3400 RPM's 5 gear (HB gear) 255/50/16's on the rear...

Look for the Miata tire size calculator to get your approx rolling dia. Get the R and P ratio. Get the HB gear ratio, then do math If you need more help, let me know and I will look it up

Sidewall 5.5 in

Radius 13.5 in

Diameter 27.0 in

Circumference 85.0 in

Revs/Mile 746.0

Now what?

Calculator:
http://www.miata.net/garage/tirecalcold.html
255/60-15 = 27"

Standard R&P = 7:31, 31 turns of the pinion to 7 turns of the ring

HB gear is 19:31 (1.632), flipped = 31:19 (.613)

Let me type up the math....

Sometimes this helps- http://www.914world.com/bbs2/index.php?aut...ach&id=3082

(RPM)(19/31)(31/7) = Wheel revolutions

Your numbers let us know that 746 revs per mile.

Divide the calculated wheel revolutions per minute (WRPM):

(WRPM)(60min/1hr) = how many times the wheel goes around in an hour (WRPH)

(WRPH)(27"/Rev) = inches per hour

(Inches per hour)(1 Mile/ X Inches) = MPH

Hey I build stuff. Math?

HB gear is 19:31 (1.632), flipped = 31:19 (.613)

31 turns of the pinion to 19 turns of the ring?

Divide the 746 wheel revolutions per 60 minutes ?= 12.43? (WRPM):

27 X 12.43= 335.61?

And now to work it:

(3400RPM)(1.63)(4.43) = 24551.06 wheel revs per minute (WRPM)

(24551.06WRPM)(60min/1hr) = 1473060.6 WRPH (wheel revs per hour)

(1473060.6 WRPH)(27"/rev) = 39772717.2" per hour

(39772717.2" per hour)/(63360"/mile) = 627 MPH.....Ok, I need to check my work

Try this
http://www.kabamus.com/garage/gears.html

Wouldn't it be a lot easier to just fly by a trooper at 5000rpms and let him tell you how fast you were flying?

OK, with Dr. Evil's R&P and final ratio for the HB I think that link is showing approx. 125.

6500 rpms would be somewhere around 232 mph... In a 914? That would make you pucker

Just checking, There was a rumor that 3 914's and a 911 Carrera passed a Prius someplace in Co. like a bullet train. I guess the GPS reading was correct. I wonder if that Prius is still in the middle of the road spinning like a top.

Oh, that wasn't the Prius. It was some other snail crawling through the canyon..

Hey, I remember that pass!
Great road!

Ya, my formula is flawed, and it is too late for me to be doing such things

another calculator. nice and simple.
www.subarubrat.com/Calc.htm

If your gear calculations are correct, then 746 rear wheel revolutions per minute= a given distance traveled in 1 minute. 746 X 85" / 12 = 5,284 feet per min. Since a mile is about 5,280 feet, then you are going about 1 mile per minute, or 60 miles per hour. Is your gearing that low?
Tom

Ah, that is where I goofed. I was using dia instead of circumference.

I remember that Prius too! He was blocking by the time I got to him... I blew by as well, and he was PISSED!!! Tried to hang in the last set of twisties, I could hear his tires howling

Hmm. Not to beat this dead horse of a tangent, but it can't be the circumference vs. diameter where you were wrong... that would increase one of your conversions such that the speed would be nearly 2000 mph!

I was thinking something was wrong when I read this line: (3400RPM)(1.63)(4.43) = 24551.06 wheel revs per minute (WRPM) That's just over 400 revolutions per second ... And I thought someone'd said already that it was 746 rpm for the wheel.

Seems like maybe the gear ratio conversion got muddled somewhere, but that's as far as my thinking takes me. I liked the simple solution of figuring out speed from the known wheel rpm of 746 ... about 60 mph at that engine rpm.

Well, the whole thing was goofed if you must at me. It was 3am and I was over confident in my math skills.


Ya Nerd

Oh, whoops. That was 746 revs/mile, not 746 revs/min. I should read more carefully.

I've got the math all right here on notes for Dr Evil's website...

MPH = ((RPM/gear ratio/r&p) * tire circumference/inches per mile)

Inches per mile = 63360
tire circumference = pi * diameter

Tire diameter = ((((width * aspect ratio)/10)/25.4)*2) + wheel diameter

I even re-wrote it for RPM's. =o)

( (MPH * IPM)/TC)(RnP)(GR) = RPM

Well, whatever the math - my speedo said 80 the last time I looked at it and someone's GPS said we did 110 - all I know is that it was a very fun drive (and the Prius never stood a chance)!

Ernie, I think your speedo is off. Did I post that?

Hey Ferg, how'd ya like that big ass "100 MPG!" Sticker plastered across the back of that Prius... Glad he's saving all that gas for us!

I was more than burning his share!

Eric and Travis, What say you....

That was obviously illegal, and I am apalled!

What...

You have to read this month's Road and Track, that was pretty much a quote from the Mustang/Camaro/Challenger writeup. Funny stuff! So how fast were you going?

Nope - electronic VDO speedo that is dead-on. I just concentrated on driving and didn't happen to look again.

My GPS said 114 mph when I was done with him. That translates to about 6 MPG I think. He could have gone another 30 miles on the raw fuel I spit all over his windshield.

Mr. DeVries hung in there admirably.

Anymore if I need to calibrate my speedo I just use my Garmin GPS. In my Audi A6 it is spot on with my speedo. On cruise control, if I increase or decrease by 1MPH, the GPS soon follows.

In my 911, I found that the speedo was off by about 3 MPH low.

Yeah the smug meter went of the charts with his sticker, I lasted about 2 corners behind his hyper miler driving style before I had enough. Since I have him two chances to pull over, and he didn't I had to do the blow by...

Thank you. Not bad for a 2 liter/4 cylinder!
FWIW, I got 31.5 mpg for the round trip Flagstaff-Fruita-Flagstaff, including this little fun run.

Really!!!?!??!

Me too... go figure!

I think we were doing 90-95 and you guys were pulling away from me.

So I say....

I think I was pulling 4200-4400 at the most.