Gearing calculation

[Classic 911]

I have a question regarding gearing

- how to calculate the percentage difference in gearing be between these tire setups.

205/55-16 + 225/50-16

VS

205/50-15 + 225/50-15

[winders]

https://www.gboxweb.com/gearchart.html

[Classic 911]

Originally Posted by winders

https://www.gboxweb.com/gearchart.html

Thank you winders

Can you explain what "Aspect Ratio" in the spread sheet is?


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[dannobee]

Aspect ratio is the middle number in the description of the tire, i.e., "P345/25/19" The aspect ratio is "25" representing height over width. The higher the number, the taller the tire (or less sidewall, if you look at it that way). The tire manufacturer usually lists data on every tire they produce. Rollout or circumference will work, as will tire diameter, too. You need to know the actual tire circumference.

[Matt Monson]

Originally Posted by Classic 911

I have a question regarding gearing

- how to calculate the percentage difference in gearing be between these tire setups.

205/55-16 + 225/50-16

VS

205/50-15 + 225/50-15

Only the rear tire matters when entering into the calculator.

[Classic 911]

Originally Posted by dannobee

Aspect ratio is the middle number in the description of the tire, i.e., "P345/25/19" The aspect ratio is "25" representing height over width. The higher the number, the taller the tire (or less sidewall, if you look at it that way). The tire manufacturer usually lists data on every tire they produce. Rollout or circumference will work, as will tire diameter, too. You need to know the actual tire circumference.

Great, thanks a lot


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[Classic 911]

Originally Posted by Matt Monson

Only the rear tire matters when entering into the calculator.

Thanks a lot


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[hcoles]

Tire Rack may have the revs per mile for each tire. Easy to calculate all after that.

[Bill Verburg]

Originally Posted by Classic 911

I have a question regarding gearing

- how to calculate the percentage difference in gearing be between these tire setups.

205/55-16 + 225/50-16

VS

205/50-15 + 225/50-15

Using the manufacturer's advertised OD can give you an approximation of the effect on gearing, a more accurate assessment is given by starting w/ revs per mile

speed in mi/hr = (rp_minute*60/(rp_mile * the overall gearing))

overall gearing = cwp ratio * specific gear ratio

if you are only interested in the gearing but not the speed then imple division is all that is necessary

for say A052;
225/50 x16 OD is 23.9" and rpm is 869 giving an effective loaded diameter of 23.2"
225/50 x16 OD is 25.0" and rpm is 831 giving an effective loaded diameter of 24.3"

23.9/25 = .956 according to this the tire gear ratio is reduced by ~4.4% w/ the taller tire
869/831 = 1.04573 this gives a truer picture the gear ratio is reduced by ~4.6% w/ the taller tire