And it kept me awake until nearly dawn thinking about getting an overdrive for the T-10 by swapping in an OD for 3rd and changing the shifter to allow it to be the new 4th.
In doing my research for this I discovered that the OE 3rd gear is already a slight OD as the ratio between the input and cluster shafts is 1.28:1, while the output from 3rd is 1.23:1, which means the ratio of the gears on the 3rd position of the transmission is already an OD just not very much of an OD, 0.96, probably just a tooth larger on the cluster than the main shaft. It’s looking like I will have to order a custom cluster with a custom gear for the main shaft that is as small as will fit. This is not an inexpensive alternative.
And it’s not like I could amortize the costs by selling duplicates at a profit until the tooling costs are covered. This application is very niche, not many people are in the market for this combination of light weight and OD top gear, and can live with only 4 speeds. Also I have been looking at the ratios in use on the transmission and most of them are not actually useful in making 3rd OD. Swapping the gear for 2nd gives an OD that is not even enough to make the output OD, the gears are 1.25:1 as they are before swapping which is less than the 1.28:1 of the input gears. So basically I’m looking at finding the smallest gear that will fit the output shaft and designing a gear to machine into the 3rd position on the cluster and hoping the resulting ratio is enough to be more overdrive than the underdrive ratio of the input shaft to the cluster. Also most of the different ratios available are just from changing the ratio of the input to the cluster, the 2.43 S cluster, and the 2.64 W cluster are the same ratios except for the input/cluster gear. In fact checking the spares list shows that most of the ratios are shared with common 2nd ratios for the S, W, CC, and Z clusters, common 3rd for S and W, and common 3rd for X, CC, and Y. So yeah, I will have to order a special cluster and gear for the main shaft if I want to hang an OD 3rd on my transmission.
And I have been going back and forth between the PDF of the spares list, the calculator app, and this post for like 4 hours, it’s time to put this one to bed.