If our speedos are off, so are our odos

[porschenut]

It's been well documented here that our 911 speedometers came miscalibrated from the factory and are optimistic. Based on all the past threads I've read, indicated speed is typically around 5 mph faster than actual speed, give or take, depending on the mph range you're driving in.

And I suspect with over two decades of age they are even more off than that. My car, for example, shows 80 mph when I'm doing about 65.

I presume the vast majority of these cars have not had their speedos recalibrated, so that would mean our cars have a lot fewer miles on them than the odos indicate. perhaps as much as 10-15% fewer. At highway speed, my odo turns over 5 miles for every 4 I actually travel.

So when somebody asks you how many miles are on her, subtract 10%.

[porschenut]

Never mind. Been discussed before.

[JFairman]

This is not always so if the speedometer is an electronic one because a seperate electric motor turns the odometer gears.

The speedometer section has it's own electric motor and the analog electronics and controllers for these old speedometer motors was not accurate back then and worse today compared to digital controllers.

[Trackrash]

Even the old mechanical ones only have a spring to "calibrate" the speed needle. The adjustment of the spring can change the speed reading without affecting the odometer.

[robertmark]

My 88 911 speedometer also reads 80 m.p.h. when my gps is showing about 65-67. I never thought that would effect the odometer, but on these older cars it probably does. How did you calculate you had driven 4 miles and it registered 5??

[porschenut]

Originally Posted by robertmark

How did you calculate you had driven 4 miles and it registered 5??

Just math. 80/65 * 4 = 5. I haven't verified it. I just assumed it because I thought the odo was calibrated to the speedo. I found some past threads where the consensus was this is not the case, at least for the electronic speedo 84-89 cars. So I believe my original post is in error.

[5String43]

Not necessarily, I think, at least not as a rule of thumb. The speedo on my daily driver is 5mph fast at 75, but the odo is right on the money.

[Matt Monson]

Are you tires sized identical to how they left the factory? Have they always been?

If not, then even with an accurate odometer it's off. Just something to think about with the number of people who change to low profile tires or put 16's on cars that came with 15's, etc. etc.

[nineball]

i am doing 70 when my speedo shows 81-82. i generally stick to tach readings now as 2900 in 5th = 70.

[RoninLB]

i adjusted the pot in my speedo and posted the info and pic

afaik there are 3 different electric designs so those that have the 1977 design can be done

[maxman]

I have the opposite problem of most of you. My speedo reads about 15% low. 50 = 57, 60=69, 70 = 81, and so on. Sounds like that might not mean I have more miles though. My car is euro spec and was officially converted in '85, and I believe they just did a speedo reface as mine goes to 180. Perhaps they refaced it wrong.

[Drisump]

Oddly enough,my 85 is the most accurate speedo in any car I've ever had and the mileage on the odo is close to perfect. I'm thinking that almost all cars have tires that are not quite the same diameter as the tire that car left the factory with (sometimes radically different). To rectify, I'm sure the electronic instruments can be calibrated, that being said, you guys must have mind modern supercar acceleration...at least in the eyes of the passenger. Cheers

[dshepp806]

I would think the circuit allows for complete independence of speedo quantification from Odo quantification?

Doyle

[rick-l]

The odometer is just a divider that increments a stepper motor based on counts from the drive flange and the reed switch.

The derivative of the counts dx/dt is derived using a one shot with separate timing capacitors and resistors. Yes they are independent.

smokey speedo

[RoninLB]

I like Warren's note of installing a trim pot