JayFi

Ok, so from everything I've read, I'm under the impression that when you swap out gears in the rear end you need adjust your tune for the change in gear ratio. My question is this:

Suppose I need to take the car in for service or warranty work (which would involve me flashing back to stock, taking off the CAI, etc) - Will the car be driveable with the stock tune and different gears? Or at this point do you just run a non-factory tune and take your chances at the dealership?

tonkpils555666sas

First off, the dealership shouldn't deny service or warranty work on your Mustang due to after market parts. It is illegal for the dealership to do this unless they can prove that the problem that needs to be resolved is a result of an aftermarket component. Second, you should be able to go to the dealership with the car reflashed and with the gears and intake still on the car. Third, I believe you can run the car with new gears without a tune, however, just keep the RPM's very low and your speedometer won't function normally/properly. Where the car redlines is affected so just drive the car at low RPM's and you should be good.

JayFi

I get your point, but I don't really feel the need to test how altruistic the dealer is either, which is why I would pull the CAI/tune. Why give 'em something extra to point a finger at?

Anyway, I was just curious what different gears would do in that scenario.

wmtheflash

I just took my '06 GT to my dealer to have the gears installed. They could have cared less about my 93 octane tune or my intake. I left them a note telling them not to flash the ECU. No big deal.

Your plan would work only if the dealership doesn't need to test drive your car after warranty repairs or while diagnosising a problem. After all the speedometer is not going to read the proper speed if it has the stock tune and new gears.

Forgive me for not doing the math, but wouldn't the speedometer read 10 to 15 mph less than actually vehicle speed on the highway? Depending of course on vehicle speed, what stock gears you had (3.31 or 3.55), if its an auto or a manual, and the new gears you've had installed. If they need to drive your vehicle to diagnose a problem and you've flashed it back to stock, then they're going to realize something is different.

There is always some risk in modifying your vehicle. If you're not comfortable with it for whatever reason, then I'd recommend not getting gears until the warranty expires.