Speedo calculated off front or rear tires?

[Ek9max]

Are the vehicle speed sensors on the front or back wheels?

Just trying to calculate how much my Speedo will be off with 245/35/20 fronts and 285/30/20 rear.

 

[Tobstertx3]

You can use this link to find out. Enter in your original tire size and new tire size and it displays the difference from 20mph to 90mph.

Tire Size Calculator

 

[M'Kehyah]

You don't say in your sig -so I mention: only do this for RWD cars. AWD cars are no Bueno for different tire heights.

 

[STINGER]

I know the LAP3 Pross taps into the speed sensor which you have to access from the bottom of the car right behind the motor, so I guess its the transmission.

 

[oddball]

That Lap3 Pro harness is grabbing the crankshaft position sensor so it can muck with ignition timing. That tells the computer where the crankshaft is in its rotation. Add in some time and you can figure out RPM.

I haven't looked it up for the Stinger, but nearly every modern car gets vehicle speed from the ABS/stability system. It's been a long time since spinning gears hanging off the driveshaft were used.

The ABS/stability system normally looks at all 4 wheels to determine vehicle speed. There's math involved.

You may have issues if the rolling height of your specific tires (check the manufacturer's tech sheet) differs by more than 5% or so. Hard to know before you do it. Some ABS calibrations are notoriously sensitive, some aren't. See if anyone on here has done a setup similar to what you want.

 

[STINGER]

That Lap3 Pro harness is grabbing the crankshaft position sensor so it can muck with ignition timing. That tells the computer where the crankshaft is in its rotation. Add in some time and you can figure out RPM.

I haven't looked it up for the Stinger, but nearly every modern car gets vehicle speed from the ABS/stability system. It's been a long time since spinning gears hanging off the driveshaft were used.

The ABS/stability system normally looks at all 4 wheels to determine vehicle speed. There's math involved.

You may have issues if the rolling height of your specific tires (check the manufacturer's tech sheet) differs by more than 5% or so. Hard to know before you do it. Some ABS calibrations are notoriously sensitive, some aren't. See if anyone on here has done a setup similar to what you want.

Thats interesting. How does the Lap 3 pro remove the speed limit?

FYI, i installed 295/30/20 and had no issues with traction or ABS. Just make sure the front diameter is the same as well. Your combo diameter is only 0.1” off between the front and back so you should be good. Difference in size between OEM and your combo is 2.7% so that will be the speedo difference, it will read 2.7% more than the speedo at any given speed.

 

[oddball]

Note that you want the rolling diameter from the manufacturer. Every tire is different. The size definition (e.g 285/30/R20) is only a category description. Like a "55 inch TV". The rolling diameter is how the tire actually reacts to the road. Your new tires may be closer - or farther - from the OE.

As for the Lap3, looks like they intercept the vehicle speed sensor (VSS) signal going to the ECM. So while the rest of the car knows how fast it's going, the Lap3 lies to the ECM and says it's not exceeding the limiter so the ECM will keep applying torque. The gauge cluster is its own computer, so it's pulling the VSS separately, most likely off the CAN bus instead of directly.
Unfortunately the bootlegged service manual doesn't include wiring diagrams, so it's a PITA to cross reference all this junk given the limited into provided.