Vehicle Theory (Wheels/Tires)
03/30/06 Update - The new TE-37's are on. Wow! Besides contibuting to the overall performance in acceleration and braking, I am now getting an additional 4 mpg on the street. That means that I should be able to pay for the wheels, just in gas mileage, in a little over 8 years. Hope they last that long.

As an engineer, I've really enjoyed linking theory to reality on this project. Searching for points to optimize The Beast, to get the most performance possible, is a great challenge. I've really learned a lot about cars from what I've done already, and I hope to continue learning as I delve deeper into the optimization.

Looking for a good wheel/tire combo:

1984 Saab 900 Turbo Gear Ratios
(from the service manual)
1st 3.80:1
2nd 2.15:1
3rd 1.44:1
4th 1.04:1
5th 0.84:1
Reverse 4.18:1
Final Drive 3.67:1

I'm currently running Pirelli P400 Touring 195 60R15 87T M&S, that came with the car. Those tires have a diameter of 24.21", width of 7.7", and a circumference of 76.1". They weigh about 19lbs. Incidently, looking at the tire rack specs, most tires weigh about 20lbs. Using the tire and gear data, I was able to compute vehicle speeds for a given engine rpm:

My current rims are stock Saab manhole covers which have a +35 offset, are 5.5" wide, 15" in diameter and weigh about 19lbs. They have a 114.3x4 (aka 4.5x4) bolt pattern.

Assuming I don't want to get larger rims, which I don't (explanation following), and I don't want to change the overall tire diameter, then I have several tire options that are currently available:

185 65R15
195 60R15
215 55R15
205 50R16
205 45R17
235 40R17
225 35R18

I'd like to run a street tire, since The Beast is a daily driver. I probably won't be able to change the tire weight too much because most tires are about 20lbs. I can get more hp to the tires by changing to lighter wheels however.

My current wheel weighs about 19lbs. There are several sites online that have tabulated sizes and weights. Unfortunately, they didn't add cost, which is a significant factor depending on what wheels you chose.

I'm going to go with the Volk Racing TE37's. They're not the lightest, but I do like the color options. Hey, it's also art. :) They're also forged, so I suspect they're stronger than the cast options.

Ok, now why the smaller rims. Primarily, they're lighter. That means less hp taken to get them up to speed. This is significant when it comes to accelleration and deceleration, which is what I do a lot of. I'm not totally concerned about top speed, which is why you'd choose a larger overall circumference. If I ever see 140mph, I'll be happy.

More to come. Waiting on the TE-37's to get here.

Page 1 - Summary
Page 2 - Tuning
Page 3 - Wheels
Page 4 - Water Injection
Page 5 - Brakes
Page 6 - Interior
Page 7 - H.I.N.
Page 8 - Racing