One more thing.
To my thinking, if the rear axle gear is numerically higher than 4.11, I would start with a rear axle gear swap to 3.73 or 3.42.
For normal driving, 1st gear X the rear axle should ideally be between 9 and 12.
My factory 3 speed has a 2.94 first gear and a 3.73 diff. That’s 10.96 and right where it should be.
The S10 T5 being considered has a first gear of 3.75 X 4.11 = 15.41. That’s essentially a granny gear and makes first gear rather annoying unless you are pulling something heavy at a slow speed. With a 3.73 diff you get 13.98. It’s a bit out of range but not too low to be useful. I know because I’ve driven one.
The T5 has a 2.18 second gear. So 2.18 X 4.11 = 8.95. So if the 4.11 diff remains, the owner will use 2nd gear to start moving, but will need to use 1st gear on a hill.
A T5 5th gear of 0.72 matched with 4.11 = 3.00. That will be a bit too low for cruising gear for highway speeds. At 65 MPH with a 29” diameter tire the RPMs would be 2260. These inline 6 engines are more efficient at higher RPMs as has already been stated.
A T5 with a 0.86 5th gear and 4.11 diff would be at 2662 RPMs at 65 MPH and seems to be a better choice.
BUT WAIT! I said earlier that I would START with a diff change! That’s because you really MIGHT be completely satisfied with how it drives with a 3.73 diff.
If you currently have a 4.11 diff your RPMs will be around 3095 at 65 MPH. Changing to 3.73 would get 2809. So you see, a diff change to 3.73 is a lot less trouble and really only 147 RPMs more than the whole T5 swap. A whole lot cheaper too.
Finally, what if you’re not happy with the 3.73 and still want a T5? Well, with a T5 plus a 3.73 you would still be able to make good use of 1st gear. For 5th gear the 0.86 OD X 3.73 = 3.20 which means at 65 MPH the RPMs would be 2416. That’s a pretty nice combo IMO.
Last edited by Lugnutz; 05/03/2020 5:12 PM.
