Re[2]: 4:10's in PDX

[Richard Welty <rwelty@domain.elided>]

On Wed, 22 May 2002 11:27:04 -0700 Norm Riffle <normriff@domain.elided> wrote:

>     Looks like I'll have to check with APE again......
>     On a related note....
>         given that the 105-115 series cars came with a 78 series tire
> and that many people have switched to a lower profile 60 or even 55
> series tire throwing the speedo way off....
>         Which differential ratio would bring the speed/RPM back to
> "normal".  Would the 4:30 or the 4:10 be closer the where the factory
> intended?

a switch to a lower profile tire won't automatically throw the speedo/odo
way off.

there is a handy little program at
ftp://ftp.krusty-motorsports.com/Autos/tire-size.c 

(there's also a lisp version for the truly sick among us; i was a
professional lisp programmer for 10 years, and still miss it.)

here is some sample output:

bash-2.04$ ./tire-size 165/78R14 185/70R14 195/60R14 205/50R14
Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
165/78-14         5.1in  12.1in    24.1in         75.8in        836        0.0%
185/70-14         5.1in  12.1in    24.2in         76.0in        834        0.3%
195/60-14         4.6in  11.6in    23.2in         72.9in        869       -3.8%
205/50-14         4.0in  11.0in    22.1in         69.3in        914       -8.5%

keep in mind that the profile is a ratio between sidewall height and tread
width; a wider tire with a lower profile can be pretty close. my 195/60R14s
are off a little, but not much.

it'd be pretty easy to build a spreadsheet for this, and rig it to include
gear ratio effects as well. tire-size.c dates from before spreadsheets were
common technology.

richard
--
Richard Welty                                         rwelty@domain.elided
Averill Park Networking                                         518-573-7592
              Unix, Linux, IP Network Engineering, Security
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