According to the published info my Fabby 1.9 TDi is doing 28mph at 1000 revs in fifth gear. This means that each increment on the rev counter = 7mph so I can calculate my exact speed as opposed to the speedo which reads fast. For instance 70mph = 2500 rpm, 40mph = 1420 rpm and 30mph = 1070rpm. Unfortunately this is to slow for the engine to function properly.

Question

You'll need to know the final drive ratio and the ratios of the other gears i would imagine , but why do you want to do it ?

All speedometers read fast. They have to as the makers are not allowed to have a tolerance that shows a slower speed than is actual. For instance at 2500 rpm in 5th my fabby is doing 70mph but the speedo shows marginally over 80mph an error of over 10%. I am assured that the rev counter is accurate. I want to be able to know what ACTUAL speed I am doing in other gears.

if you tell me your gearbox code and the size markings off the side of your tyres i'll work it out for you.

Hi teflon tom ur a star. My tyres are 185/60/R14 Goodyear. That's the easy one where do I find the gearbox code?

Cheers Gibbo

the gearbox code will be printed on the label inside the front cover of the car handbook. also on the label under the carpet in the boot. it's a 3 letter code... i cant remember if it's after or before the engine code. but it is marked anyway

Get a GPS device, Talex or Tom Tom etc.

Hi Teflontom

The three letters before the engine number are ATD hope that's what you need Cheers mate

ATD is the engine code not the gearbox code.

I'm with Lummox - borrow a GPS and check it. Rev counters are NOT accurate, in fact likely to be less so than the speedo (what would the rationale be for them being calibrated better than the speedometer?)

The way you are trying to work it out is still likely to give you a few percent error, apart from any rev-counter inaccuracies, there is also the problems of slightly different rolling radius' of the tyres, etc. etc.

I'm in Peterborough, I see you're in Lincs, if it's not far I have a couple of GPS devices if you wanted a short drive to see what it's telling you/what GPS says it is.

Thanks for the offer franV

How does GPS work indetermining exact spees?

Cheers Gibbo

Hi Lummox,

I have checked the Gearbox code and it apears to be EWT = -s that nay help?

Cheers mate Gibbo

How does GPS work indetermining exact spees?

Satellites

I won't go into huge detail of the exact workings of GPS, basically it works out your position by triangulatin your distance from several satellites orbiting the earth, which the US military kindly put there for us. Each satellite knows where it is itslf, so the GPS merely calculates where all these points coincide.

Anyway, once it knows exactly where you are, the GPS unit also knows where you are a moment later, so by working out how far you were from the first point to the second, and how long ago you were there - its simple distance over time equals speed.

GPS accuracy is pretty good, I was talking to a cartographer the other day (someone who makes maps) and she was saying GPS is pretty accurate, it's certainly the most accurate method of determining speed short of attachng a 'fifth wheel' arrangement of old or one of the optical type ones.

Hi,

yes I know about Global Position Satellites but exactly what process? Even the SAS have difficulty with it!

Cheers Gibbo

Hi,

yes I know about Global Position Satellites but exactly what process? Even the SAS have difficulty with it!

Cheers Gibbo

Stick the gadget in your car, turn it on, it tells you your speed..lol

Hi

Sounds wonderful - is it accurate and where can I buy one?

Gibbo

based on the gearbox code EWT and with tyres which are 185/60/r14

your gear ratios are as follows

1st. 3.77:1

2nd 2.06:1

3rd 1.30:1

4th 0.90:1

5th 0.70:1

final drive 3.15:1

so therefore the overall gear ratios (including the final drive) are as follows

1st 11.88:1

2nd 6.49:1

3rd 4.1:1

4th 2.84:1

5th 2.2:1

a tyre with 185/60/r14 has a curcumference of 71.44 inches approx

here's the formula

........... rpm X Tyre circumference(in inches)

mph = ________________________________

........................ overall ratio X 1050

a quick example for fifth gear

...............................2000 rpm X 71.44

61.85 mph = ________________________

..................................2.2 X 1050

you can work out the rest because i'm too lazy..... hope this helps

Dear Teflon

You are more than a star, you are a firmament!!!!!

You should be a maths teacher

Thanks a bunch

Gibbo

This might help

http://www.briskoda.net/forums/fabia/fabia-1-9-tdi-100bhp-gear-ratios/53041/

Have a look at the attachment i put on #4

This might help

http://www.briskoda.net/forums/fabia/fabia-1-9-tdi-100bhp-gear-ratios/53041/

Have a look at the attachment i put on #4

ah that's good! i didn't know about that!!! should have done a search really....

Hi Grant and Teflon Tom,

Thanks for the help.

Tom, in your calculator there are two constant factors, firstly the circumference of the wheel (no prob) the other is the figure 1050 that is multiplied by the ratio. What is this factor, what does it refer to, how is it calculated?

Sorry to be a pain but I am intrigued.

I have worked out all the speeds and note that your figure of the mph per 1k revs in 5th is slightly different from the published figure but not much - 30 to 28mph (this was taken from the road test in DieselCar magazine)

Grant I'll see how I get on with the calculator - sadly my math in primitive.

Thanks chaps - this is what makes Briskoda a great facility.

Cheers Gibbo

Tom, in your calculator there are two constant factors, firstly the circumference of the wheel (no prob) the other is the figure 1050 that is multiplied by the ratio. What is this factor, what does it refer to, how is it calculated?

it is factor based on the fact that the tyre circumference is measured in inches, it can be substituted for a different number(which i cant remember off the top of my head??) when the tyre circumference is measured in cm.