Speedometer Reading Correct?

[rcgt350]

I took my GT 350 for a drive this morning, I've 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 it's 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]

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]

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]

  I didn't do any figuring, but, at 70MPH my tach reads 3500 RPM.

[gt350bp]

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
gt350bp

[Coralsnake]

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.

[rcgt350]

Ok, well I'll try getting that app loaded and make another run, tires are the original Goodyear Speedways too.
Thanks

[S7MS427]

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.

[deathsled]

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.

[FL SAAC]

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

[texas swede]

I always use the program by Bowling and Grippo. Easy and exact.
Texas Swede

[Road Reptile]

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]

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

[69mach351w]

  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.