Pagoda SL Group
W113 Pagoda SL Group => Electrical and Instruments => Topic started by: behnamlaw on May 11, 2020, 03:43:50
-
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
-
Tire size... https://www.sl113.org/wiki/WheelsTires/TireSize
-
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
-
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?
-
that diff between 100 and 104 is less than mine mine is +10mph but a member suggested tire size
-
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.
-
well it s relief as I was thinking some bigger issue
Calibrate the speedo and foogetaboutit :)
-
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?
-
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
-
https://tiresize.com/calculator/
-
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