how to calculate top speed

[Quetzolcotl]

this is in response to an earlier thread: http://www.automotiveforums.com/vbulletin/t33557.html

you can calculate your car's top speed in this following way:

you will need to know your car's final drive ratio and final gear ratio, size of your tires, and your engine's peak power rpm.

on a d16z6 engine the final drive ratio is 4.25:1 and the overdrive 5th gear ratio is .702:1

multiply the ratios together to get the absolute final drive ratio
(4.25 X .702 = 2.98:1)

to calculate the tire diameter have your tire size. ex: 185/65 14
multiply sidewall (185mm) by the aspect ratio (.65) to get 120.25mm. this is the sidewall on one side so multiply by 2 to get the top and bottom sidewall (120.25 X 2 = 240.5)
divide this number by 24.5 to convert mm into inches. (240.5/24.5 = 9.47)
add this number to the wheel diameter to get a total rolling diameter of 23.47 inches. (9.47 + 14 = 23.47)

here, i'll brake it down again: 185/65 14 size tires
185 X .65 = 120.25
120.25 X 2 = 240.5
240.5/24.5 = 9.47
9.47 + 14 = 23.47 this is your rolling diameter in inches.

for the d16z6 we'll use 6600 rpm instead of redline for peak power

now that you know the absolute gear ratio, tire diameter and maximum engine rpm you can apply this formula:

mph = (rpm X tire dimeter) / gear ratio X 336

(6600 X 23.47) / (2.98 X 336) = 154.7mph

but we all know a stock d16 wont go 154mph, its because of the gearing. top speed is done in 4th gear. the 4th gear ratio is .909:1. so that works out to be an overall gear ratio of 3.86:1 (.909 X 4.25 = 3.86)

so this works into the formula to equal 119.43 mph top speed on a stock engine (on level ground)

now you can play around with tire sizes and gear ratios to see how it will affect your top speed.

i dont know why 6600 rpm is used for this example, i got all this from the honda/acura handbook by scc but its info worth getting out there. a moderator might want to post this in the tires/wheels forum. also remember too that even though bigger tires will mathematically let you go faster they are also wider and will cause more drag and you need the hp to back them up and push them along.