hawker Posted January 5, 2011 Report Share Posted January 5, 2011 Manufacturers generally tend to set their speedos to read approx 10% too fast. Ie. when your doing 30mph, your actually doing about 27mph. Does this mean the mileage readings will be 10% over actual mileage? And an obscure one: Does driving in reverse clock up mileage? Link to comment Share on other sites More sharing options...
ams Posted January 5, 2011 Report Share Posted January 5, 2011 i think on older cars - you would see the clock go backwards in reverse - on newer cars where its a signal rather than a drive probably not best way to check if you think your mileage is more - stick a journey in sat nav check how much it should be and then follow it and see how mnay miles it is on your clock Link to comment Share on other sites More sharing options...
cooperman Posted January 5, 2011 Report Share Posted January 5, 2011 1. yes and 2. yes. 10% is the limit on the plus or minus side for the speedo. so the tacho could be out by the same amount. reverse does rack up millage as well im sorry to say. (as seen in the film ferris bueller's day off lol) andy Link to comment Share on other sites More sharing options...
dublet Posted January 5, 2011 Report Share Posted January 5, 2011 1. yes and 2. yes.10% is the limit on the plus or minus side for the speedo. so the tacho could be out by the same amount. Wrong. The mileage is spot on. The speedo is adjusted to be a certain margin above the speed to avoid any lawsuits in speeding cases ("My speedo said I was doing X") but the mileage is recorded in terms of revolutions of the wheels, so that actually changes if you put on different sizes wheels. Link to comment Share on other sites More sharing options...
Gen6GT Posted January 5, 2011 Report Share Posted January 5, 2011 Wrong. The mileage is spot on. The speedo is adjusted to be a certain margin above the speed to avoid any lawsuits in speeding cases ("My speedo said I was doing X") but the mileage is recorded in terms of revolutions of the wheels, so that actually changes if you put on different sizes wheels. Surely speed is also measured in terms of revolutions of the wheels? Otherwise how else is it measured? Link to comment Share on other sites More sharing options...
princy Posted January 5, 2011 Report Share Posted January 5, 2011 In a roundabout way it is taken from the wheels, the sensor is fitted to the final drive so whatever the ratio per revolution will be picked up by the sender. Part of the reason that there's an allowance is to make provision for things like tyre wear. One full revolution of a wheel with a brand new tyre on would give a rolling distance of roughly 2 inches more than the same tyre just above the wear limits. Link to comment Share on other sites More sharing options...
dublet Posted January 5, 2011 Report Share Posted January 5, 2011 Surely speed is also measured in terms of revolutions of the wheels?Otherwise how else is it measured? It's measured the same way, the needle's just pointed a bit too high (or in the case of digital speedos, speed = realSpeed * 1.10). Link to comment Share on other sites More sharing options...
antmax7 Posted January 5, 2011 Report Share Posted January 5, 2011 thought most cars had speed sensor in the gearbox???? Link to comment Share on other sites More sharing options...
nairn man Posted January 5, 2011 Report Share Posted January 5, 2011 like someone said above a different rim size does affect the speedo because it takes a bit more rotation. so something like 18/19 inch would probally dramatically change the speedo rating as most cars are fitted with 16s as standard Link to comment Share on other sites More sharing options...
Liam 190 Posted January 5, 2011 Report Share Posted January 5, 2011 Surely speed is also measured in terms of revolutions of the wheels?Otherwise how else is it measured? As someone else said, think the sensors in your gearbox Link to comment Share on other sites More sharing options...
Old Red Celica Posted January 5, 2011 Report Share Posted January 5, 2011 It's not the size of wheels that affects your speedo readings, it's the size of the tyres on them!! Link to comment Share on other sites More sharing options...
Edd1Ninety9 Posted January 6, 2011 Report Share Posted January 6, 2011 It's not the size of wheels that affects your speedo readings, it's the size of the tyres on them!! It's the combination of the two. So a large diameter wheel with a tiny profile could actually have the same rolling radius as stock. Conversely, if you had a small wheel with a massive tyre it could be larger than stock. Like a tractor tyre or something! haha Link to comment Share on other sites More sharing options...
tlee05 Posted January 7, 2011 Report Share Posted January 7, 2011 thought most cars had speed sensor in the gearbox???? My old car did and the sensor snapped inside the gearbox Link to comment Share on other sites More sharing options...
silver gt Posted January 7, 2011 Report Share Posted January 7, 2011 It is with hesitation that I contradict someone who describes themself as "Winner of 3 CCUK awards. Sex god. Master Debater.", but Dublet's answer above is misleading. There is one sensor (on the gearbox) that counts revolutions. The signal is used by the speedo and the odometer. If the speedo is reading 10% fast, then the odometer is reading 10% fast too. The law allows for speedos to be up to 10% fast, but never slow. To be safe, most manufacturers aim for 5% fast. Wheel size has no relation to the speedo, the speed/mileage is derived from the revolutions and the outside diameter/circumference of the tyres. Bear in mind that tyre wear affects accuracy, the diameter reduces with wear. A worn tyre will read about 2% faster than a new one. Link to comment Share on other sites More sharing options...
planc Posted January 7, 2011 Report Share Posted January 7, 2011 (edited) thought most cars had speed sensor in the gearbox???? Older cars yes, but as newer cars have ABS what is the point of putting an extra sensor on to measure speed when you already have 1 on each hub? Edited January 7, 2011 by planc Link to comment Share on other sites More sharing options...
antmax7 Posted January 8, 2011 Report Share Posted January 8, 2011 my gen 5 from 1991 had ABS but it still had a speed sensor in the gearbox? Link to comment Share on other sites More sharing options...
planc Posted January 8, 2011 Report Share Posted January 8, 2011 my gen 5 from 1991 had ABS but it still had a speed sensor in the gearbox? Indeed you do, thats why I put "Older cars yes", Its the same with many circuits in a car one sensor can provide the same info for more than one aplication. Link to comment Share on other sites More sharing options...
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