Tire diameter and gearing
06-23-2010, 07:53 AM
#16
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M Kibort - We are aware that smaller wheel/tire diameter doesn't increase engine power or torque. But I believe smaller diameter wheel/tire combo provides potential to use overdrive gear on long straightaways.
06-23-2010, 08:13 AM
#17
On my previous car, I would switch between tire sizes for the purpose of changing shift points/gearing at different tracks. On that car, the tire circumference was about 4 inches different which made an immediately noticeable difference. 80.5 inches versus 84.2 inches
However, currently I am looking at at difference of 1.4 inches in circumference between potential tire sizes. 78.1 inches versus 79.5 inches
to make it more complicated, I am going from a 17 at 80.1 inches I have always run on this car and looking at going to 18's at 78.1 inches for a 2 inch difference.
Query, will it really make any noticeable difference with gearing? I suspect it will be negligible but would like to hear from folks with real world experience.
However, currently I am looking at at difference of 1.4 inches in circumference between potential tire sizes. 78.1 inches versus 79.5 inches
to make it more complicated, I am going from a 17 at 80.1 inches I have always run on this car and looking at going to 18's at 78.1 inches for a 2 inch difference.
Query, will it really make any noticeable difference with gearing? I suspect it will be negligible but would like to hear from folks with real world experience.
diameter and circumference are mathematically linked, if you know one you automatically know the other but the loaded radius and even worse the dynamic loaded radius are a little more elusive.
The important thing wrt to what the driver feels during acceleration and shift points on track is the dynamic loaded radius of the tire. You can infer that from the revs/mi data if the manufacturer publishes it or do a roll out on the tire in a parking lot.
Unfortunately Hoosier doesn't publish the rpm, you'll notice that they do publish circumference and approx. dia., this is because the unmounted tire is round and easily measured, but when the tire is mounted on the car it is no longer round, it is flat on the bottom, how flat depends on the wheel width, air pressure weight of the car etc and this changes dynamically.
from the specs you provided it looks like you are comparing a 285/30 and 295/30 x18 R6s? Is this for you 993? If so consider that the 285/30 will fit better
06-23-2010, 04:01 PM
#19
Rennlist Member
I dont know about that, as the diameter change has to be pretty substantial to allow for a different gear to be used or one to allow for a greater top speed. I mean, if you are banging the rev limiter on a long stretch, just before turn in, and that extra 2% of smaller diameter allows for an extra 2mph at 137mph vs 135mph before, then yes, it probably can help you. But that would be an extreme, example of where it would pay to change out a tire diameter. (example was going from a 25.5" diameter tire from a 26" diameter tire, as in going from a 305x35 vs a 335 or 305x30)
