Speed vs Rpm

RPM Gear Ratio

= Tailshaft RPM

Tailshaft RPM = Axle RPM Diff Ratio

Axle RPM x 60 = Axle Revolution per hour (RPH) Axle RPH x rolling diameter in KM = Speed So, let’s say I want to calculate my theoretical top speed: Rpm limiter is 8500 5th gear ratio in the W57 is .860 Diff ratio is 4.3 Tires are 225/50R15 Therefore: RPM Gear Ratio

= Tailshaft RPM

8500 0.860

= Tailshaft RPM

= 9883.7 Tailshaft RPM = Axle RPM Diff Ratio 9883.7 = Axle RPM 4.3

= 2298.5 Axle RPM x 60 = Axle Revolution per hour (RPH) 2298.5 x 60 = 137912.4

Axle RPH x rolling diameter in KM = Speed 137912.4 x .001904KM = Speed = 262.6 KM/H so I have a theoretical top speed of 262.6 KM/H. but what use is this? Not a lot really. So let’s look at something more useful. Like changing a gearbox. Or changing tyre size. Or both ;-) NOTE: to calculate rolling diameter I use the tyre bible found at http://www.carbibles.com/tyre_bible.html and scroll down a little beyond halfway. You will find a section where you can enter your current tyre size, and tyres you want to change to and you can then calculate the difference these will have. I just use the first section, where it will provide you with current circumference. This is in mm, to convert it to KM, just put 0.00 in front of it. AKA dividing it by 1,000,000. So, lets say I decide I want to change to W58 for the longer 5th gear ratio to drop my RPM on the highway. So, with the W57, I know from experience she revs around 3500 RPM at 110 km/h. Let’s do the exact calculation: Axle RPH x rolling diameter in KM = Speed Axle RPH x .001904KM = 110KM/H Therefore: Axle RPH = 110 .001904 Axle RPH = 57773.1 Axle RPM x 60 = Axle Revolution per hour (RPH) Axle RPM x 60 = 57773.1 Axle RPM =57773.1 60 = 962.9

Tailshaft RPM = Axle RPM Diff Ratio Tailshaft RPM = 962.9 4.3

Tailshaft RPM

= 962.9 x 4.3 = 4140.4

RPM Gear Ratio

= Tailshaft RPM

RPM 0.860

= 4140.4

RPM = 4140.4 x .860 = 3560.7 RPM @110KM/H in 5th gear. So my tacho and my eyes are doing pretty well 

NOTE: I use http://www.club4ag.com/faq%20and%20tech_pages/RWDgearboxes.htm to find the gear ratios of gearboxes. However your repair manual and numerous other sources on the net will tell you your gear ratios. Ok, so. Let’s see what my RPM will be with a W58 gearbox in place which has a 5th gear ratio of .768 From previous calculations ^^ we know the Tailshaft RPM is 4140.4 So: RPM = 4140.4 x .786 = 3254.4 A drop of 300 RPM. Nice start me thinks 