Tire size/gear ratio and width theory
Posted: Thu Jun 28, 2012 12:11 pm
Long story short, my non turbo subie currently has 225/50 16 toyo RA1's. Love them......LOVE THEM. They are at least five year old, and still give me plenty of stick. However, I'd guess this will be there last season. Overall diameter is 24.9 inches.
At Ascutney and Burke I don't get beyond 3rd gear, and there are points where 3rd gear is barely holding speed. GPS data says 71mph is as fast as I got at burke (right before check 3 and right before check 7). Calculating the gear ratio, at 6500 rpm, the car should be at 81 mph. It really looses any steam by 6000 rpm(or lower).
At Burke for example, I'm in third gear in the large sweeper before check two. I'm at 69-70 going into that sweeper, I loose 1 or 2 mph in the sweeper with the pedal on the floor. At the point where I hit the brakes before check 3 and down shift, I'm back to 69 or 70. I pretty much am only able to hold my speed there. There were other points where I held 2nd gear pinging the rev limiter rather than up shift to 3rd, since I'd have to downshift immediately after, and felt it wasn't worth the lost time in shifting.
I am contemplating switching to a 15 inch rim which is lighter, and going with a 205/50 15in toyo RA1. That drops the diameter to 23.1 inches(compared to 24.9).
At 6500 rpm in third gear, speed would drop from 81 to 75.
Advantages I perceive:
Reduced weight in both tire and rim(4-5lbs per corner). I'd have to go to a smaller diameter rotor setup, which is also slightly lighter. 205/50 15 is spec miata tire size, take offs are readily available and dirt cheap(less than $200 for 4). Tough to find used 225/50 16's used, new are ~$170 per tire.
Disadvantages:
Tire is narrower. However, I don't feel like I've exceeded the corner holding ability of the 16 inch setup, given my power levels.
Must put on a smaller brake rotor(caliper stays the same). This would be the oem size for the drivetrain. I have the parts for all of this, so the cost to do it is nill(other than time).
My thoughts are that this car is not a high horsepower car, so I must preserve momentum through corners when possible. Taking tire width away may harm that. I also feel like I need to get more power getting out of the corners, I think this might help. I think this will help acceleration on the steeper parts of the hills.
What do folks think? I'm leaning towards it just from a cost savings perspective, if at worst there is no performance gain. 15 inch allows for many option in tires at reduced prices, including rallycross tires.
Smaller brakes should be a non issue other than one venue(sccv track event at slmp). I can still change brakes easy enough.
I know Terry runs a smaller diameter tire. Unsure if you still have the stock gear box(4.111 final) or not.
At Ascutney and Burke I don't get beyond 3rd gear, and there are points where 3rd gear is barely holding speed. GPS data says 71mph is as fast as I got at burke (right before check 3 and right before check 7). Calculating the gear ratio, at 6500 rpm, the car should be at 81 mph. It really looses any steam by 6000 rpm(or lower).
At Burke for example, I'm in third gear in the large sweeper before check two. I'm at 69-70 going into that sweeper, I loose 1 or 2 mph in the sweeper with the pedal on the floor. At the point where I hit the brakes before check 3 and down shift, I'm back to 69 or 70. I pretty much am only able to hold my speed there. There were other points where I held 2nd gear pinging the rev limiter rather than up shift to 3rd, since I'd have to downshift immediately after, and felt it wasn't worth the lost time in shifting.
I am contemplating switching to a 15 inch rim which is lighter, and going with a 205/50 15in toyo RA1. That drops the diameter to 23.1 inches(compared to 24.9).
At 6500 rpm in third gear, speed would drop from 81 to 75.
Advantages I perceive:
Reduced weight in both tire and rim(4-5lbs per corner). I'd have to go to a smaller diameter rotor setup, which is also slightly lighter. 205/50 15 is spec miata tire size, take offs are readily available and dirt cheap(less than $200 for 4). Tough to find used 225/50 16's used, new are ~$170 per tire.
Disadvantages:
Tire is narrower. However, I don't feel like I've exceeded the corner holding ability of the 16 inch setup, given my power levels.
Must put on a smaller brake rotor(caliper stays the same). This would be the oem size for the drivetrain. I have the parts for all of this, so the cost to do it is nill(other than time).
My thoughts are that this car is not a high horsepower car, so I must preserve momentum through corners when possible. Taking tire width away may harm that. I also feel like I need to get more power getting out of the corners, I think this might help. I think this will help acceleration on the steeper parts of the hills.
What do folks think? I'm leaning towards it just from a cost savings perspective, if at worst there is no performance gain. 15 inch allows for many option in tires at reduced prices, including rallycross tires.
Smaller brakes should be a non issue other than one venue(sccv track event at slmp). I can still change brakes easy enough.
I know Terry runs a smaller diameter tire. Unsure if you still have the stock gear box(4.111 final) or not.