Fast Speedo

[behnamlaw]

Hi members
1970 280SL
My Speedo reads about +/-10mph more than it should.
I noticed it first when I was on the freeway and then confirmed it by using my maps app on my iPhone that shows your speed. 

I’m taking it to a local Speedo NHS shop to be tested and calibrated but curios why it’s doing it. 
Swapped incorrect Speedo ?
Swapped differential?
Defective Speedo cable ?
TIA

[Peter van Es]

Tire size... https://www.sl113.org/wiki/WheelsTires/TireSize

[MikeSimon]

Consider one thing: In Germany it is illegal to have a motor vehicle speedometer indicate a lower speed than you are actually going. That's why, virtually forever, German manufacturers make their speedometers to go "fast". In the motorcycle community, BMWs are notorious for showing more than you go.
As per laws in Germany, a "calibrated" speedometer for a vehicle that requires one (taxis, police cars etc) has to be set at between -0 and +4%. So, even a calibrated instrument could show 104mph at an actual 100mph

[behnamlaw]

This was the result
I am running 195/75-14

Tire Size Comparison
Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
185/60-14         4.4in  11.4in    22.7in         71.4in        887        0.0%
195/75-14         5.8in  12.8in    25.5in         80.2in        790       12.2%

so the rev miles is 998 compared to 790
a drop?

[behnamlaw]

that diff between 100 and 104 is less than mine  mine is +10mph but a member suggested tire size

[Alex D]

Agree with Peter, tire size, I had the exact issue similiar numbers a few months ago.  If you get the correct tire size, 185 R14 you still will have to recalibrate speedo. 

[behnamlaw]

well it s relief as I was thinking some bigger issue
Calibrate the speedo and foogetaboutit

[MikeSimon]

If it is the tire...Making the speedo show faster than actual would be caused by a smaller tire! Smaller tires turn faster at a given speed than a larger tire thus making the speedometer go faster.
How can a 195/75-14 cause the speedo show faster?
I have a little trouble - as usual -  with the calculation. Why compare with a 185/60, and where does the 998 come from?

[behnamlaw]

I tend to agree with you as if this was a 195/75-13 or 15 or 16 then it makes more sense- - or + mph
from what I know the 195/75 denotes (195) tire width and (75) side wall height so I dont see how it makes sense as Peter suggested  but I am no expert hence why I posted
I called a speedo shop and they asked at what speed do i see the difference and when I replied at 70mph it reads 80mph they said it cant be the tire size
maybe i should check if the differential was changed to a diff ratio

[jeffc280sl]

https://tiresize.com/calculator/

[behnamlaw]

Diameter
22.7" 25.5" +12.3%

Width
7.3" 7.7" +5.5%

Sidewall
4.4" 5.8" +31.8%

Calcs say revs per mile is less for my tire size than mfg recommended tire size

See calcs below

Circum.  71.4" 80.1" +12.3%

Revs/Mile 887 791 -96