There are a few factors you need to consider before selecting a particular T5 model and a ring and pinion ratio to match.
Most importantly, you want your final setup to optimize your engines potential and to fit your driving style. In my particular case, I have a built, low-end torque 235 engine coupled to a 1985 NWC .72 OD T5 turning a 88 Camaro 3.42 R&P and running a 28" tall rear tire. My truck cruises at 70 mph at 2000 rpms and it is a daily driver. My particular .72 OD came from behind a 2.8L V-6 truck.
This particular combo lets me drop down to 45 mph in 5th gear without a downshift, and accelerate with traffic. In all honesty, and after putting on 5000 miles, I would have been a bit happier to have had a R&P ratio in the 3.55 to 3.73 range. It cruises fine at 55-65 mph at 16-1700 rpm range, but my vacuum gauge tells me she would be more efficient at closer to 2400 rpms at 60-65mph. With fuel economy an issue today, the shorter R&P ratios might be just the ticket, especially if you like to putz along, or are in real hilly country.
The T5's come in several OD ratios...the .72 and .86 are common for the Non-World (NWC) class S10 boxes. Also the Pre-89 boxes are set up for a mechanical speedo while the later T5's are for electronic speedos. My 3.76 1st gear has me shifting at about 15 mph....the .86 OD NWC T5 from behind the 2.5L 4 cylinder with a 4.03 1 st gear would drop the 1st gear shift point even lower. I consider my .72 box to be an ideal setup.....3.42's Camaro diffs (62" wide) are plentiful while the 3.73's are a little more scarce.
Of course, there is another option with the World Class (WC) T5's that are found behind the 305 SBC Camaro. You didn't mention what your engine size was going to be....the NWC T5 would be fine behind the 235-261. If you're running a stock 350, the WC T5 would be okay for normal driving applications. However, you will need to change out the tailshaft to the shorter NWC S10 version to get proper location for your floor shifter. Also getting the speedo "drive" gear located on the output shaft so that you can have a speedo, takes some special effort. The gear ratios are a little closer together but by the time you pay for the WC box, the short tailshaft housing and get the speedo drive gear properly positioned, the NWC S-10 T5 will be your least costly and less difficult install.
I've got a fairly comprehensive photo documentation of my T5 install in my Webshots albums "Shoebox" and "Tips and Tricks." It will outline all of the modifications you'll have to make and I've documented the process with pictures.
Upgrading to an overdrive transmission has transformed my truck into a great driving machine that utilizes the full potential of the 235 engine. I've done most all of the R&D to getting it installed and after 5K without any problems, I'd highly recommend the upgrade.
Dave
