Tire diameter calculation while tuning and tire growth with speed?

[TheNormsk]

I was looking at some calculators for determining optimal shift point based on gear ratio, torque and so on but part of that needs tire diameter for calculating speed at shift point along with rpms.

Trying to figure out tire diameter, I calculated this for the M6 GT3 and found that in the gear ratio/speed calculation in GT7 that the tire diameter calculation changes with gear.

At first this does not make sense to me, but could GT be increasing the diameter of the tire as speed goes up to allow for case ovalization/expansion with the centrifugal force?

If this is the case, is there some sort of expression I can use to allow for this?

 

[Greycap]

At least the 6th gear difference is explained by the top speed for the highest gear being calculated at the rev limiter unlike the others that are shown at the redline.

For the rest the differences are so small that it can be a matter of the exact value being, say, 219.49 km/h which is rounded down to 219 km/h but if it was 219.51 it would be rounded up and the figure would be more in line with the ones surrounding it.

 

[TheNormsk]

At least the 6th gear difference is explained by the top speed for the highest gear being calculated at the rev limiter unlike the others that are shown at the redline.

For the rest the differences are so small that it can be a matter of the exact value being, say, 219.49 km/h which is rounded down to 219 km/h but if it was 219.51 it would be rounded up and the figure would be more in line with the ones surrounding it.

Thanks! The sixth gear at rev limiter makes sense now. That was the one that perplexed me the most.

Yes, I agree the others are probably just rounding errors.

 

[gti_sdn]

you could collect the data using Motec and check what your speed is at 6500rpm in each gear more accurately
GT7 to Motec i2

Or you could dive a little deeper into Motec and find the best shift point based on G-Force
Using Telemetry to know when to upshift