Math Problem:
 

I've read my car's top speed is 126, which may or may not be true with 60 series tires, but for argument, let's say that is the top speed with this height tires. (I'm not certain of the tire height on stock 1974 911s were).

If I change to 50 series (with the decrease in height), the top speed will be less. But how much less?

This is taking into consideration whatever the percentage of "less tire" there is with 50 series as opposed to 60 series.

Hope someone can help.

Thanks.

Here's a calculator:

http://www.miata.net/garage/tirecalc.html

Cheers,

Joe

mph of new tire = (mph of old tire) * (old tire diameter/new tire diameter)

If you are from LA does it really matter? Ihave never been able to exceed 80MPH on the freeways. On the way to Vegas perhaps?

Depending on your power curve, your top end could be slightly higher (maybe a few mph). I doubt you are redlining at 126 mph, and lowering to 50 series tires slightly lowers your gearing. Lower gearing means more torque to the wheels at any rpm. I doubt if it's enough for you to feel when you hit the gas, and it shouldn't really make any difference (or a very small improvement) in your top speed unless you do hit your rev limiter (if not, you'll be revving slightly higher at 126 mph).

Brad

one less calculation if you could give the tire width and rim size.

126 miles /hour x ( 5280 feet /mile ) x ( hour/3600 seconds ) = 184.8 feet/second

i fill assume a 205/60 15 which would give a rolling radius of

205 X (60%) =123 ( divide by 25.4 to meteric ) 4.8425"

plus rim radius 7.5" + 4.8425" = 12.3425" rolling radius (1.0285')


now the rpm of the tire.
perimeter 2 pi r...... 2 x ( 3.1416 ) x 1.0285'= 6.4625' /rev

velocity/perimeter= rev/time

(184.8 feet/sec) / (6.4625'/rev) about 28.59 rev/second


with the new aspect ratio for a 50 series.

205/50/ 15 rolling radius is 102.5 /25.4 = 4.0354 + 7.5 =11.535" radius
rolling diameter 11.535 x 2 x pi = 72.476"/rev


assume 28.59 revolutions /sec x 72.476"/rev = 2072"/second

117.73 mph if you have 205 /50 15's

probably some bad math somewhere. plug in your tire size




i'm loosing it. 3 posts before i could figure it out

If you are running a 195 tire for a 15" rim, and are considering 60s and 50s, the 50s will drop your top speed by 8 mph to 118.

The process:
Take the width in mm and multiply it by the aspect ratio. Multiply that by 2 and then add it to your rim dia. Take the new number and the old number each and multiply the overall diameters by pi. That is the circumference or length rolled/revolution. Divide the new one by the old one and you will get a decimal. Multiply that decimal by your old top speed=NEW TOP SPEED.

as a note, the smaller tires will help you accelerate faster.

Thanks for the help, guys...
 

This is why I was a lit. major and not math. :)

I re-iterate....unless your're already hitting your rev limiter at 126 mph your top speed should NOT change with slightly smaller tires, you'll just rev higher. Top speed is a function of HP and DRAG. Your speedo will read erroneously (well porsche speedos are notoriously inaccurate to begin with) but if you measure your top speed before and after with a radar gun, I'd bet anyone $100 that the speed will be about the same.