Porsche Enthusiasts Club Forum

I was giving my new car some hoot this evening and noticed that the old dears in thier cars were travelling at an average of 80 to 90mph?

I then realised that my car has had the original 16" wheels changed for 17" jobbies.

Anyone care to work out how much this has effectd the speedo accuracy?

I had an indicated 130mph (on a private road of course), but what would it really have been?

The OD of the wheel if anything should be slightly smaller. My speedo over reads by about 5% in theory. No bad thing as using the sat nav for reference it's bob on at 40. 75 cars doing 70. 140 cars doing. Well I never go that fast. Theres a box of tricks on the gearbox that sends a signal to the speedo head. Maybe you have the wrong one. There are quite a few part no's. Pet should give you the part no & it's embossed onto the sensor. If that parts right then you could have the wrong speedo in the car.

Someone posted this useful link not long ago, which gives you the speedo difference

http://www.1010tires.com/tiresizecalculator.asp

Sorry for stealing the thunder of whoever posted this originally

Generally the larger wheels come with lower profile tyres so the overall rolling radius stays the same.
This site shows the differences http://www.miata.net/garage/tirecalc.html

Those were good sites. Question though, which tyre sizes should I be putting in? I take it the front ones. On those sites my front tyres which are now 18" have a speed difference of 0.9% so 60mph on the speedo would be 60.6mph on the road. My rear tyres came up as 2.5% difference though so 60mph on speedo would be 61.5mph. 1.5mph doesn't really bother me but should I be concerned or am I reading the results wrong?

Only one thing to do to check it is to get someone, with a reliable speedo, to drive at 70mph in front and see what yours reads. It maybe that all the old dears that day are ex-racing drivers!

Not sure what could really go wrong to make the speedo read faster, if the pick up in the gearbox was missing the odd pulse it would read slower.

or a gps sat nav... thats what i use... its amazing how far out they can be, 60mph, which is just under 100kmh, well i have to do 110 to be doing sixty...

i went past one of those speed indicators today and it showed 28 mph, whereas my 944 was indicating 32 mph, i went back and tried it in the pug, showed 30 mph whereas the pug speedo showed 31/32 mph

On a late oval dash 944 it is the rear wheels as it is driven electrically off a sensor in the box. On the early 944 & all 924 & variants it is driven from a front wheel hub.
The 944 is miles out, basically I think the technical expression is the speedo is shagged I do have a spare one which is a roundtuit job.