I recently put the 15" Lace wheels on my 1985 GT and took it racing and noticed my times were quite a bit slower. When I had the tires installed, I had them put the same size tires as the reccomended size for my 87 GT. When I put the 14" wheel next to the new wheels, I noticed that the 15" wheels were about 1 inch taller. Will this affect my 1/8th mile times? Will it effect my speedometer? and why is there such a diffrence in hight?

I have p205-55-15s on my lace rims... I think the 55 series tire size is the way to go for non-86-87 GT Fieros that have the 86-87 GT rims. The tires have a lower profile which may increase performance. I've read an article either in Car-n-Driver or Road-n-Track in which the same tests were repeated with different size rims/tires - the larger the rim, the lower the profile tire. If I recall correctly, the 16in rims/tire combo outperformed both the 15in rims/higher profile tires and the 17in rims/lower profile tires.

The height of the 15in 55 series tire is about the same as the 14in 60 series tires that are stock on the 85 GT.

Here's pics of my 87 SE with the 55 series tires...


1987 SE 4-cyl, 5-spd (factory Iron Duke cruiser w/ dual exhaust)

[This message has been edited by Kelvin Vivian (edited 03-05-2002).]

http://www.secret-secret.com/turbo/DIY/tirecalc/turbotire.html

If this works, this is a tire calculater to see the size difference.

The tire with the smaller radius will give your axle torque better leverage to accelerate the car.
Also, the 15" wheels have a larger moment of inertia than the 14" wheels. The larger wheels will require more power to spin up, which means less power accelerating the car.

They are taller because the rim is 1 inch larger in diameter.

The tire size (215 60 R 15) tells you that the tread width is 215 milimeters, the aspect ratio is 60% meaning the rubber is 60% as tall as it is wide, and the 15 means you have a 15 inch diameter rim.

_______ __
| |
| | 129mm =(215 x 60%)
| | __

< 215mm>
wide
(Tire cross section from rim to tread)

The only difference in the 215 60 R 14 and the 215 60 R 15 is the diameter of the rim it is attached to (the cross section of the actual tire from where it contacts the rim to the tread is the same on both).

So a 215 60 R 14 will be 1 inch smaller in diameter than the 215 60 R 15.

Consequently the speedometer will read about 5% too slow and the odometer will read about 5% to short. You'll travel about 5% farther for each revolution of the tire than the instruments read.

So much for trying to include a drawing. The little graphic shows the cross section of the tire from the rim to the tread. It does not diaplay as it was drawn. Sorry

[This message has been edited by Carrolles (edited 05-22-2002).]