Tire size effecting transmission shifts

cummins195900 · **Joined:** Thu Feb 04, 2016 5:57 am **Posts:** 5

My local mechanic told me if I run Oversized tires my transmission performance would be effected,as well as speedometer accuracy.I get the speedO issue ,but will the transmission shift as it should?

CATCRD · **Posted:** Thu Jan 07, 2021 9:46 am

Mine does. I had the speedometer adjusted, so that's fine now, but the transmission just shifts at slightly higher road speeds than before. Where it might have shifted to 5th at 58mph before, now it does it at 61. Other than that there have been no issues for 70k miles with oversized tires.

thesameguy · **Joined:** Sat Oct 01, 2016 8:24 pm **Posts:** 652

Yeah, I've never heard of tire size having an adverse effect on transmission, unless of course you're talking about *dramatic* changes that affect effective gearing in a tangible way.

A larger tire OD requires a corresponding drop in gear ratio in order to maintain shift point vs. vehicle speed. For a small change, no big deal... but if that goes really out of whack, you'd end up stuck in a gear much too long. My friend's old Toyota with 35" tires on stock gears was kinda funny that way. Also, VERY SLOW.

WWDiesel · **Posted:** Thu Jan 07, 2021 5:29 pm

Larger/Taller tires can affect the shift points of the transmission due to the change in transmission output shaft speed. A taller tire in essence raises the final drive gear ratio slightly. So shift points would be slightly raised due to a higher MPH speed on the speedometer when installing a taller tire.
But the indicated speedometer speed does not control shift points.

From the FSM: wrote:

OUTPUT SPEED SENSOR DESCRIPTION
The Input and Output Speed Sensors are two-wire magnetic pickup devices that generate AC signals as rotation occurs. They are mounted in the left side of the transmission case and are considered primary inputs to the Transmission Control Module (TCM).

OPERATION
The Input Speed Sensor provides information on how fast the input shaft is rotating. As the teeth of the input clutch hub pass by the sensor coil, an AC voltage is generated and sent to the TCM. The TCM interprets this information as input shaft rpm. The Output Speed Sensor generates an AC signal in a similar fashion, though its coil is excited by rotation of the rear planetary carrier lugs. The TCM interprets this information as output shaft rpm.

The TCM compares the input and output speed signals to determine the following:
• Transmission gear ratio
• Speed ratio error detection
• CVI calculation

The TCM also compares the input speed signal and the engine speed signal to determine the following:
• Torque converter clutch slippage
• Torque converter element speed ratio

Tire size effecting transmission shifts

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