No clue how fast I'm going....

BAD67FUN

Founding Member

Oct 31, 2001

589

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0

Wisconsin

Apr 14, 2004

#1

• Apr 14, 2004

• #1

Okay.. .somebody can figure this out hopefully.

I have a 67 that has original C4 and came with 2.79 gears.
No clue on the factory tire size.

I now run 275-40's (17's if you have to know) and 3.80 gears.

I'd like to know how fast I'm going at different RPM's. Or at least a ballpark guess. 30mph, 40mph, and 50mph would be good to know. Since I'll be driving it now like a stock car on pit road (no speedo's... just tach's).

Thanks.

 

65fastback2+2

New Member

Aug 4, 2003

1,229

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0

Louisville, KY

Apr 14, 2004

#2

• Apr 14, 2004

• #2

I have this problem too. When im doing about 55mph, the speedo says 80. How do I fix this, i know its a gear in the tranny for the speedo thing, but what should I change it to??
67 Coupe
Factory C4
2.83 rear gears
245/60R14 rear tires

Thanks

 

G

gp001

Founding Member

Jun 30, 2001

4,401

0

66

So. Cal.

Apr 14, 2004

#3

• Apr 14, 2004

• #3

OK, first you need your tire diameter in inches

Tire Dia. in Inches = [2 x (section width / 25.4) x (aspect ratio / 100)] + rim dia
*Gear Ratio = Transmission Gear Ratio * Rear Gear Ratio

Then you can use

MPH = ( RPM x Tire Dia) / (Gear Ratio x 336)

-OR-

RPM = (MPH x Gear Ratio x 336) / Tire Dia

Ex. 50 mph, with a C4 in 3rd gear, 2.79 rear gear, and 275/40-17 tires

TD = [2 x (275/25.4) x (40/100)] +17 = 25.66 = 26

RPM = [50 x (1 * 2.79) x 336) / 26 = 1803

 

G

gp001

Founding Member

Jun 30, 2001

4,401

0

66

So. Cal.

Apr 14, 2004

#4

• Apr 14, 2004

• #4

Forgot to add, Print yourself out a little piece of paper with the common speed limits on them and the corresponding RPM so you can make sure you're at the right speed if you are being "monitored"

Hope it helps guys

 

V

vrooom

New Member

Apr 17, 2003

139

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0

Midwest

Apr 14, 2004

#5

• Apr 14, 2004

• #5

http://pw1.netcom.com/~sgalaba/mph.htm

EDIT: THis is the one you want. Do you have a tach?

 

V

vrooom

New Member

Apr 17, 2003

139

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0

Midwest

Apr 14, 2004

#6

• Apr 14, 2004

• #6

Here is your chart based on 275x40x17, 1:1 (3rd gear c4) and 3.80 rear.

 

Attachments

• speedo.webp

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65fastback2+2

New Member

Aug 4, 2003

1,229

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0

Louisville, KY

Apr 14, 2004

#7

• Apr 14, 2004

• #7

someone do that for mine??? thanks

 

V

vrooom

New Member

Apr 17, 2003

139

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Midwest

Apr 14, 2004

#8

• Apr 14, 2004

• #8

65fastback2+2 said:

someone do that for mine??? thanks

Click to expand...

you can use that link I posted and print out the results. But...here you go.
have fun!

 

Attachments

• speedo2.webp

  16.4 KB · Views: 161

65fastback2+2

New Member

Aug 4, 2003

1,229

0

0

Louisville, KY

Apr 14, 2004

#9

• Apr 14, 2004

• #9

now i just need a tach