower gearing will always increase torque mechanically as Cowboy described.

Now your only real problem is trying to decide between 380s or 411s

how much driving is at or above 70 mph?

-Gun