Morning quiz: Guessing my Speed.
 

Some of you know that i just passed the Inspection Saturday in DC witch consisted of An emission test at idle. for about 40 sec.

And that was it.

So after that i when driving on River rd MD and knowing that my speedometer doesn't work, and being in 4th gear and @4500rpm what could be my estimated speed?

Thank you guys.

I'm guessing north of 80 mph?

Congrats on passing the emissions test. Victories must be celebrated....even the little ones.

As far as speedo estimate, that is a very open ended question. I guess someone in a similar/identical car can chime in. But fundamentally, gear ratios (possible differences between tranny models/versions/modifications), final drive ratio and overall tire & wheel size, will have an effect on the actual speed. If you know the car is totally stock, the owner's manual (and prolly some online resources, too) can confirm the gear and diff ratios. A number of online calculators are available where you can plug in the variables, and it will calculate your speed.

Or.....stick a GPS on your dash. ;)

Whatever it was, you were speeding! In my '86 Targa, 3100 RPM in 5th is about 65-70 MPH, so figure 4100 RPM in 4th, add 10% for 4500 and you get 72-77 MPH if you're geared like my stock gearing.

Instant-g.com has spreadsheets that show RPM/MPH in all gears based on trans type and tyre size

thanks guys, from was i know the car is stock g50 with 15 inchs.
Definitly will go for the GPS
Then look at the speedometer "sensor" on the gear box (didn't look to good when the engine was out)

G50 was in '87-89 911s. Yours is more likely the 915 trans.

If you have one, there should be a chart in the ownersmanual showing gear/speed/rpms.

I have an Application on my cell phone that uses GPS to calculate speed; but the cool part is it works as a HUD HeadsUpDisplay on my windshield!


The 'phone screen' displays the MPH backwards, which when reflected on the windshield... Well you guys get it...

Pretty cool though, and it was free!

What's the app? Damn I miss this feature from my old Bonneville SSEi!

In round number estimates, figure about 1k rpm differential in the upper gears.
So 4500 in 4th would be 3500 in 5th.
Then for estimating speed in 5th double the tach reading. 3k ---estimate 60mph. 3500 estimate about 70 mph.