Speed Calculator

[shaneth]

There is always questions about the accuracy of the speedo so I wanted to do this for a long time. this may not be 100% accurate but is just a guide to see how far out the speedo is. I have done a bit of research for this formula and this was about the easiest i could find.

Speed Calculator

Equation :- (0.00006) * (RPM * r) / (R1 * R2) = vehicle speed in km/hour

Where :-
RPM = engine speed, in revolutions/minute
r = loaded tire radius (wheel center to pavement), in mm
R1 = Gear Ratio
R2 = Diff Ratio

Polo Tire Radius
299.95

Polo Gear Ratios
1st – 3.30
2nd - 1.94
3rd - 1.36
4th - 1.03
5th - 0.82

Polo Diff Ratio
3.65


Now I have done one going off the Motor Mags Figures Which is 1km/hr out compared to the equation I found which is above. So either one is close enough for me. I will be testing this out later this week and see.




Disclaimer :- This is not ment to replace your speedo. It is a guide only. Testing this is at your own risk and neither I or VWW.org.au takes any responsibility

Now before everyone asks no im not back yet I have wanted to do this for a long time and when Motor Mag published the Gear Ratios I could finish it. If anyone has any questions or queries I suggest you email it to me as I can only check it when I go over to my parents place. I will be back in the new year.

[Eddy]

Hoyhoy.
SSSSSSSSHHHAANE we miss ya.
P.S. Speaking for myself that is.

[PeterHBne]

Welcome back! (kind of....)

[Timbo]

Welcome back, Shaneth....but now my head hurts!

[jasn78]

geez shane talk bout way too much maths

good to have u back mate

so wheres the next group buy? lmao

[95vr6]

Why dont you plug in a GPS nav unit and it will till you your exact speed in any gear.

[VWindahouse]

He Lives!!!!!

[Wand Weaver]

Nice work Shaneth.

For those complaining about 'too much maths' - I've knocked up a JavaScript calculator for in-gear speeds.

At the moment, only the Polo's there, but the script's modular enough for me to add more cars if I find out their tyre size, diff ratios and gear ratios.

If you're wondering how a different sized tyre might affect your speedo (not that the Polo's speedo's particularly accurate) - just change the tyre size at the top.

As with my post in the 'Top Speed' thread, the assumption of a perfectly round tyre is made, so the values are probably marginally high.

http://home.people.net.au/~kenny-f/h...us/calculator/

All thanks goes to Shaneth for providing the gear ratios.
Edited to add: Found the ratios for the Golf

[Timbo]

Good work! but some of us with 17" wheels use 40 profile tyres and I guess some of the Golf guys do, too

[Wand Weaver]

Good work! but some of us with 17" wheels use 40 profile tyres and I guess some of the Golf guys do, too

Thanks

Point taken on the profile - I've added a few extra options to the form. I think all tyre widths end with '5'?

or at least I hope so!