Author Topic: Speedometer Reading Correct?  (Read 1738 times)

rcgt350

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Speedometer Reading Correct?
« on: September 30, 2020, 02:10:42 AM »
I took my GT 350 for a drive this morning, Iíve ever been real sure how accurate the speedometer is. So I wrote down a few numbers when I got back. Do these numbers seem close? I know first gear is long, but doing 60mph feels a bit slow, but maybe itís different in an older vehicle.
I really need to get side by side with a new car to compare, but just wanted to throw this out there first.

 3.89 rear end

 20 mph @2000rpm in first gear
 30 mph @3000 rpm in first gear
 60 mph @2700 rpm. In 4th gear

rmarble57

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Re: Speedometer Reading Correct?
« Reply #1 on: September 30, 2020, 07:18:56 AM »
Download the DigiHUD app on your phone and compare to your speedo.  That's short for Digital Heads Up Display.
Of course have someone else holding your smart phone while you drive.

gt350bp

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Re: Speedometer Reading Correct?
« Reply #2 on: September 30, 2020, 08:30:47 AM »
Depends on your tire diameter. I used a conversion chart available on line to plug in the gear ratio and actual tire diameter and it will provide speed for every 100 rpm in fourth gear based on a 1.0 fourth gear ratio. I have a chart that I taped to the dash when I drove from Daytona to Pittsburgh for the SAAC
Convention the first time. I know use a GPS, as I hooked up the cigar lighter.

Don
gt350bp

roddster

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Re: Speedometer Reading Correct?
« Reply #3 on: September 30, 2020, 10:56:38 AM »
  I didn't do any figuring, but, at 70MPH my tach reads 3500 RPM.

gt350bp

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Re: Speedometer Reading Correct?
« Reply #4 on: September 30, 2020, 11:02:40 AM »
I have 245-60 x 15 rear tires and they are 26.6" diameter. Chart indicates 71 mph @ 3,500 rpm with 3.89 gears.

Don
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Coralsnake

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Re: Speedometer Reading Correct?
« Reply #5 on: September 30, 2020, 11:55:33 AM »
While rpm is related to speed, the best way to determine if your speedometer is correct is to measure the speed over a measured distance.

Time yourself on the highway. It should take you exactly 60 seconds to travel one mile at 60 mph.

If you speedometer reads incorrectly, you can change easily change the driven gear. From under the car, remove the cable from from the side of the transmission. Increasing the number of teeth on the gear will slow your speedometer reading. Decreasing the number of teeth on the gear will make your speedometer read faster.

I maybe wrong but isnt rpm also related to compression ratio? You are also (incorrectly) adding in the fact the tachometer may not be accurate.
« Last Edit: September 30, 2020, 12:05:18 PM by Coralsnake »
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rcgt350

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Re: Speedometer Reading Correct?
« Reply #6 on: September 30, 2020, 12:39:53 PM »
Ok, well Iíll try getting that app loaded and make another run, tires are the original Goodyear Speedways too.
Thanks

S7MS427

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Re: Speedometer Reading Correct?
« Reply #7 on: September 30, 2020, 07:28:36 PM »
I maybe wrong but isnt rpm also related to compression ratio?
I've never heard of a direct mechanical link between RPM and CP.  CP is a factor in how much power the engine is capable of producing.
Roy Simkins
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deathsled

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Re: Speedometer Reading Correct?
« Reply #8 on: September 30, 2020, 08:46:04 PM »
If your state posts unmanned mobile digital radar speed signs by law enforcement to advise you of your speed use that and make a few passes. The app would be far easier though so I vote app.
"Low she sits on five spoke wheels
Small block eight so live she feels
There she's parked beside the curb
Engine revving to disturb
She's the princess from his past
Red paint gold stripes damned she's fast"

FL SAAC

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Re: Speedometer Reading Correct?
« Reply #9 on: October 01, 2020, 09:47:30 AM »
Download any of the apps that clock your vehicle when it's moving.

True story, got stopped by a police officer

Officer : you know how fast you were going ?

Me : very bold of you to assume that the speedometer works in this piece of crap....

Officer : License and registration

It ended well he called his supervisor and we ended having lunch and talking about war stories
These cars are meant to be driven so enjoy the hell out of all of it. Not just the look of it when its all cleanCS

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texas swede

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Re: Speedometer Reading Correct?
« Reply #10 on: October 01, 2020, 11:28:12 AM »
I always use the program by Bowling and Grippo. Easy and exact.
Texas Swede

Road Reptile

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Re: Speedometer Reading Correct?
« Reply #11 on: October 01, 2020, 05:09:15 PM »
Hi Randy &  All 67 Fans,
Just checked and found a great article on this in the BOSS 302 Registry 1992 Edition
Also checked and counted 16 teeth on the driven gear (on the speedo cable)  As stated the biggest change is the tire circumference which must match the original tire to maintain any
accuracy. Many tire manufacturers have a chart that gives you Revolutions Per Mile With this info and some simple math it is easy to figure out. The odometer is what must be accurate. (speedo is
usually close when odometer is within the 3% allowable range.
Really great to know you have driven the car !!!
Regards R.R.

Richstang

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Re: Speedometer Reading Correct?
« Reply #12 on: October 03, 2020, 10:39:44 AM »
We received copies of an original owner customer complaint sent to SAI 7/27/67 (for car 67 #00244).
The owner was a mechanical engineer for Ford. He addressed this speedometer issue and many others.

https://groups.google.com/d/msg/shelbyresearch/9qQFHmagzRU/iM_41vgnCAAJ

relevant excerpt:
The speedometer and odometer readings were approximately 10 percent low when the car was first received.
It was necessary to remove the 18 tooth (natural color) drive gear and install a 16 tooth (orange color) drive gear.
Low odometer readings are tough on gas mileage figures and low speed readings are great for receiving speeding tickets (especially with a G.T. 500).   

Rich
« Last Edit: October 03, 2020, 10:43:23 AM by Richstang »
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69mach351w

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Re: Speedometer Reading Correct?
« Reply #13 on: October 04, 2020, 04:21:59 PM »
  I didn't do any figuring, but, at 70MPH my tach reads 3500 RPM.
That sound right. My 289 standard build with 3:00 rear gear original C4 trans at 60mph will be turning 2300rpm.