15" rims for GenI Lumina

[Jprice90]

im looking to get rid of my MC rims and tires, im looking to go back with 15", what would you guys say would look good on my car? i have a White Lumina for you guys that dont know

[19Cutlass94]

Why would you want to go with 15's? Its like going back 30yrs in technology.

[Jprice90]

not really ..would it accelerate different? it kinda feels like the tires hold it back

[WrathOfSocrus]

The last critical number to figure out your true overall ratio between the engine and the ground, besides gear and differential, is the tire diameter. I did a quick search for Static Loaded Radius and found this link http://www.vibratesoftware.com/html_help/2011/Diagnosis/Tire/Static_Loaded_Radius.htm that explains some of the dynamics of the relationship between the tire and the road. Think of how a breaker bar works, but in reverse. If you put a socket on a breaker bar, bend it to 90* and hold it by the socket, the bar has a good amount of leverage. Add a short section of pipe hanging off the handle and it will be increasingly more difficult to hold on.

 

The same physics work on a car. The more feet between your hub and the ground, the more ft/lbs of torque needed to get the same mass moving. Changing the tire diameter will throw off the speedometer and can move shift points to different speeds which can alter fuel economy or just be plain annoying. I had a beat to hell old Ford Fairmont that would constantly shift when holding a steady 40 MPH which was a nightmare living in a hilly area that had a lot of back roads with a 40 MPH speed limit. Since it had a straight six, I knew it had enough torque to move around pretty well, so I went from 205/65 to 215/75. That allowed me to keep it in 2nd and not have to downshift to 2nd on every small uphill section. The silly 1 barrel carb never seemed to use much more fuel no matter how much you pushed on the pedal. The extra width plus the limited slip was sweet for the snow!

[urbex]

Can that really alter it that much by getting a tire 1 inch smaller? When I went from 15" to 16"s on my HHR I never noticed a difference other than the speedo is off by 1.

[WrathOfSocrus]

It is a ratio of your old tire size to your new tire size. If you are going 25 MPH it isn't much. 1% of almost nothing is just a lot closer to nothing. When you get to highway speeds, depending on the ratio, it can throw off the speedo by several MPH. Might not be a ton but if your tire has a smaller circumference, and the speed sensor is reading X revolutions, then it 'thinks' X multiplied by the stock circumference is the speed you are going. Even with differences from tire to tire of the same size from different manufacturers, compounded by the fact many of us have ~20 year old cars, there are a number of variables that can add up. Here is a tire calc http://www.miata.net/garage/tirecalc.html. Going by the ratio of the small tire my car came with compared to what is on it now, the calc says when my speedo would have read 60 I am really doing 65.5, had I kept the stock computer and wiring. Not an insane difference but probably enough to get me a ticket in the right situation if i wasn't cognizant of the discrepancy. Going to a smaller tire would do the opposite. For example, you see a cop on the freeway and drop your speed to what you think is the speed limit, and start seeing cars getting backed up behind you because you are really going 5MPH under the speed limit.

 

My main point is that many people only think about tires for how much grip they have for launching and braking, or what they look like. However, tire diameter does effect the final ratio between combustion and the road. In my case I had a weak 2.8 on a base CS with little tires on 14" rims. Since I have a 3500 now, I will have a ton more torque to get me moving. With 16" aluminum wheels and smaller rubber between the rim and the ground, my overall wheel weight probably didn't change much. Since I took the entire transmission, computer, and wiring harness from a vehicle with a tire much closer to my current size, my speedo and shift points should end up close enough for my needs.