Miata v8 7.5 diff
 

I am hoping for some feedback on the best street 7.5 ford diff ratio using a t56 5speed with 16 in tires and rims. The gear in my car now either a 3.08 or 3.27 and is hard to drive at low rpm's. Car has a 5.0 ford crate motor. I use it for strickly pleasure.

Thanks for any feedback.

My 7.5 is a 3.27. It works fine with my T5 and 331 Ford although the jump from 4th to 5th is a bit much on the track.

-Jason

thanks.

You are saying t56 5 speed but the t56 is a 6 speed. So is your car a 5 or a 6 speed car?

If it is a t5 the 3.27 that is probably in the car now is a great gear but it would be too low for a 6 speed.

It is a t5 5speed. I guess a better tune might be the area I need to focus. Thanks for your help.

What's it doing you don't like? Or what are you trying to change for the better?

I am running 16' tires, T-5 Trans, & 327 gears. What type of driveability issues do you have?

It does not want to run smoothly in traffic. Hard to drive and enjoy. Seems to want to lurch when driving under 50MPH. Always having to downshift and go in a lower gear (4 to 3rd) and keep over 2500RPMs. Hard to drive at 2000 RPM's in even 4th gear. Makes for terrible gas milage when driving always in 3rd or 4th gear. That is why I think I need to go back and put it on the dyno and maybe retune. The cam is a 498 intake and Exhaust lift with 220 degrees duration, 30 lb injectors, Walbro 255 fuel pump, 73MM throttle body, 76maf, reworked ported Cobra upper and lower and puts 296RWHPand 310 lbs torque. I have put about 2,000 miles on this new motor.

Runs great when you just excelerate and blow through the gears. Just awful when driving around 1800-2400RPM's.

Thanks for everyones help and replies.

My first build was exactly the same way. There are two things that can be done to help eliminate what is essentially "driveline slop":

1: Make sure the VSS (vehicle speed sensor) is hooked up and operating.
-The VSS tells the ECU whether the car is moving or not. The ECU then uses this information to decide if it needs to implement the "dashpot function" when you let off the gas. If you let off the gas when the car is still the ecu will just let the rpm's drop naturally. If you let off the gas while moving the ecu will slowly let the rpm's down using the IAC valve. If the VSS is not hooked up the ECU always figures the car is not moving and lets the RPM's crash. This may not be noticeable in a car with low compression but the more performance oriented the engine is the more braking effect the engine will have causing what you're describing.

2: Reinforce the transmission crossmember.
As designed, the Monster Miata crossmember is simply a piece of flat steel which has the tendency to act like a leaf spring (also a piece of flat steel) when you let off the gas. Again, this is not such an issue with a low compression (stock) 5.0 but with higher compression the braking force is going to want to make the rear of the trans move up and then shot back down about 1/4 inch. To stop this in my most recent build I drilled a hole down through the passenger floor pan, through the crossmember and bolted it to the car there. This can be seen in this thread:

https://www.v8miata.net/build-thread...age5/#post4307

This was by far the best $2.00 I have ever spent on the car. Before this mod I could not cruise below 2500 rpm's even with my VSS hooked up. After I can let it down to almost 1000 rpm's without much drama.

Try both of those fairly cheap fixes before looking at changing out your trans or rear gears.

Hope that helps,
-Jason

Ha Jason posted at the same time I did. Basically I was going to say he would have the answer anyway!

thanks for the tips. It is awesome to have a web site to ask for HELP. I will let you know how I make out.

You're welcome, we aim to please!

Do please let us know how it works out,

-Jason

Thanks Jason! I love this solution!
R,
Chris

Surging
 

I know that this is an old thread. But one thing that was not mentioned that will cause surging and or hesitation at sub 2200rpm speeds especially in taller gears (4th/5th) are bad O2 sensors. When the sensors fail or are out of range they will wreak havoc with the fuel trims and make the car a pest to drive slow on very light throttle sub 2200 rpm (closed loop) . Full throttle (open loop running) miraculously fixes the issue.

So always invest in good quality (NTK or Bosch) and NEW O2 sensors for any build. They need replacing every 50,000 miles or so to stay accurate enough for the ECU to make the right adjustments.

cheers

Cheers

As engineer mentioned, this is an older thread, but since he posted and it popped up for me today I thought I would drop a piece of info in here. Just saying you have a T5 transmission when asking about gear ratios is not enough. Not all T5's have the same gear ratio's for each gear. If you got your trans out of a truck for instance, it will have a totally different first gear than a car T5. And there are many different versions of the T5 for cars. Here is a link to at least one chart showing the different gear ratios in T5's. TREMEC T5 Transmission -Modern Driveline. Hope this helps. I know I was very careful when buying my T5 and diff.