How to calculate speed using RPM and gear ratios

[Jonas_R]

Hi,

Anybody that can tell me how you calculate the cars speed based on the RPM, gear ratio, tire diameter etc?

/Jonas

[quattro]

buy a gps!

[Jonas_R]

Very funny, but that is not the reason why I'm asking.

[andy10v]

There are a few calculators on the net that allow you to enter gear ratios, final drive, rpm and tyre sizes or circumference

Or measure your tyres circumfrence by marking with chalk and rolling it.

[Jonas_R]

I don't want a calculator I want the math behind it

[driftin jo]

Ok easy

THIS IS SECOND GEAR IN AN S2 TAKING 1.89 AS SECOND GEAR RATIO AND 4.11 AS FINAL DRIVE RATIO.



RPM / GEAR RATIO / FINAL DRIVE RATIO = WHEEL RPM
EG 7000 / 1 .89 / 4.11 = 901.14

THIS GIVES YOU WHEEL RPM OF 901.14


NOW A 225/40/18 TIRE DOES 804 REVS PER MILE

So
804 WHEEL RPM IS 60MPH

804 / 60 = 13.4 REVS PER MPH (13.4 = 1 MPH)

901.14 / 13.4 = 67.24 MPH


HERE IS WHERE YOU CAN FIND THE REVS PER MILE FOR TIRE SIZES.
http://www.miata.net/garage/tirecalc.html

[S2central.net]

I did this from 1st principles a while back... Added in the 'wheel rpm' column tonight.

[4WheelDrifter]

Nice one Paul. That's a bit neater than the one I did a while back.

It reminds me that I'll need a 7500 rpm rev limit to hit a theoretical 200 mph. Even though I feel 168 mph in 5th will probably be enough in the old Coupé, unless I improve the aerodynamics a bit.

[Jonas_R]

Anyhow this was what I was doing

I combined S2central excel sheet with A80Vant Excel sheet and did some rewriting of both.

Numbers etc are very bad but that is because I just used some crap data.

Anyhow you can now calculate your horsepower based on RPM's combined with a sample freq.

In Zeitronix you can log RPM at 10Hz and then get it out as comma separated file that you can export into excel.

Excel sheet

[Jonas_R]

S2central:

Do you have the gear ratios for the 5 speed?

[S2central.net]

SURE

[UrS4boy]

Hi, Anybody that can tell me how you calculate the cars speed based on the RPM, gear ratio, tire diameter etc? /Jonas

I have attached the Excel spreadsheet that I developed to do this for my old (gone now) 86 VW Jetta and my still (since 98) 93 S4. The basic equations are C= Pie*D, etc. with the various gear ratios built in. I have also included the wheel diameter (in inches) and tire size, e.g. 225/50 as variables.

Enjoy (or not) (see attached)

[s2avant]

Strange question I know but has anybody got any details of calculations to work this out for an auto box? I can't get my head round the ratio setup with primary ratio, individual gear ratios and final drive ratios. I've tried some calculations but they come out with silly numbers.

Ben

[UrS4boy]

Strange question I know but has anybody got any details of calculations to work this out for an auto box? I can't get my head round the ratio setup with primary ratio, individual gear ratios and final drive ratios. I've tried some calculations but they come out with silly numbers. Ben

Automatic transmissions have four gear ratios and a final drive ratio. Find them and enter them into the spread sheet that I posted above.

[Sonic]

Cool!

Using Paul's spreadsheet i can see the max speed of my s2 is 186.59mph, with a theoretical max speed of 578.429mph subject to enough power, aero and rolling losses and mentioned