Ford Kuga Gear Ratio - Top Speed

[pizzaice]

Hi guys,

I am a bit stuck on the following issue:
I've got some gear ratio data on the Ford Kuga but by using it (6th gear, 0.564 and final drive 3.238) and the overall wheel radius (17 inch, 235/55 wheels = 345mm) I get a top speed of 89 m/s, which is far to high for this car - should be around 50 m/s. I am aware that obviously the top speed is depending on the road load and power supplied by the engine and therefore wont be that high in reality, however, I am wondering if I missed something out as I can't see the point in gearing your 6th gear so that it is theoretically possible to reach this speed. Wouldnt that just ruin all you acceleration? The engine develops its max power at 4000 rpm, my top speed calculations are based on 4500 rpm.

It would be great if you could comment on this. I am not working in this field, so therefore am not that knowledgable.

Best regards,

Chris

ps. The Calc I've done: (4500rpm*2*pi*0.345m/60)/(0.564*3.238)=89 m/s

 

[patprimmer]

Just use the first four gears then.

Seriously. The overdrive ratios are there to keep rpm down for noise an fuel consumption reasons.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[dgallup]

I know nothing about this vehicle (never even heard of it) but just from the extreme overdrive ratio in 6th it looks like it is optimized for fuel economy, not acceleration or top speed. Lots of vehicles are geared that way due to mandated fuel economy regulations.

 

[Cargeke]

I'm pretty sure highway fuel economy, not theoretical top speed, was the major factor in deciding the top gear ratio. The idea is to keep the engine at a low speed, say ~1800 RPM, at a common highway speed, say 70 mph (31 m/s). Any hard acceleration will likely drop the transmission into 5th or even 4th gear, if it's an automatic. If it's a stick, yes, the car will most likely accelerate like an ocean liner in 6th gear.

 

[pizzaice]

Thanks for the replies. Does anyone have any information on dynamic tyre diameter? It seems I am unable to find any info on it and I guess that could influence my results by a large amount?

 

[patprimmer]

If you have drag slicks it has a large effect, but with 55 series tyres the effect will be minimal, especially if you run high tyre pressures.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[NormPeterson]

Effective rolling diameter is about 96.5% of the advertised diameter.

As Pat has mentioned, centrifugal effects in some tires can be huge (and, I suspect, intentionally so as a means of getting around certain competition rules involving engine rev limits and mandated axle ratios), but not so in more normal road tires. The Kuha does not sound like a vehicle that would need much slip in order to develop the traction required to attain 50 m/s, and slippage works opposite to centrifugal effects anyway.


Norm