KenF27
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| posted on 1/6/08 at 09:14 PM |
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tyre ratio
Going to le Manns next weekend in the F27. It's running Zetec, Type 9, T34 axle with 3.9 Diff 195/50/15s. Seventy mph runs at about 3500 rpm.
Should I think about uping the tyre profile to say 195/70/15s if they will fit under the rear arch's. Theoreticaly 70 mph should run about
3000rpm. Is this going to far or will the cost of the tyres outnumber the fuel saving.
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hobbsy
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| posted on 1/6/08 at 09:27 PM |
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I think you'll struggle to offset the cost of the tyres plus I think 195/70-15's might be an odd size that could be a lot more expensive
than you think (if available at all). I don't think 500rpm will make all that much difference MPG wise.
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Paul TigerB6
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| posted on 1/6/08 at 11:17 PM |
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totally agree ^^^ - dont think spending maybe £200+ on a set of tyres just to try and impove fuel economy by lowering revs is a wise move. Would think
you would be hard pushed to spend £200 in fuel anyway - never mind save the same. Will raise the ride height and so on also, so dont do it.
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mr henderson
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| posted on 2/6/08 at 07:12 AM |
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You won't necessarily save fuel by running at any particular speed with a lower engine rpm rate.
The amount of energy that is required to overcome the friction and other energy losses caused by driving a particuar vehicle at a particular speed
over a particular route is fixed.
If you had a higher gear in your car, or an overdrive, or tyres with a bigger radius then you would need to open the throttle more in order to
maintain the same speed.
The point of overdrives is to reduce engine rpm/noise for the comfort of the occupants, not to save fuel (although the marketing people would
disagree)
John
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will121
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| posted on 2/6/08 at 07:17 AM |
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how about looking at buying a second hand set of mondeo 15" wheels/tyres. they had quite high profiles can pick them up of ebay for £50, and
even sell them off again afterwards!
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nick205
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| posted on 2/6/08 at 08:26 AM |
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Leave earlier and drive slower - cruise there at 55mph, enjoy the journey and save on fuel 
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MikeRJ
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| posted on 2/6/08 at 05:13 PM |
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quote:
Originally posted by mr henderson
If you had a higher gear in your car, or an overdrive, or tyres with a bigger radius then you would need to open the throttle more in order to
maintain the same speed.
The point of overdrives is to reduce engine rpm/noise for the comfort of the occupants, not to save fuel (although the marketing people would
disagree)
Petrol engines generaly run more efficiently at lower RPM with more throttle to achieve the same power (it raises the effective compression ratio).
In this case I suspect the savings will be marginal.
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