fast Ed
Registered User
Re: I thought the calculation was...
You have the right formula, but you're plugging in the wrong numbers !!
255, or whatever the first number of your tire sizing is, is the section width of the tire in millimetres, not the diameter. You need to calculate the diameter using this formula :
section width X aspect ratio (the 2nd number in the tire sizing) X 2 / 25.4 (converts millimetres to inches) + rim diameter.
So for my example ... ( 255 x .45 x 2 / 25.4 ) + 18 = 27.035" , which is very close to the spec listed in the Goodyear catalogue for my tires, 27.05".
Then, for trans. ratio in 5th gear, that's 0.75, not 3.27. Axle ratio in my case is 3.27, not 8.8, that's the ring gear diameter in inches, which doesn't matter for this calculation.
So if you plug in 4600 x 27.05 / 3.27 x 0.75 x 336 , you will get the 150 mph figure I came up with.
cheers
Ed N.
acros323 said:Doesn't the top speed calculation go like this:
MPH = (RPM * TireDiameter)/[(TransRatio * AxleRatio) * 336]
For Instance
MPH = (4600RPM * 255)/[(3.27 * 8.8) * 336]
MPH = 1173000/(28.776 * 336)
MPH = 1173000/9668.736
MPH = 121.318
????
Not trying to pick on your calculation Ed (or that you didn't reach 150MPH) but you had all the available information to calculate. Is this calculation accurate or am I missing something? Just curious
You have the right formula, but you're plugging in the wrong numbers !!
255, or whatever the first number of your tire sizing is, is the section width of the tire in millimetres, not the diameter. You need to calculate the diameter using this formula :
section width X aspect ratio (the 2nd number in the tire sizing) X 2 / 25.4 (converts millimetres to inches) + rim diameter.
So for my example ... ( 255 x .45 x 2 / 25.4 ) + 18 = 27.035" , which is very close to the spec listed in the Goodyear catalogue for my tires, 27.05".
Then, for trans. ratio in 5th gear, that's 0.75, not 3.27. Axle ratio in my case is 3.27, not 8.8, that's the ring gear diameter in inches, which doesn't matter for this calculation.
So if you plug in 4600 x 27.05 / 3.27 x 0.75 x 336 , you will get the 150 mph figure I came up with.
cheers
Ed N.