Date:         Mon, 5 Dec 94 22:16:17 EST
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         dcarment@ccs.carleton.ca (David Carment)
Subject:      Re: Tire size question

Joel Walker writes:
> 
> On Mon, 5 Dec 94 20:10:26 CST David Carment said:
> >  A simple question for any one with time on their hands...what effect
> >does running a larger tire have on the odometer, speedometer and trip
> >meter? If for example there are 195s instead of 185s on the front wheels would
> >it be necessary
> >to increase the distance travelled and speed by say 18% (the difference
> >betwen 185s and 195s)? I am curious and also looking for an excuse why my
> >mpg is lower than it should be. TIA - DC
> 
> well, this is what my dippy little program puts out.
> assuming we have a 185/70R14 ... it has
> circumference of 76.02 inches, makes 833.50 rev/mile, indicated 60 mph = 60mph
>   (assuming NO speedometer error. ;)
> now, changing only the width of the tire to 195/70R14, we have
> circ. 77.75, 814.94 rev/mile, indicated 60mph = 61.37 mph actual, 2.28% off
> which is not a whole lot off. and, given the normally erroneous low reading
> of most car speedos, it might actually be more correctly indicating the true
> speed.
> if you change the tire again, to 205/70R14, we get
> circ. 79.48, 797.18 rev/mile, or 60mph indicated = 62.73, 4.5% off.
> 
> this is what my calculations indicate. :)
> joel
> 

Joel: Thanks :) - so does this mean that I can inflate my mpg figures by
2.3%? 

Just
got back from a 600 mile trip and noticed that cruising above 60mph and
the mpg drops dramatically, below 60 and it goes back up to 21mpg. Now
that the rings have finally "settled"; the engine seems to run well above
65 except for going up big hills and against the wind... DC -- 

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David Carment
School of International Affairs
Carleton University
Ottawa, Ontario
K1S 5B6
voice - (613) 788-2600-6662
fax - (613) 788-2889
Email address: dcarment@ccs.carleton.ca
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