EEC, Gears and Speedometer Concepts

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Staff member
Sep 12, 2023
North Ridgeville, OH
Vehicle Details
1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Before we get into the EEC and axle ratios, here is an explanation 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 speed. 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 EEC 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.

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