Will I need a new tune??

[97xl]

Ok just so I have things clear in my head...the shift points are controlled by the speedo...the VSS gear in the truck right now is the stock one so I know the speedo is off but if I change the VSS to match say my 265s that will correct the speedo error and since the shift points are based on the speedo then the truck should shift at the proper point without having to modify the tune??

[Jackpine]

Originally Posted by 97xl

Ok just so I have things clear in my head...the shift points are controlled by the speedo...the VSS gear in the truck right now is the stock one so I know the speedo is off but if I change the VSS to match say my 265s that will correct the speedo error and since the shift points are based on the speedo then the truck should shift at the proper point without having to modify the tune??

Yes, almost. The part throttle shift points are controlled by speed AND throttle position. At a fairly open throttle, the shift is delayed to a higher speed, and vice versa. But yes, having the correct VSS for the tires AND gears will make the shifts occur where they should.

- Jack

[97xl]

Ok thanks for clearing that up. This mod is already costing me big $$ and I was hoping not to have to spend more to get new tunes.

[97xl]

Well the rear end has been installed and my speedo is waayyy off...when I know I'm going close to 60 kph(based on tach rpms) the speedo says 80ish. The trutrac has a whine to it whenever you let off the gas. My mechanic said to bring it back in a week so he can check to make sure its ok. Now to get that VSS gear.

[Power Hungry]

Once the VSS gear is corrected, the shifts will all return to normal, at least based on MPH. However, they may still be a little high RPM-wise since you've now changed your RPM/MPH ratio. This isn't necessarily a bad thing, but might not exactly be what you're looking for... especially if you're looking for fuel economy.

If you can provide old gear/tire combo and new gear/tire combo, I can run it through the calculator and let you know just how much to expect your RPM to change. Of course, you've probably already noticed the difference.

[97xl]

Thanks Bill,

Old:

3.08 open diff 265/70r16 tires 30.61"

New:

3.73 LS 265/70r16 tires 30.61"

Update. I spoke with Eaton directly and they said 99.9% that the reason I've got the noise is because they used synthetic diff fluid. They said only use conventional. Was told it will make a small amount of noise when turning but other than that there should be no noise if the gears are setup properly. Also that its possible to take the exciter gear from the old rear diff and install it on the new one so the ABS will work. I called the shop today to make sure they still had my old parts, which they do, so this should be getting fixed tomorrow.

[Power Hungry]

The gear change with no tire size difference equates to a 21% difference.

I've attached a really neat calculator that will take tire size, rear gear ratio and transmission ratios to provide an RPM per MPH value for each gear. Hopefully it's useful.

Take care.