Jon Steiger wrote:
>
> At 08:32 AM 4/4/97 -0400, you wrote:
> >Thanks for the info, Jeff. I've got a set of 235/70's on the truck
> >now. I imagine the 245/60's would probably be close to them in
> >height, don't ya think? I like the height of the truck now, so I
> >don't want to change it much. Thanks again.
>
> BTW: I didn't notice if anyone mentioned this, but here's the
> "formula" for tire sizes... It can be used for a bunch of stuff,
> such as keeping two different size tires the same height, for
> example...
>
> The first number (i.e. 245) is the width of the tire in millimeters.
> The second number (i.e. 70) is the ratio of sidewall height to tire
> width. This ratio is expressed as a percentage. (70%) So, the sidewall
> height is 70% of the width of the tire) So, to find the height of the
> sidewall, you multiply the two numbers together. (That'll give you the
> height in millimeters)
>
> To find the height of the tire, take the sidewall height times 2, then
> add the diameter of the wheel.
>
> To convert those useless millimeters into nice 'merican inches, divide
> by 25.4. (With apologies to all our Canadian members.) ;)
>
> So the heights of the tires in this example would be:
>
> 235/70:
>
> (235mm * 70%) = (235mm * .70) = 164.5mm
>
> 164.6mm / 25.4 = 6.476377952756in
>
> (6.476377952756in * 2) + 15 = 27.95in
>
> 245/60:
>
> (245mm * 60%) = (245mm * .60) = 147.0mm
>
> 147.0mm / 25.4 = 5.787401574803in
>
> ( 5.787401574803in * 2) + 15 = 26.57in
>
> You'd have a difference of 1.38 inches.
>
> Wether that's a lot or not depends on your point of view, I guess. :)
>
> Keep in mind that any change in the circumference of your tire will
> throw off your speedometer and odometer.
>
> I think that the formula for circumference is Pi * diameter. (Its
> either that or Pi * r squared, but I think that one is area) Hey,
> its been a while! :)
>
> Anyway, assuming I'm right about the formula, the circumference of
> a 235/70 tire would be 87.81617260034in, and the circumference of a
> 245/60 tire would be 83.4872063454in. So, for every revolution, the
> new 245/60 tire would travel about 4.33 inches less.
>
> If we assume a speed of 60mph:
>
> I took 60mph / 60 mins per hour * 5280 feet per mile * 12 inches per feet
> to get 63360 inches per minute. Dividing this by the circumference of
> each tire, we determine the revolutions per minute that tire has to turn
> at to make the truck go 60mph:
>
> 235/70: 721.5071908037 rpm
>
> 245/60: 758.9186747712 rpm
>
> If we're using the 235/70 as our baseline, we can divide that by 60mph to
> determine the rpm nescessary to sustain 1 mph.
>
> 721.5071908037 / 60 = 12.02511984673 rpm per 1mph
>
> So, every mile per hour needs about 12.025 rpm. If we divide the
> rpm of the 245/60 tire at 60mph by 12.025 we see that the speedometer
> would actually read 63.1 mph.
>
> Of course, the Dakota's speedometer is calibrated for a 215/75 tire.
> An rpm of 728.1735685438 will cause a 60mph reading on the speedo.
> Doing the numbers again from a baseline given by the 215/75 we get:
>
> Indicated speed Actual speed
> 215/70: 60 60
> 235/70: 60 60.55
> 245/60: 60 57.57
>
> So, it would appear that the 235/70 is a pretty good match with the stock
> tire. That's what I've got on my truck now. I never bothered to go
> through with this calculation though. Looks like those of us who
> picked those tires did a pretty good job! :) I just did a quick
> calculation for a 245/65 tire, and that would read 59.66mph at 60mph.
> (Height of 27.539 in.) That's even better than the 235/70 for accuracy,
> but I have no idea if they make a 245/65.
So, what you are saying is that the overall diameter, given the mass-
circumference, divided by the mean foot print, equals two fat black
patches of
synthetic rubber compound on hardened tar & gravel. Right?
Do you take into account the additional frontal area of each units
width?? This additional frontal load resistance, together with the
additional
rolling resistance, might necessitate the assistance of an injection of
nitrous-oxide?? :)
(Sorry, too much Jolt Cola...)
..Sam '95 SLT
BTW:
I was bummed yesterday, I almost picked up a extremely clean (as
what I could tell from the picture, and the ad) slide in camper
for my Dakota :( Asking price $995... It was a ?Dalvin? brand
made especially for a short bed Dakota (1987 was the year)...
SOLD before I got there...
Oh well, things for the Dakota are OUT THERE (Dakota-x-files...)
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 12:07:37 EDT