I am trying to put together a spreadsheet that details roughly what speed I should be doing in each of the 5 gears, at different revs, so I can make sure the speedo is reading roughly correct before SVA, but the numbers don't look right.

I wondered whether someone elso could sort me out:

1st Gear ratio is 3.65:1
Diff Ratio is 3.92
Wheel size is 195/15/45 which equates to 1.748 metre circumference (distance travelled for each revolution)

This is where I get confused.

If the engine is running at 1000rpm, thats 60000 revs per hour

In that hour the prop will rotate 3.65 x 60000 = 219000 times

The wheels will rotate 219000 x 3.92 = 858480 times, at 1.748 metres a time thats 1500865.131 metres

divide by 1609.344 metres in a mile

Equates to 932.5943561 MPH

Even I know that's not right.

Where have I been stupid????

You need to divide, not multiply!

60000 / 3.65 = 16438 approx

16438 / 3.92 = 4193

4193 x 1.748 = 7330

divide that by 1609 = 4.56mph
(most of the sums rounded very roughly due to laziness.)

David



[Edited on 23/8/06 by David Jenkins]

[Edited on 23/8/06 by David Jenkins]

gear/speed calculator

try quaife.com (quaife usa website) look in downloads, there is an excellent program for calculating speeds/ gear ratios. mja

Thanks,
I'm pretty sure I went the divide route, but numbers still didn't look right.

OK now

I did the calculation (See Dave Jenkins' post above) then took my car to a disused airfield and checked it against my GPS and made the necessary adjustments. It was spot on at SVA. If you're planning to drive it to your SVA you could do the GPS comparison on the way.
Cheers
Jon

Found this exel sheet somewhere on the net.

Caber