Couple Questions
 

After I install the new gears, I'll have to get a speed cal right? And that corrects the speedo? Now what about the mileage? Will they accumilate faster since the axle is spinning faster?

I'll have mine installed in 2 weeks and I'll let you know. I hear from others that the fuel milage loss is marginal.

I'm talking about miles on the odometer....

99 and up mustangs, get a tuner to fix the speedo, 98 and below cars need to get a speedcal or a chip burned for their car.
The speedcal, you will have to wire it to the vss, by moving some wires from that connector to a connector they supplied, also one wire goes to your cig lighter wire for power, plug it all in, set the dip switches and your good to go. to set the dip switches, the place you buy it from should have a calculator, like dallas mustangs, they also have a forum were you can ask the tech what to set them to. I had to do this for my Tranny swap, i went from a mechanical to a electrical vss, so it works for tranny swaps too.
If you don't get one or the other at first, then your speedo will say your going faster than you really are, which you will add more miles, than what your really doing, to your car.

Thanks, So a tuner is another option? I could take the $100 I would be using for the speed cal and put it towards a tuner. That would calibrate my speedo and odometer? It's just that messing with the cars computer scares me a little bit.

gotcha...should have specified.

a tuner will be more $$, but will correct your speedo and give you a little hp as well. xcal2 perhaps.

The speedcal 100 bucks, the tuner 299 and maybe up. The speedcal is only for gear or tranny changes. The Tuner can do the gear changes, shift points for autos, performance tune, tire selection, and others.

Once you do one or the other, it will fix your speedo problem so you are going the right speed while driving. The speedcal tricks the tranny vss electrical pulses, the tuner tells the computer that you've changed the gear ratio. Both works, it's just the price/options you want to go with.
I've known several people with the tuners and no computer problems yet.
I had to rewire several wires on my 95 GT for it to accept a 89 mustang's A9L computer, now that's scary.

I planned on getting a tuner sooner or later. So I guess this gives me a reason to get one. I like tuners especially the SCT but they are expensive.

expensive, yeah ok..but not when you consider you can use it over and over and it can make your car better everytime you upgrade. That's why I am getting one at least. Seems like a good performance investment.

But when you use the performance tune, you have to use premium gas. I guess thats good if you already use premium. But I use regular and prem. pretty expensive. And it seems like a hassle that everytime you get a new mod, you have to get it re-tuned.

I was on diablo's website and it says it does a Performance Tune for 87 octane. Does SCT do this also?

I think with the SCT you can have it tuned both ways...one for 87 and another for 93 that you can switch between on your own at home.