Hi guys does anybody know how to work out your speed from revs gearing and tyre size?? I know there is a formula.
Today at bathurst down conrod I pulled 7800 in fith 0.83:1 overdrive with 4.625:1 diff and 225/50/15 tyres. Wondering what sort of speed that equates too. I estimate around 215-220
The only reason I know what revs I was doing is it hit the rev limit on two of my runs.
The datto did me proud run strong all day blew and intercooler pipe on one run best time of 49.0 over one second ahead of second place outright in a late model big $$$ porsche.
here is a picture from today
1200 bathurst

You are doing both yourself & all of us proud. Good bloody fortune to you for the future.
Sorry i don't have a formula, but it would give you a speed that is probably a whisker short of the true speed, as it would not take onto account tyre growth at that speed. Hey, that's gotta be worth a couple of extra K's

I've got a program for that let me work it out.

i have worked it out once before, here's my rough recolection. its probably wrong but hopefully you'll see my mistake and be able to work it out.

RPM x {final drive of gearbox(1/.82)}
divide by diff ratio (4.625)

this gives you revolutions of your tyre per minute.

then times by the circumference of your tyre (you'll need to measure this)

this will give you a meters per minute figure. then times by 60 and this gives you metres per hour. then divide by 1000 and you'll get kilometers per hour

any more input people?? if i guestimate your wheel size i got 254kph but i'm very doubtful a 4.625 would get that high...

by the way, are those road tyres????

I got 231 kph. I worked the tyre size out to be 60.5cms. Thats a dam good top speed if you ask me.

Thanks killer1200 thats sounds about right I just watched the incar video I got Holly crap its a wild ride LOL.

Measure your tyre size for us, (Either dia or circunference) and we should be able to work it out exactly) not allowing of course for the loss due to the tyres not trnsmitting 100% of the energy to the road.

good stuff Andrew - winning outright is awesome! looking good in that photo too.

was that top speed from the same starting point they used back in '03 when I came down? bloody impressive. where'd you hit the limiter - approaching the Chase or Murray's corner? (I think that's what its called - onto the pit straight)

How did it feel at those speeds? I'd imagine a front spoiler would make a noticeable difference at those kind of speeds - keep some of that air out from under the front of the car and reduce front end lift/float.

rolling circumfrence of a 225/50/15 tyre in mm is 1903.8mm.

Benny

I just measured the outer rolling diameter of my tyres its 1860mm.
found a nifty little program for working it all out on the net put all my info in came up with 223.6 kmh down conrod and 206.2kmh down into murrays corner.
speed calc

Thanks harry for the compliments it wasnt as bad as I expected the front end wasnt to floaty I hit the limiter before the kink down conrod then again coming down onto the chase i was only pulling 7200 in fith down onto murrays. the bob tail helped a bit through the kink could carry a lot more speed through there
I plan on fitting my 4.3:1 for next time. I dont want to go too tall or it wont pull as well out of the corners.
Apparently the track is a bit longer as they start it back a bit for the dip and also around another 100 meteres past the normal finish line.
I run 47.89 back in 2000 with a 3.3 diff the shorter track and a long way off the speed I was doing on sunday