Tyres/wheels/speedo reading

[matt-me]

Finding this a little complicated to get my head round. Ideally I'd like my speedo to be as accurate as a 47 year old speedo can be. I have standard 14" wheels at the moment, if I go 185/55's on the front how does this affect the speedo reading?

Thanks
Al

 

[matt-me]

Sorry, with new wheels 15" diameter...

 

[sparkywig]

The stock tyre is 185/C/14 which works out about 185/82/14. This has a rolling radius of 1987.5mm.
If you change to a 15" wheel with 185/55/15 tyres the rolling radius changes to 1762.81mm. This is a difference of -224.69mm (or -11.31%).
So when your speedo reads 70mph you'll actually be going 62.08mph.

To get the stock rolling radius with a 15" tyre you'll need a 195/70/15 which is just under at -15.08mm (or -0.76%).

 

[matt-me]

thanks sparky, appreciated

Al

 

[nobbly]

If money is no object fit this....

https://www.vwheritage.com/ac957035-smiths-digital-speedo-t2-68-73-90-mph-oe-style-grey-face

... and whatever the hell tyres you like :lol: :lol: :lol: :lol:

 

[Happystamps]

Bear in mind that not all speedos are created equal, they're within a tolerance. I have found that my camper speedo reads about 7% slow, so if it says I'm doing 60, i'm really doing something like 54. You can compare using a satnav, and if you want it to read higher or lower you could always see what size of tyre is around