Speedo reading

Reply Subscribe

Thread Tools

Search this Thread

Mar 8, 2016 | 05:07 PM

prestige6

Thread Starter

Safety Car

Joined: Sep 2006

Posts: 3,540

Likes: 138

Speedo reading

With 370 gears what should my tach be reading at 60mph???

Mar 8, 2016 | 05:13 PM

vark_wso

Melting Slicks

Joined: Aug 2004

Posts: 2,058

Likes: 554

From: Sun Prairie WI

What size are your tires?

Mar 8, 2016 | 05:46 PM

irwiny

Racer

Joined: Jun 2013

Posts: 337

Likes: 27

From: Fremont CA

Quote:

Originally Posted by prestige6

With 370 gears what should my tach be reading at 60mph???

That would depend on which gear and if you have a 4 or 5 speed. If you want a more accurate answer for your car, maybe use a portable GPS to determine your actual RPM vs Speed.

Mar 8, 2016 | 05:51 PM

Viet Nam Vett

Le Mans Master

Joined: Apr 2002

Posts: 5,537

Likes: 280

From: Egg Harbor Township NJ

2015 C2 of Year

St. Jude Donor '04-'05-'06-'07-'08-'09-'10, '14-'15

Also keep in mind that the older gauge speedo has a 5-7% speed error. They are not that accurate.. This I found out from a "WIllcox" Speedo repair Video.

Mar 8, 2016 | 05:52 PM

Donald #31176

Melting Slicks

Joined: Jul 2004

Posts: 2,869

Likes: 196

From: Maryland

Using 27 inch diameter tire and 4th gear ratio of 1=2800 rpm

Mar 8, 2016 | 06:04 PM

vark_wso

Melting Slicks

Joined: Aug 2004

Posts: 2,058

Likes: 554

From: Sun Prairie WI

Very simple calculation:

Tire revs/mile X rear axle ratio is ALWAYS engine rpm @ 60 mph in high gear with 1:1.

Assume you're running 7.75-15 bias ply or perhaps 205/75R-15 radials, then your tire diameter is ~27.2".

Assume you're equipped with 3 or 4 speed manual or Powerglide in high gear (1:1), then --

(27.2 x 3.14)/12 = 7.12' tire circumference
5280'/7.12' = 742 tire revs/mile and ...
742 x 3.70 axle ratio = 2744 engine rpm @ 60 mph

Last edited by vark_wso; Mar 8, 2016 at 06:36 PM. Reason: typo

Mar 8, 2016 | 09:34 PM

buns

Safety Car

Joined: Feb 2006

Posts: 4,142

Likes: 704

If anyone needs information along these lines, I still have my first computer purchased in the seventies, made by Iskendarian. It even goes up to 12,000 RPM!!!!!!

.

Attached Images