Gear ratio/tire/tranny=>>>RPMs. ? Any thoughts...

[joelliott]

I have been thinking about the gear swap again. Due to realising just how much HW I drive, i have been investigating different options. I got some 1985 T5 tranny ratio numbers from a guy on stangnet and are as follows.
1st=3.35
2nd=1.99
3rd=1.33
4th=1.0
5th=0.68

I used Randys Ring and Pinion Calculators to get the following: using a 255-40-17 tire (25in tall), and 5th gear=0.68, and 80MPH...
2.73 gears @ 80 =1996 rpms
3.08 gears @ 80 =2252 rpms
3.27 gears @ 80 =2391 rpms
2.45 gears @ 80 =2522 rpms
3.73 gears @ 80 =2727 rpms

I also did the above with a 275-40-17 tire I hope to run soon (25.7"):
3.27 gears @ 80 =2326 rpms
3.45 gears @ 80 =2454 rpms


I have always thought that I had 3.08 stock gears. But on my tach @ 80 mph Im turning about 2K. So this looks more like I have 2.73 rear end. Hopefully this weekend Ill get time to try the wheel spin/driveshaft count test. That should lock down my ratio questions. Im leaning to 3.45 gears due to the decent rpms at HW speed. Any tips or comments on my numbers or info. Im trying to get all this straight but not guarenteeing this info is right. Any help is appreciated.
Joe E. :D

[mfpmax]

Why not 3.55s? Or is this in a 7.5 rear?

[joelliott]

yeah 7.5. My engines not crazy so Im not worried about causing failure to the axle. I know I know get an 8.8. But for the time being, Im sticking with my stocker thatll get rebuilt with the gear swap.
Joe E.

[joelliott]

Are these number reasonable?
Joe E.

[old lx]

Yes, those RPM's look reasonable.

I put the data for the 3.45 gears into my spreadsheet, using both the 25 and the 25.7 tire diameters, and I got the same RPM's as you listed in your posts for that gear and those tire diameters.

[joelliott]

OK thanks. Im trying to get the swap together and I appreciate the help. Have a good one.
Joe E.