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Old 10th June 2006, 02:28   #1
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Post What is the difference between speed and speedo ??

Guys I know dis might look a very dumb question, but while reading speed tests of cars I have come across these 2 words many a times. Ex: My speed was 150 kms/hr but my speedo was 165kms/hr. Can ne1 plz elaborate this difference to me ???
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Old 10th June 2006, 02:36   #2
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By Speedo one means the Speedometer. There is many times an error because of the difference between the actual speed of a car and the speed shown in the speedometer. One of the main factor for this error is the rim size.
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Old 10th June 2006, 10:43   #3
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but den how does 1 gets to know the real speed .... like if ur speedometer is showing 120 there's no other option, but to believe it coz there is no other device to test ur speed in ur car.
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Old 10th June 2006, 13:53   #4
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Quote:
Originally Posted by 0-100
but den how does 1 gets to know the real speed .... like if ur speedometer is showing 120 there's no other option, but to believe it coz there is no other device to test ur speed in ur car.
Get a GPS system. It will tell your actual speed after you are cruising at it for a few sec
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Old 10th June 2006, 13:56   #5
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Yes you can.
Least expensive is bicycle computer gives a fairly good accuracy with an error of +-5 kmph. The accuracy depends on the wheel circumference you feed into the computer.

Expensive and accurate measurement can be obtained by using a GPS unit. Or something like GTech.

Or if you just want to know the top speed of your car use a speedgun.
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Old 10th June 2006, 14:07   #6
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i think you have to deduct 10 to 12% to get the real speed
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Old 10th June 2006, 14:08   #7
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Quote:
Originally Posted by Sankar
Yes you can.
Least expensive is bicycle computer gives a fairly good accuracy with an error of +-5 kmph. The accuracy depends on the wheel circumference you feed into the computer.
How can you install a bicycle computer on a car. I think its going to be really tough. They are designed to be installed on bicycle wheels, not car!
Quote:
Originally Posted by Sankar
Or if you just want to know the top speed of your car use a speedgun.
The speed gun will tell you your current speed. Somebody has to stand outside while you approach him, or your copassenger has to aim at a stationary object right in front of you.
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Old 10th June 2006, 20:34   #8
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Just an inquisitive mind.

Would like to know of cars that show your speed while reversing also on the odometer.

So long....
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Old 21st July 2006, 20:30   #9
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Speedo error !!! This is one term that keeps popping up a lot on tbhp. Have a couple of queries about this. Experts, your views solicited.

1. My thinking is that speedo error happens only on changing the stock wheels/tyres. Or is it that a stock car will also have speedo error ? If so, what is the reason for such error ?

2. MUL does quite some crazy things. One of them being putting different tyres/wheels on different models of same car (eg. 165/80/R13 tyres on the Baleno Lxi, and putting 185/60/R14 tyres on the Vxi). So, do they actually set the speedo/odo differently for the 2 models, to be in tune with the tyre/wheel fitted ? If not, I would expect one of the models to have speedo/odo error, even in stock form.
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Old 21st July 2006, 23:06   #10
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Quote:
1. My thinking is that speedo error happens only on changing the stock wheels/tyres. Or is it that a stock car will also have speedo error ? If so, what is the reason for such error ?
Usually speedo errors are used for safety reasons. Make the driver believe he's doing 160 when he's actually doing 140. This keeps the driver satisfied and prevents him from doing the speeds he'd intended to (without his knowledge).

Another reason could be that people usually tend to drive at the recomended speed limit (for a certain road section) and not below it. Speedo error reduces his chances of overspeeding.

But sometimes, it's also due to bad speedo cables or wrong caliberation.

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Old 22nd July 2006, 21:39   #11
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Tyres and wheels have some preset combinations that one can mix and match, to get the most accurate reading. Now, a 165/80/R13 setup will have a total diameter of 594.2mm, while a 185/60/R14 would have a total diameter of ony 577.6mm, which means, that the same car has different models running on wheels of different diameters, but the speedo is calibrated to one setting, thus giving us speedo error, between models(one could have a greater speedo error than the other). Now to understand speedo error we have to understand how a speedometer works.

It is a very basic system, there is a dynamo that connects to the wheel(usually the free wheeling one), that runs an electro-magnet which in turn deflects the needle to show you the speed. The difficulty, is in the calculation of the friction involved, the magnetic force's angle when the needle is verticle and horizontal, the power produced by the dynamo, the strength of the magnet, the distance between the magnet and the needle, gravity, the tire's circumference, diameter and many other complications. So, by the time the calculation is done, it might be accurate for a particular speed but less accurate for another speed reading, hence producing speedo error. Infact you would have noticed that the speedo might have only 2km error at say, 30km/h, 3km error at 40km/h, and back to a 2km difference at 50km/h- this happens mostly due to the angle of the needle, where its weight pulls it closer to the other side due to gavity. Then, the task of printing a perfect speedo dial. That's not a hard task at all, but given the different shapes and sizes it takes, the speedo error occurs.

TC

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Old 22nd July 2006, 23:10   #12
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Quote:
Originally Posted by Shan2nu
Usually speedo errors are used for safety reasons. Make the driver believe he's doing 160 when he's actually doing 140. This keeps the driver satisfied and prevents him from doing the speeds he'd intended to (without his knowledge).
Agree with Shan here.
OD's road test reports gave the degree of speedo errors for each vehicle.

Quote:
Originally Posted by 0-100
..Ex: My speed was 150 kms/hr...
How would you calculate this ? On the road, I've noticed that using the milestones as a measure of distance is very often, not accurate.
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Old 23rd July 2006, 10:44   #13
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Hi, one fabulous site i came across when changing tyres on my Jeep. I guess the biggest concern is when you are faced with speed cameras, and you really need to know how much over the speed limit you will be going..

This easy calculation helps you plan your speed.. Due credit to www.miata.net, who have hosted this calculator

http://www.miata.net/garage/tirecalc.html

Enjoy
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Old 23rd July 2006, 11:12   #14
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Actually, just calculating the tyre diff isn't gonna help. First, you'l have to calculate the speedo error with the stock tyres and then add these results to it.

I've already mentioned the formula i use (on some other threads) but what the heck. "Ek aur sahi".

Engine Rpm / Gear Ratio / Final Drive Ratio * Wheel Circumference in Inches * 2.54 / 100000 * 60 = vehicle speed in Kmph (+/- 2%)

Try it.

Shan2nu
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Old 23rd July 2006, 23:35   #15
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Manufacturers would build in speedo error intentionally to make their cars seem faster. And more fuel efficient!

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