the baron Posted September 20, 2018 Report Share Posted September 20, 2018 So My boxster has been running standard wheels with 255.40.17 rears and 205.30.17 front tyres. Ive recently bought a set of Cayman S wheels that came with 235.50.17 rears and 205.35.17 front tyres. With the old wheels/tyre combo the speedo was always about 10% higher mph then the Garmin Satnav that I presume is more accurate. With the new wheel/tyre combo the speedo is spot on with the Garmin mph right up to 100mph (private road) Did Porsche get the tyre sizing wrong as the extra 10mm seems to have brought the speed in line with the actual speed now being a larger circumference. Thoughts. Link to comment Share on other sites More sharing options...
PaulQ Posted September 20, 2018 Report Share Posted September 20, 2018 You mean 55 profile fronts, so 987 sizes? Link to comment Share on other sites More sharing options...
the baron Posted September 20, 2018 Author Report Share Posted September 20, 2018 11 minutes ago, PaulQ said: You mean 55 profile fronts, so 987 sizes? Sorry yes 55 fronts Link to comment Share on other sites More sharing options...
PaulQ Posted September 20, 2018 Report Share Posted September 20, 2018 Under 5% https://www.willtheyfit.com/index.php?width=255&aspect=40&diameter=17&wheelwidth=8.5&offset=48&width2=235&aspect2=50&wheel_size=17&wheel_width=8-5&offset2=40 Link to comment Share on other sites More sharing options...
Bike Loon Posted September 20, 2018 Report Share Posted September 20, 2018 Manufacturers deliberately produce speedos that over read by between 5 - 10%. The reason is simple, if they set it to be spot on and then it was out by a couple of mph you could get caught speeding and blame the speedo / manufacturer. So with it over reading, when your speedo is reading 44mph you would be doing 40mph so to get caught speeding in a 40mph limit for example your speedo would be reading nearer 50mph, onus of responsibility for speeding squarely on the driver. Link to comment Share on other sites More sharing options...
PaulQ Posted September 20, 2018 Report Share Posted September 20, 2018 How do you find the looks on the 987 tyres.? I thought they were too chunky when I flitted them to my car, but that was when it was lowered 30-35mm Might try them again now the car is back on standard springs, as I think these are a bit nicer looking wheel then the twists. Link to comment Share on other sites More sharing options...
the baron Posted September 20, 2018 Author Report Share Posted September 20, 2018 I love the look of the actual alloys, they work really well on the 986 however they do expose a bit more brake disc and calipers hence my other post showing my refurb of brakes. Re the chunkier tyres I think they look great, they fill the wheel arches out much better and the ride is more compliant, I don’t get as much crashing over bumps which in turn creates less rattles etc especially with the soft top roof. My car is running the M030 suspension so its lower than normal so works well with the slightly higher profile tyre. There has been some debate over the 235 v 255 rears, when i compared both when off the car the 235 Michelin Pilots were actually wider than the 255 Nexan tyres which I don’t understand. Handling wise no noticeable difference on the road, I haven’t tracked the car with the new tyres yet so that’s when I’ll probably notice the difference if any Link to comment Share on other sites More sharing options...
Southy Posted September 20, 2018 Report Share Posted September 20, 2018 7 hours ago, Bike Loon said: Manufacturers deliberately produce speedos that over read by between 5 - 10%. The reason is simple, if they set it to be spot on and then it was out by a couple of mph you could get caught speeding and blame the speedo / manufacturer. So with it over reading, when your speedo is reading 44mph you would be doing 40mph so to get caught speeding in a 40mph limit for example your speedo would be reading nearer 50mph, onus of responsibility for speeding squarely on the driver. Exactly this, took a ride out in a mate's 850t5 ex police car, it had a calibrated speedo he use to drive just the police on the motorway at 70, everyone thought he was speeding except the police. Link to comment Share on other sites More sharing options...
S8ked Posted September 20, 2018 Report Share Posted September 20, 2018 This is a really helpful thread it explains a mystery. I've noticed that my 98 Boxster always seems to be going around 5 MPH slower than the Speedometer indicated. The car has it's original 17" turbo twist wheels and is running on original width 17" tires. If only I'd read this information before I bought new tires last month... Link to comment Share on other sites More sharing options...
Pothole Posted September 21, 2018 Report Share Posted September 21, 2018 All car speedos read a bit high. Well, they do if they are running in factory specification. Link to comment Share on other sites More sharing options...
Nickyg63 Posted September 22, 2018 Report Share Posted September 22, 2018 agree that cars are doing this. Does that mean that the car will do 10% more miles than it actually has?? Link to comment Share on other sites More sharing options...
Nobbie Posted September 22, 2018 Report Share Posted September 22, 2018 35 minutes ago, Nickyg63 said: agree that cars are doing this. Does that mean that the car will do 10% more miles than it actually has?? Interesting point, it's the law that causes speedo to deliberately overread, but I'm not aware that the law has anything to say about the mileometer. It would be straightforward for the manufacturer to have the speedo overhead and the mileometer read accurately and I've not noticed any discrepancy on mileage between road signs and the cars I've driven over the years on long journeys. Link to comment Share on other sites More sharing options...
Nickyg63 Posted September 22, 2018 Report Share Posted September 22, 2018 i had this with an audi i had a while back. if you think that it works out 25 mins to get home and for those 25 mins i average 50mph but actually im really doing 45mph average. I had this when i used a sat nav attached to the screen Link to comment Share on other sites More sharing options...
headless Posted September 22, 2018 Report Share Posted September 22, 2018 7 hours ago, Nickyg63 said: agree that cars are doing this. Does that mean that the car will do 10% more miles than it actually has?? No, the speedo reads high but the mileometer is correct!! Link to comment Share on other sites More sharing options...
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