I have an AWD car that has Front tires 245/45R18 and Rear tires 335/40R19.
In the past I’ve always done a quick approximation in my head the Final and then eye-balled the other gears, but what is the proper way to do the calculation when you have different wheel sizes on an AWD car?
6-speed gear box (Racing)
Max speed = ~180 mph
HP = 440 hp
Torque = 419 lbs-ft
Torque max = ~4500 rpm
Redline = 6500 rpm (but I tend to go over a bit since it is still responsive post-redline)
Power = ~5000 rpm (guesstimate)
Front = 245/45R18
Rear = 335/40R19
Weight = 3502 lbs / 46% front
If by ‘proper’, you mean mathematically, then yes. However, it’s a lot of math that’s probably not worth the time when you could save time adjusting your final drive to see what works. If you really want to do all the math though, I’d be glad to help you.
Ah, that explains why you love pain so much lol. Aight, here you go.
Front tires: (((((RPM/Gear Ratio)/Final Drive)(3.14(2*(245mm0.45)+18in)))/12in)/5280ft)60min
Rear tires: (((((RPM/Gear Ratio)/Final Drive)(3.14(2*(335mm*0.40)+19in)))/12in)/5280ft)*60min
So basically you take what your desired shifting RPM is and multiply it by the current gear you in. Once you multiply that by current gear’s ratio, and then once again by the final drive, you’ll have your desired axle speed. Multiply that by the tire circumference, convert to MPH, and you’ll have the approximate desired tire speed. Reverse the equation so that you use tire speed and the gear you’re shifting into to find the approximate RPM you’ll be at after shifting.
Now keep mind that I obviously don’t have the torque factored into everything, so you’ll have to do the rest yourself, and this is only because I’m missing your degree lol. Just remember that you’re going to have the coefficients of resistance for track surface, moving vehicle weight, aerodynamic resistance, gravitational forces on each tire and any other variables you’ll need to factor in, and you’ll have an approximate idea of what to set the ratios at. If you eventually decide that’s what you want to do, I would love to see the rest of the equations needed to figure this out, because I’m curious as to how you’d factor in the rest of the variables in to that lmao.
