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RPM and MPH


FJFOOL

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My 2000 V11 Sport's numbers just don't seem right. 6th gear, 4,000 rpm shows 55 mph. Sure seems like I'm going faster than that. What do yours read at that rpm ? Thanks!

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According to the workshop manual for the V11 Sport the "Total ratio (engine-wheel)" is 1:4.1737 in 6th gear.  In other words for every 4.1737 turns of the engine you get 1 turn of the rear wheel.  The biggest uncertainty is the circumference of your rear tire but a 170/60-ZR17 should be about 78.64" according to a tire size calculator.

 

At 4000 rpm (engine) you would have 4000/4.1737 = 958.38 rpm of the rear wheel.  At 78.64" around, that gives you 958.38 * 78.64" = 75367.18 inches/min.  Now just convert to miles/hour...

 

75367.18 inches/minute * 60 minutes/hour = 4522030.81 inches/hour / 12 inches/foot / 5280 ft/mile = 71.37 miles/hour

 

So unless your rear tire is WAY smaller than it should be, either your tach or your speedo (or both) is way off.  Speedo is easy to check with a GPS.

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My tacho is all over the place so is no use in these equations, but my engine is definitely lugging in 6th anywhere below 80mph on the level and I know I can trust the speedo within about 5mph.

 

It'll take me up to an indicated 130+ (good day, following wind, off a cliff etc).

 

I have a 160 section tyre so it'll be spinning more rather than less for the same number of revs than a 170 so 70-ish sounds a bit on the low side to me at 4000 rpm.

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She just really wakes up at 5,000!

hell yeah... 6 even beter LOL

 

At 4000 rpm Mine shows 120 kph which = 74.5 mph

 

same if not a bit faster

According to the workshop manual for the V11 Sport the "Total ratio (engine-wheel)" is 1:4.1737 in 6th gear.  In other words for every 4.1737 turns of the engine you get 1 turn of the rear wheel.  The biggest uncertainty is the circumference of your rear tire but a 170/60-ZR17 should be about 78.64" according to a tire size calculator.

 

At 4000 rpm (engine) you would have 4000/4.1737 = 958.38 rpm of the rear wheel.  At 78.64" around, that gives you 958.38 * 78.64" = 75367.18 inches/min.  Now just convert to miles/hour...

 

75367.18 inches/minute * 60 minutes/hour = 4522030.81 inches/hour / 12 inches/foot / 5280 ft/mile = 71.37 miles/hour

 

So unless your rear tire is WAY smaller than it should be, either your tach or your speedo (or both) is way off.  Speedo is easy to check with a GPS.

HUH? fark happened just then ;-) 

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