Speed ??

[HarryM1BYT]

Originally Posted by GadgetBoy

SatNav is not accurate. It certainly shouldn't be used to "calibrate" your speedometer. If you want a better indication of your speedometer accuracy use a proper handheld GPS unit. While not 100% accurate it will be much more accurate than SatNav or a GPS equiped phone.

Your Odometer will be relatively accurate if your car has the original factory specified tyres and wheel sizes. When fitting new wheels/tyres you need to compare the circumference of the old combination to the new to work out how it will effect your speedo/odometer.

The difference in the workings of a satnav and a GPS is what?

They both use the same principles, but the satnav has the addition of a mapping and routing capability. I have never been able to discern any inaccuracy in the indicated speed of my satnav, assuming it is checked on a reasonably straight road and at an absolutely steady speed. Bends and variations in speed will introduce errors, the later because they work on a one second interval of position calculation. There is also the 10yds limit on measurement accuracy which might cause some slight dither, but generally they are spot on with the true mph.

[HarryM1BYT]

Originally Posted by SD1too

Rene,

Another member has found an adjustable circuit which corrects speedo errors. It's on my list of things to do.

Simon.

There is such a gadget, they use them on imported motobikes bikes to recalibrate a kph speedo to read in mph. They take the speedo pulse in, and output a slightly diffferent rate of pulse out to the speedo.

I have a gadget on the bike, which takes in the speedo pulse plus the rev pulse and then converts that into a display so you know which gear you are in. I didn't install it :-)

[James.uk]

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

Put in details of your cars original tire size, then the size of your intended new wheel and it gives you the correct size of tire you need to maintain the correct ratio..
...

[SD1too]

Originally Posted by James.uk

... tire ...

Really James, we're British you know! We have tyres.

Simon.

[revor64]

Simon, then it is on my list now too. Please let me know what you find, or what will be needed.

Great.

Rene.

[ChineseRover]

Minor point but all the official Rover wheels for the 75/ZT all have the same circumference so changing the wheel size from, say, 15” to 18” will have no impact on the accuracy of the speedo. More metal in the alloy means less rubber in the tyre so firmer ride.

GPS speed measurements are subject to their own inaccuracies. Firstly they do not measure current speed, it measures what you were doing. They will be adversely affected by hills as the GPS is measuring your speed over an assumed flat earth (back to school trig!). They will also “smooth out” road twists in its calculations. Finally the satnav is only accurate to around 25 feet.

In short GPS is great for dropping bombs on your enemies but not great for measuring your speed accurately in a car.

[J1MBO]

Originally Posted by ChineseRover

Minor point but all the official Rover wheels for the 75/ZT all have the same circumference so changing the wheel size from, say, 15” to 18” will have no impact on the accuracy of the speedo. More metal in the alloy means less rubber in the tyre so firmer ride.

In my experience not quite correct as I've found that cars on 15" and 16" rims the speed displayed can be up to 10% fast, whereas the cars on 17" and 18" rims the speed displayed is less than 5% fast.

My previous KV6 on 16" rims, displayed 70mph - gps displayed 64-65mph
My current V8 on 18" rims, displayed 70mph - gps displayed 67-68mph.

Granted using gps as a bench mark is not perfect solution to the problem/question but it does seem to prove that there is a variance dependant what size of rims/tyres are fitted.

Cheers
Jim

[crashmarks]

Most police areas will have several marked out measured miles they use them to callibrate some of their speed equipment. If you ask one of your local traffic cops they should be able to tell you where they are, usually found at the side of the Mway or a good stretch of dual carriageway. Drive the mile at a constant indicated 60 which is harder than it sounds unless you have cruse and time it with a stop watch. Obviously if it takes 60 sec then its bang on but I doubt that because its an offence to use a speedo that reads low hence why they always read fast. Every 1 sec over the 60 is 1.66%

[revor64]

In short GPS is great for dropping bombs on your enemies but not great for measuring your speed accurately in a car.

car speedo seems also not great for measuring your speed accurately in a car.

Actually, for road safety it is nessesary to know exactly how fast I go. Now if I spot a camara, I hit the break. Since I have no clue about the real speed, so 10 Km less than the max is sure a safe value for passing a speed camara

Measuring with a gps might not accurate, but since the deviation should be the same, it is possible to compare different cars with it.

[spyder]

Unless you spend a lot of money on a rally type setup, and have that calibrated to your car, there is some guess-work. To check the accuracy, find a nice long road, with mileage markers on the side travel as long as possible, and check the time with a stop-watch. Convert the seconds and minutes correctly to distance traveled and it will give you a good average speed. You will of course need to travel at a constant speed-Sunday mornings might be good somewhere.

This method is used on close odo rallies, where the time is measured in seconds.

Craig