Speedos all wrong!!

[flipper]

My speedos out of wack. @70 on the dial, I am only going about 62-63mph. I am running stock wheels with 195/60 15's all the way around. Any ideas? Or is it because my car is brown?

[Porsche virgin]

Mine is about the same amount off, despite using the correct tire size. I think that there is a thread on the subject, so you might want to do a search for more info. I have decided that I'm too lazy to pull it out and get it recalibrated.

[Early_S_Man]

The problem isn't your speedo ... your tires are 1.3" shorter [24.2" vs 25.5" for 185/70-15s] so more revs per mile = higher indicated speed than you are actually travelling!

[flipper]

So because of the tires, my car has less mileage than it actually shows on the odometer? Is the factory size the 185/70-15?

[Early_S_Man]

Yep to both questions!

[Paulporsche]

Flipper and Warren,
When I do the math, I get 25.2" for 185/70/15 [(185x1.4/25.4) + 15], and 24.2" for 195/60/15 [(195x1.2/25.4) +15]. Since circumference is 3.14 x diameter, I only get a 4% difference between the two tires. That is less than 3 mph difference, not 7 or 8. Are you counting on the fact that the speedos are a little optimistic to begin with? Did I leave out something here?

[LakeCleElum]

I got my speedo back in line using larger tires. Used this web site to find the correct size:

http://www.miatanet.com/garage/tirecalcold.html

[87coupe]

BTW, if your speedo is reading fast, you're tacking on more miles than you are travelling. Not the other way around.

[toolman]

I think all German speedos read fast. The 911 is about 6mph fast at 70 and the 740 is 5mph fast at 70.

[jstobo]

On my early car with a cable Northhollywood Speedometer told me that the Porsche Spedometers were set to read for tire revolutions of 800 per mile. I am not sure about the elecric ones. But 185/70-15 size are 800 rev/mile. The best way to check your rolling circumference (since there is differences between tire manufacturers) is to mark the tire and pavement where it touches the pavement, roll the car forward(or backward) one revolution untill the mark on the tire again touches the pavement, mark the pavement and measure the distance. Divide 5280 ft by the feet you got from the measurement and you get rev/mile. By the tire calculator my tires were supposed to be 861 rev/mi but turned out to be 880 rev/mi(6 feet circumferance). So speedo reads exactly 10% high.

[BoogieOnSkis]

Mine is about the same amount off, despite using the correct tire size. I think that there is a thread on the subject, so you might want to do a search for more info.

I'm not finding the thread you mentioned. Do you mind posting a link?

Thanks!

[Rod.911S]

If you look at road test reports on 911s from the '70s, the speedos are always indicating high. For example, on a '76 911S, R&T had 56 mph true for 60 indicated, 66 at 70, and 76 at 80, with 9.8 miles for 10 indicated on the odo. On the Turbo, it was even further off: 43 for 50, 53 for 60, 63 for 70, and 72 for 80 mph.

Gonna check mine against a GPS tomorrow.

[darkbeer]

speedo ? speedo ?
who ever reads their stinking speedo ???

[dmarv]

I thought my car actually was traveling 200mph. You mean may speedo my be off?

[Rod.911S]

I checked my 911 against a GPS (Garmin which is spec'd at 0.1 mph accuracy). First-off, my tires are 5.2% smaller than stock which means 5 kph higher indicated than true. Plus, there's also some reading inaccuracies -- seems my wife had never done 170 kph before...

Anyway, here's indicated kph / measured kph:
30 / 25.5
40 / 36
50 / 43
60 / 52
70 / 61
80 / 71
90 / 81
100 / 89
110 / 100
120 / 107
130 / 120
140 / 129
150 / 139
160 / 149
170 / 155

So, through the normal highway speed range, I figure I'm around 10 kph indicated lower than true. Also checked my Saab and its 5 kph lower.

"Tell it to the judge, son..."

[Carl Bauer]

Originally posted by Rod.911S
If you look at road test reports on 911s from the '70s, the speedos are always indicating high. For example, on a '76 911S, R&T had 56 mph true for 60 indicated, 66 at 70, and 76 at 80, with 9.8 miles for 10 indicated on the odo. On the Turbo, it was even further off: 43 for 50, 53 for 60, 63 for 70, and 72 for 80 mph.

Gonna check mine against a GPS tomorrow.

Correct. By law speedometers in German cars (even in cars that were for sale in the US) have to read fast. There is an article in a 70's Christophorus issuse that discusses the matter throughly. I'll see if I can find it.

[stv951]

My

Sorry about the pic quality
My speedo reads almost 210 klms/h close to the cut-off in forth(6800rpm). Is that correct ?
Other C3,2s dont get uo to 6800 rpm.

[stv951]

Thats 4th gear BTW