Speedo/reading too high!!

[climbasky]

Happy smile on face I see the speedometer on my just bought 1973.5 T as I drive it home to Chicago from Pittsburgh read 90 then 100 then 110 mph and even higher!! This is not right, the reality quickly sinks in.
The car has larger tyres. I am guessing that this is the reason. The speedo is showing about 10 to 15 % too high. How exactly does the speedo work on these cars? I thought it worked on the RPM times the circumference of the tyres that would be original spec, and in that case the different tyre would have no impact!
Thanks!

[Won]

10% to 15% seems about right for the "safety" they put in from the factory. You know, it makes the driver feel better 'cause you think you're going faster, and still keeps you under the speed limit even if you're doing 5-over so the police are happy too.

[DW SD]

Originally posted by climbasky
Happy smile on face I see the speedometer on my just bought 1973.5 T as I drive it home to Chicago from Pittsburgh read 90 then 100 then 110 mph and even higher!! This is not right, the reality quickly sinks in.
The car has larger tyres. I am guessing that this is the reason. The speedo is showing about 10 to 15 % too high. How exactly does the speedo work on these cars? I thought it worked on the RPM times the circumference of the tyres that would be original spec, and in that case the different tyre would have no impact!
Thanks!

speed is directly proportional to tire size. stalling larger tires would actually cause the speedo to read low(er)!

Your speedo, like most in older cars is pretty far off, maybe the problem is more acute than some.

Doug

[Natog]

Just confirming, but does the optimistic speedo also reflect in high readings on the odometer?

[Won]

Someone on the board checked the odometer reading with the mile markers and they matched, no optimism there.

[Por_sha911]

Go here: Tire Size Calculator to check your tire size and see if your readings are off.

[ruf-porsche]

Get a GPS to check the top speed and the acutal speed.

[rdrr]

I have checked my speedos in both my Porsches and found that they read about 10% too high. On my Carrera I have check the odometer and found that is was the same - 10% high when checked against kilometre markers.

[climbasky]

Thanks for your input.still hoping to understand how the speedo works! And yes, the odometer also showed that it was a longer drive home than what Mapquest had estimated, by about 10 to 15 %.
Is there an inexpensive, practical, fix?

[jstobo]

If you have the mechanical speedometer. it is factory spec to read 1 mph per 800 revolutions of the rear tire. A 185/70-15 tire gives exactly 800 rev/mile. So any tire size that is different in rev/mile will affect the speedo. Best way to calculate is to mark the pavement and tire, roll the car until the tire mark is again facing the pavement, mark the pavement at the new location and then measure the distance between the pavement marks. For example if the marks are 6 feet apart. that works out to 880 rev/mile and the speedo will read exactly 10% low. You can have the speedo calibrated. They change the internal gears. But then if you change tire sizes you have to do it all over again. Hope this helps.

[bkreigsr]

Palo Alto Speedo has a calibration calculation on their website. Perform the tests, record the results, and send them you speedo to re-gear. Not sure on pricing, but count on spending $$.

http://www.paspeedo.com/ratio.htm

[ben parrish]

Sooo...., reason would tell us that your car has between 10 and 15% lower total miles on it. Not a terrible thing after all.
Ben

[Natog]

Originally posted by ben parrish
reason would tell us that your car has between 10 and 15% lower total miles on it.

That was the gist of my question. On low mile cars, it makes a big difference. I just hit 60K on my 87 cab, and I know the speedo reads high.

[bkreigsr]

not to mention, that it is also receiving regular servicing at shortened intervals. +++