Hi all: the actual difference between "actual" and "rolling" is not a
significant factor in this equation. Assuming 1 inch of "compression" while
rolling, there is not a significant difference in mph. For example, using a
circumference of 71 inches and a 3.89 rear end with a 1:1 final drive, at
60mph the rpm's are 3500(according to my Sunbeam SpeedCalc software). With
a circumference of 70 inches at 3500 rpm it calculates 59 mph. OK, so it's 1
mph different. I think most of us would be delighted if we had something
close to this kind of accuracy.
The difference between the 3.89 and 4.22 rear end is more dramatic. Again,
using a 71 inch tire circumference, the 4.22 rear end requires 3765 rpm to
make 60 mph compared to 3500 with a 3.89.
My 5 speed with a .783 final drive ratio and 3.89 rearend requires only 2948
to make 60.
my 2c worth
Paul
-----Original Message-----
From: owner-alpines@autox.team.net
[mailto:owner-alpines@autox.team.net]On Behalf Of Thomas Wiencek
Sent: Friday, June 29, 2001 9:13 AM
To: alpines@autox.team.net; Ron Tebo; owner-alpines@autox.team.net
Subject: Re: Overdrives and rpm]
Wouldn't the rolling diameter be less since the tire is compressed?
On 6/29/01, Ron Tebo <tebomr@cadvision.com> wrote:
All:
Could it be possible that some people are using true diameter to
calculate while others are using rolling diameter?
Ron Tebo
Thomas Wiencek wrote:
>
> Where did you get your diameters from? They seem a little high. Check
out this site for all size tires.
> Welcome to the Turbo! Saab tire size calculator!
> Thanks to our friends at Miata.net, We've ...
> www.secret-secret.com/turbo/DIY/tirecalc/turbotire.html
>
> On 6/27/01, J Arzt <humber_snipe@hotmail.com> wrote:
> Don't forget the value of pi in the equation, Jan - there's a pretty
> significant difference in the circumference of the two tire sizes. The
> diameter of a 560x15 measures out at about 25.90", and the 560x13 at
23.60".
> That can be calculated to a circumference of about 81.35" and 74.10".
> Right off the bat, you have to increase the revs by about 10% just to make
> up for the tire change.
>
> So, if you compare percentages, the 4.55 Minx axle, less the 10% advantage
> of the 15" tire, cuts to about a 4.10, or just slightly longer-legged than
> the 4.22 Alpine. The 3.89 Alpine rear would only be about 5% better than
the
> Minx, when corrected for tire size.
>
> My goal is a 4.22 Alpine rear in my 58 Minx convertible (with 15" tires),
> along with a 1494 or 1592 engine. That will give me a comparative ratio of
> about 3.80 to 1, (slightly better than an Alpine with a 3.89 and 13"
> tires), and enough power to push it.
>
> Of course, then the trick is to find a non-synchro first overdrive trans
to
> put in............. after all, I DO live out here in the middle of
nowhere,
> where it takes hours of interstate travel to get to anything! lol
>
> Jon Arzt
> Omaha, NE USA
>
> ----- Original Message -----
> From: "Jan Eyerman" <jan.eyerman@usa.net>
> To: <Jay_Laifman@countrywide.com>; <alpines@autox.team.net>
> Sent: Wednesday, June 27, 2001 7:48 PM
> Subject: Re: [Re: Overdrives and rpm]
>
> > I am not sure if those numbers are right, I have calculated 3500 RPM for
a
> '59
> > Minx with a 4.55:1 rear with 5.60x15 tires. Even with smaller wheels, I
> would
> > assume an Alpine with a 4.22:1 rear would have to do better, and a
3.89:1
> rear
> > MUCH better.
> >
> > Jan Eyerman
> > 1959 Hillman Minx Series III DeLuxe
> > 1973 Hillman Avenger DL
> >
> >
> >
> >
> > Jay_Laifman@countrywide.com wrote:
> > Tom writes:
> >
> > >With all this talk of Overdrives, ...
> > >does anybody know how fast the Series
> > >V engine is approximately turning at
> > >65 mph?
> >
> > My speedo is completely accurate, as it was just calibrated, but my tach
> is
> > probably off. I do recall others reporting certain RPMS, etc. I do
> > believe though that before I changed to the OD (and the speedo was not
way
> > off), my tach would read about 4000-4100 at 65. Now, with the OD and
the
> > calibrated speedo, at 65 I'm reading about 3600. I believe others have
> > reported 3300 rpm at 65. For a long time, I've believed my tach to be
> > about 250-300 high. So, this makes sense.
> >
> > That all said, I think someone had and equation for all this, and thus
had
> > the exact numbers, perhaps taking into account some tire size change
too.
> >
> > Jay
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