Does anyone has a gear ratio calculator to work out top speed

I am trying to work out the best ratio diff to get for a bike engine car. I have found some posts about one, but the link is no longer available.

Any help would be greatly appreciated.

this one is still available: https://locost7.info/gearcalc.php

but the calcs aren't all that hard to do manually (and/or stick in a spreadsheet)

you just need to know the number of rev's pre mile for your tyre size (which you can lookup or calculate)

This website provides all the tyre data that you'll need

Tyre Size Calculator

in its simplest form its just converting rpm of the wheels into mph

if you know the effective diameter of the tyre (diameter of the rim + [2*profile*width] once you covert all to either mm or inches)
then Pi*D gives the circumference (probably in mm or inches depending on your preference)
then just convert 1mile into the same units (1,609,344mm or 63360" ) and divide by the circumference

but JAG's link above is easier

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so you've got rev's per mile for the tyre (one way or the other)

an engine speed in rpm

a gear ratio

and a diff ratio


so for a given gear the calc goes a bit like this:

engine rpm (max. or maybe just after peak power - whatever your aiming for) - say 9000rpm for this example

gear ratio lets call it 0.82 (so 0.82 turns of the crank to 1 turn of the output [aka overdrive] typical for top gears on cars etc )
then the final reduction at the diff - say 3.62 (typical sierra V6 etc...)

so you can either do it step by step or multiply the ratios first and do it in one hit > 0.82*3.62 = 2.9684 (the overall gear ratio) then 9000rpm / 2.9684 = 3031.9rpm at the wheels

3031.9 * 60 = 181916 revs per hour

tyres - say 195/55 R16 with 862 revs per mile - so 181916/862 = 211 mph



but of course none of that accounts for your engine power or aerodynamics which will easily dominate at anything over 100mph

[Edited on 10/8/2022 by mcerd1]

if you want to get an approximation for actual top speed so that you can try to find gear ratio's to best fit it - then you need a calculator that works out drag and power...


the basic maths goes like this:

P = 0.5* p * Cd * A * V³

where P is the power in Watts,
p is air density (1.226),
CD is the coefficient of drag,
A is the frontal area in square metres,
and v is the velocity in m/s

so rearranging: V = cube-root ( P / 0.5* p; * Cd * A )



I ran the numbers based on my Dax:
P = 140hp = 104kW
Cd = 0.7 (approx. for most sevens / replica's some estimates are 0.65 some 0.75)
A = 1.85 (with windscreen)
V = 50.7m/s = 114 mph

and running it the other way aiming for a 150mph top speed:
150mph = 67m/s
Cd = 0.7
A = 1.85

P = 239kW = 320 hp

and thats all power at the wheels too.....

you'll find online calculator for this too, but they are mostly American drag racing sites

also I found this site which does the gear calc bit for you:
https://www.boosttown.com/gearbox_differential/speed_calculator.php

my previous link does still work, but the program doesn't like win10+ much



[Edited on 10/8/2022 by mcerd1]

There is a 'sticky' literally the first line of the BEC section on the forum for this exact thing :-

Link to Sticky

I used it and after road testing it turned out to be spot on.

Cheers
Tony

Thanks for the links and help. Appreciated. I shall look at it more in-depth later

Edwardo - that link unfortunately isn't working anymore, unless someone has the spreadsheet downloaded and willing to share with me also..

That's strange - I clicked the link and it opens up fine and downloaded the spreadsheet again this morning?

If you U2U me your email address I'll send it over

You can try this one
http://www.mintylamb.co.uk/gearspeed/
it is set up for Triumphs, but you can put in any gear/diff ratios you like. Does 4speed plus OD, and you stick in the tyre size.

https://www.blocklayer.com/rpm-gear

quote:

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Originally posted by m8kwr
Thanks for the links and help. Appreciated. I shall look at it more in-depth later

Edwardo - that link unfortunately isn't working anymore, unless someone has the spreadsheet downloaded and willing to share with me also..

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