fsp31

I'm doing some math to try and calculate speed and rpm's for a given wheel size and at a given gear ratio. So far, my figures just aren't coming up right. Can y'all see if I'm missing something? Wheel diameter measurement is a bit rough btw, due to tire deflection, poor use of a tape measure, etc...

First: Calculate the gear ratios. Let's use 1st gear in a D16Y8. According to what I've found, 1st gear is 3.250:1. Using a final drive ratio of 4.250:1 I get 13.8125:1 (engine rotations : wheel rotations, in 1st gear)

Next: Calculate the circumfrence of my wheel. A rough measurement gave me a diameter of 22 inches. Using 2(Pi)R=C I got 5.7596 feet per wheel revolution, which equals 916.73 revolutions per mile.

Then I applied time to the calculation using 40 mph for a point of reference. 916.73*40/60 = 611.153 wheel revolutions per minute. (not sure about this step)

Finally: 611.153 (wheel rpm's) * 13.8125 (engine:wheel ratio) = 8442 rpm's at 40 mph on a D16Y8 tranny. That ain't right. So, what am I missing????

Crazydave

3.25x4.25 = 13.8125 RPMS for every 1 Wheel Revoltion
13.8125 RPMS = 5.76 feet traveled
so for 8442 revolutions in one minute means you turn 611.185 wheel rpms which equals 3520 feet in one minute
so conver 3520ft/min x 1mile/5280ft x 60 mins/1hour = 40 Miles an hour

yeah.. worked for me..
PLus you can take your civic... until you hit your redline.. and look at your speed.. should be about 33 or so.. I redline at 7000




[Modified by Crazydave, 4:32 PM 1/16/2002]

fsp31

>>so for 8442 revolutions in one minute means you turn 611.185 wheel rpms which equals 3520 feet in one minute
so conver 3520ft/min x 1mile/5280ft x 60 mins/1hour = 40 Miles an hour<<

Herein lies the problem! I guarantee that a D16Y8 is not spinning at 8442 rpm's in 1st gear at 40 mph. It's closer to 7442 rpm's in reality, so where is the problem??? All I can come up with is that these tranny gear ratios are wrong.

Newsflash! I just came up with a site that claims 99 Civic Si (B16A2) gear ratios are 3.230:1 for 1st gear and 4.230:1 Final Drive. If you plug these numbers in for a Civic Si traveling 40mph in 1st gear, the RPM's come out right. Looks like the gear ratios I used are waaayyy off. So, does anyone know ACCURATE gear ratios for the D-series transmissions???



[Modified by fsp31, 12:50 AM 1/17/2002]

Crazydave

The gears are correct, check the manual
Tire size also effects this calculation..
the taller the tire the higher the speed
so different tires and rims will change the rpms and speed..
They make sense to me..

[Modified by Crazydave, 4:58 PM 1/16/2002]


[Modified by Crazydave, 4:58 PM 1/16/2002]

Mista Bone

rpms/FD ratio/gear ratio/336 * tire OD

7200/4.250/3.250/336*22.7

35.21 mph
http://nonsense.netfirms.com/perlstuff/

Greyout

the thing is, taller tires will not change your indicated speed at redline. you can have 14" rims, or 32" superswampers, and at redline your speedo will always say the same in 1st gear.

fsp31

Thanks Mistabone!!!

Now I've figured out my problem. I assumed my calculations were incorrect because I didn't think that the engine would gaine another 1000+ rpm's to go from 35 to 40 mph. But according to the gearing calculator, I was right on target.

fsp31

Sorry for whipping this dead horse, but there's still a problem!!!

According to the gearing calculator, my D16Y7 hits the 6200 rpm rev limit at 34.63 mph. This is assuming a 1st gear ratio of 3.250 and a final drive of 3.722. This is way wrong. I know our speedometers have some error factored into them, and we need to allow for errors in tire diameter, but not this much.

My D16Y7 hits the 6200 rpm rev limit at 43 mph according to my speedometer. I've also had a radar gun check my speedometer for accuracy. It reads about 2 mph off at speeds under 40 mph. My car was dynoed last weekend and the 6200 rev limit is spot-on. In order to attain a speed above 40 mph my wheels would have to be something like 26 inches in diameter. SOMETHING IS MISSING.

earl

Are u sure on your ratios (especially the final drive)? Clutch slippage at high rpm? Just guesses...

tinkerbell

using the table on http://nonsense.netfirms.com/cgi-bin/speed.pl,

with FD of 4.4 and a fifth gear ratio of 0.848 with 23.26in tyres - it calculates that in fifth 7000rpm i would be doing 130mph?

wow! now i just need to power to pull 7000 in fifth!

maybe down a hill eh?

t..

fsp31

>>Are u sure on your ratios (especially the final drive)? Clutch slippage at high rpm?<<

I used to be sure. The clutch slipping scenerio can't be right either because that would cause the results to be skewed higher. Well, they're already way too high. WAAAYYYY too high.

Mista Bone

I think the Y7 has a higher redline, also makes sure are geeting a reading at a constant rpm, not on a 1-2 shift.

Now if you had a 3.25 FD, it would be alot closer.