MPH@Engine Speed
 

I have a '81 911SC (3.0L,915trans, USspec) with a stock speedometer (85MPH). If possible I would like to compare MPH @ RPM. I have stock tires on Fuchs.

Right now I am reading the following:

- 3000RPM = 75MPH

Would like to get examples at the following:

- 2000RPM = ???MPH
- 4000RPM = ???MPH
- 4500RPM = ???MPH
- 5000RPM = ???MPH

Thanks

If you look in the tech articles section here on the PP site I have a program called GEARS and it runs on PCs and will do just about what you are looking for. Good luck.

Thanks John,

I have a problem in that I don't have a manual (on order). I would appreciate if anyone has the info for the ratio for each gear and the final drive ratio. Again it is a stock US '81 SC.

Thanks in advance.

Hi 911SC
-
On your reading above, are you in 4th or 5th gear? With my 83SC, in 5th, 3,100-3,200 gives me 80 mph.
-
In 4th gear, doubling the first two digits of the tach reading gives me my speed - e.g., 3,000 rpm = 60 mph, 3,500 rpm = 70 mph, etc. In fourth, this reminds me of driving my old 356s, where the same ratio worked. Of course, the 356s, at least mine, only had 4-speed trannies.
-
BTY, I am running Fuchs 8x16 in the rear with 245/45s - but the readings were identical with my old Fuchs 7x16s with 225/50s. See the thread below entitled SO-2s.
chuck

I'm not quite sure what gear that you want to know rpm-mph for but you can figure 5th easily. RPM/MPH is a linear relationship.
If 3000 RPM = 80 MPH then 4500 RPM = ((80/3000)*4500) = 119.9 mph. sub any other RPM for 4500 to get speed. Hope this gives you what you need.

911SC

I have a 1982 Euro 911SC, I also get 75mph @ 3000 rpm in 5th gear. Based on that your car should run 50 mph @ 2000 rpm and 100 mph @ 4000 rpm, and 125 @ 5000 rpm. all this assumes that your speedometer is accurate to begin with. I think Porsche speedometers of this vintage are a few mph optomistic. I'd be curious to see what other's experience is.

Seth

911SC

I just checked out an old issue of Road & Track Magazine (July 1985). For the 1985 911 which I believe has the same gear ratios they show 24 mph per thousand rpm in top gear. So....
1000 rpm = 24 mph
2000 rpm = 48 mph
3000 rpm = 72 mph and so on.

The car they tested had 215/60-15 tires on the rear, my car has the more common 225/50-16 tires.
There is a great tire size calculator @ www.miata.net/garage/tirecalc.html
I used this and calculate our cars should be approx. 23.7 per thousand in top gear.

Seth

I must have some oddball gear in my '78 SC 915 tranny, 60mph = 2900RPM in 5th. Speedo has been checked and is true, tach functions properly. Normally runs 3400 RPM on the interstate going with traffic @ 70mph.
To answer 911SC's original guestion you just do the math (as Scott and Seth show). If 3000 = 75 then 6000 = 150, etc.

Rarlyl8,

I'm hav'n some fun going through my old magazines and just came across a road test of the 1978 911SC in the April 78 issue of R&T They show the same overall final drive ratio as the 85 model but a calculated 23.2 mph per 1000 rpm in top gear(with 225/50-16 tires). They state 60mph should be 2590 rpm. It is possible that your car's final drive is different as they are available.

Seth

Hey everyone,

Thanks for all the input. I took John's suggestion and used the "gears" program that he recommended and came up with the following:

RPM MPH
3069 75
3478 85
3887 95
4092 100
4910 120

I used the following gear ratios:
1st - 3.181
2nd - 1.833
3rd - 1.261
4th - 1.000
5th - 0.785
final drive ratio - 3.875

If anyone needs the ratios for their specific model please let me know. They are outlined in the book "Porsche 911, Forever Young" by Tobias Aichele.

Hi,

I have a european 81SC Targa.

The owners manual show the following for 185/70 VR15 - 215/60 VR15

gear 1 2 3 4 5

rpm speed MPH
2000 12 20 31 39 50
3000 19 31 45 57 75
4000 24 40 60 77 96
5000 30 52 76 96 121
6000 35 62 90 115 146
6500 39 67 98 124 N/A

I have taken liberty in visually deciding what the speed is at the rpm for the gear. So please do not attack me for my visual estimates.

The maximum speed shown is 140mph although the text states 146mph. 0 to 62 mph is 6.8 seconds (at curb weight and half load capacity).

regards

Neil

Note this is the 204hp version.