I am looking to regear my ride (auto GT) to 4.10s and want to get a chip burned to compensate for the gear change, among other things. I noticed that a lot of places offer the chip and custom programming. Who do you guys/gals recommend? JMS?
I don't think a chip will work on a 96 auto to correct speedo on that particular model. If you have some work to be done on your car look up McMillion Performance in the phone book. He's really good and cheap. He's out of Axton Va. Takem Easy
If the only mod you have is 4.10 gears I would just get the SpeedCal for 90 bucks. Also if you have plans on adding more mods then I would also wait on the chip. This is just an idead to save you from spending 300bucks then about 75-100 to reburn your chip later for other mods. Hands down I would only go JMS Chip. JMSCHIP.com
EES, gears and speedos - concepts
Before we get into the EEC and axle ratios, here is an explonation to a few things.
For a given tire size and axle ratio, there is a value that's called N/V (said N over V). That is the engine speed (or better yet, driveshaft speed) divided by vehicle. This basically says the driveshaft RPM required for every 1 MPH.
On a 3.08 axle T-bird with stock tires this value is 40, with a 3.27 axle, it's 42.5. This means that at 100 mph a 3.27 axle bird has a driveshaft speed of 4250 rpm (100 times 42.5).
The calibration in the car is set-up for a given N/V. In 1994/5 this was 40 since the base axle for the car was a 3.08. If you change axles and change the speedo gear so the speedo reads correct, the EEC always knows the correct N/V. It knows output shaft speed via the OSS sensor (this is driveshaft speed) and it knows vehicle speed via the vehicle speed sensor. Divide the two and you have the actual N/V.
Now, the EEC takes the N/V the car was set up for and divides it by the actual N/V. It then comes up with a correction factor. For example, if you had a 1994/5 car the N/V it was set up for was 40. But if you ordered a 3.27 axle, the actual N/V is 42.5. It then divides the two 40/42.5 and gets a value of .94. Then, the EEC takes this number and multiplies the entire shift schedule by this number so the car shifts at a lower vehicle speed with a higher numerical axle. But, if you think about this, even though it's shifting at a lower vehicle speed, it's shifting at the same driveshaft speed. (At the same vehicle speeds a 3.08 axle car's driveshaft is turning 6% slower than the same car with a 3.27 axle).
So if you change the axle and speedo gear, the trans should shift at the same driveshaft speed, but a lower vehicle speed. No problem.
But, there are limits that are set to how much the EEC is allowed to correct for. These limits vary by model year and calibration. They are usually set to allow to go up 1 or 2 axle ratios.
The reason your engine may hit the rev limiter when changing axle ratios is a totally different story and has nothing to do with this.