carl0s Posted March 19, 2005 Share Posted March 19, 2005 Most cars speedometers overread by ~10% (says 33mph when it's actually doing 30). Putting larger wheels on a car would obviously change this, but what are peoples general thoughts on this old wives tale? Quote Link to comment Share on other sites More sharing options...
Homer Posted March 19, 2005 Share Posted March 19, 2005 There is a small difference between the wheel size and the Speedo reading. Unfortunately I had the opportunity of checking this recently due to going through a few different sets of wheels: - Blitz 17's 265/40/17 F1's: 100mph Speedo reading = 95mph on road angel - Stock supra 16's 245/45/16 Bridgestone’s: 100mph Speedo reading = 92mph on road angel - Stock supra 17's 265/40/17 F1's: 100mph Speedo = 96 mph on road angel Not sure if that helps though!! Editing to add: Obviously these are rear wheels sizes! Quote Link to comment Share on other sites More sharing options...
Amazing grace Posted March 19, 2005 Share Posted March 19, 2005 it is true the differenc on speedo and Road Angel seems to become further apart the faster i travel??? I dont understand this have checked Angel is working fine but i do know the speedo is not the place to read for correct speed reading!!! I have 18" wheels on low profile tyres . Regards, Sheena Quote Link to comment Share on other sites More sharing options...
Terminator Posted March 19, 2005 Share Posted March 19, 2005 Factory MKIV speedo will be calibrated to either stock 16 " or 17" wheels, unless you match the circumference of these exactly there will be a difference. Quote Link to comment Share on other sites More sharing options...
Soop Dogg Posted March 19, 2005 Share Posted March 19, 2005 Car speedos are normally out and are normally optimistic.(read high) Manufacturers will usually err on the side of caution with this sort of thing. Better to have it reading slightly too fast than read too slow. If it was proven that their speedos all read slow, they might find thtemselves being sued by people who got done for speeding, or worse,by accident victims. As for changing wheel sizes, fitting larger wheels shouldn't make a huge difference as when you put larger wheels on, you should come down a size or two on the tyre profile. The rolling radius of tyres of different sizes is rarely exactly the same, but there are plenty of tyre size calculators on line that you can download and work out the closest tyre size on say, an 18" rim, to the original 16" or 17" rim. Try these and see just how far out your aftermarket wheels have put you! http://www.tyresave.co.uk/tyresize.html http://www.miata.net/garage/tirecalc.html Obviously tyre width make a difference too as the profile is expressed as a percentage of tyre width. Quote Link to comment Share on other sites More sharing options...
Gaz Walker Posted March 19, 2005 Share Posted March 19, 2005 Car speedos are normally out and are normally optimistic.(read high) Manufacturers will usually err on the side of caution with this sort of thing. Better to have it reading slightly too fast than read too slow. If it was proven that their speedos all read slow, they might find thtemselves being sued by people who got done for speeding, or worse,by accident victims. Thats right, the manufacturing tolerance's allow them to over read by upto 10% but not under read at all. Gaz. Quote Link to comment Share on other sites More sharing options...
Green Peace Posted March 19, 2005 Share Posted March 19, 2005 ahhhh so maybe the supra isn't as fast as we thought...I know when I have been doing 165...the GPS reads it as around 150-152 (on a private track obviously) But I guess in order to get a true reading on GPS you have to hold the speed steady for a few seconds. Quote Link to comment Share on other sites More sharing options...
Pete Posted March 19, 2005 Share Posted March 19, 2005 It's all down to the calibration of the speedo. It'll be out by a ratio - therefore reads further out of synch the faster you go. Your speedo will be out of you change your wheels, diff ratio, gear box ratios, tyre profile etc etc. Quote Link to comment Share on other sites More sharing options...
carl0s Posted March 19, 2005 Author Share Posted March 19, 2005 that's why we say 10% or whatever - a percentage. So when you're doing 30mph, it'll display 33mph. When you're doing 100mph, it'll display 110mph. or thereabouts. bit more difficult working it backwards. When it displays 30mph you're probably doing something like 27.3mph, if it were in fact 10%. Quote Link to comment Share on other sites More sharing options...
supra dan Posted March 19, 2005 Share Posted March 19, 2005 how accurate is a road angel? Quote Link to comment Share on other sites More sharing options...
carl0s Posted March 19, 2005 Author Share Posted March 19, 2005 +/- 2% at worst I should think. Or do they say "within 1.5mph"? Quote Link to comment Share on other sites More sharing options...
Homer Posted March 20, 2005 Share Posted March 20, 2005 how accurate is a road angel? Its a GPS device so is 100% accurate - provided of course the speed is maintained to provide an accurate reading... Quote Link to comment Share on other sites More sharing options...
damouk Posted March 20, 2005 Share Posted March 20, 2005 i checked my speedo about 3 moths ago and it was reading 10% over with stock wheels Quote Link to comment Share on other sites More sharing options...
Chiefgroover Posted March 20, 2005 Share Posted March 20, 2005 Its a GPS device so is 100% accurate - provided of course the speed is maintained to provide an accurate reading... I have a mate in Mountain Rescue, his GPS tells his location within a 6 feet tolerance, thats pretty good. Question is, does anyone know if a fast moving vehicle e.g. @ 150mph gives the same accuracy of reading as a 5mph one?. Quote Link to comment Share on other sites More sharing options...
supra dan Posted March 20, 2005 Share Posted March 20, 2005 Question is, does anyone know if a fast moving vehicle e.g. @ 150mph gives the same accuracy of reading as a 5mph one?. thats what i was thinking. most people who own one seem to take the reading as gospel, i just wonder if they really are that good Quote Link to comment Share on other sites More sharing options...
michael Posted March 20, 2005 Share Posted March 20, 2005 What I wonder, more than the speed issue, is how many extra miles our cars are showing on the clock than they really need to be? Has a 100k car really done that many miles? Quote Link to comment Share on other sites More sharing options...
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