I went scurrying to the workshop manual. The original Tiger tires were
listed as 5-90x 13 with a rolling radius of 11.5 inches (Diameter 23
inches). I'm not interested in mileage necessarily, more interested in a bit
more ground clearance, so I was shooting for a diameter @ 24". I could
probably get some of that with new springs, but I need the tires anyway;
mine are thirty years old.
Either way if you go to a larger tire the speedometer will be off by a bit ,
but I'm sure there are shops that can calibrate it for me (Nisonger does
it). Anyhow, a 22 or so inch diameter tire will run at a somewhat higher
engine speed than a 24" tire. Wiki tells me C = Pi x D, so a tire with a 22
inch diameter rolls a bit more than 69 inches per revolution, a 23 inch
diameter tire rolls @ 72 1/4 inches per revolution,a 24 inch diameter tire
rolls a bit less than 75 1/2 inches per revolution.
I'll leave it to the engineers to complete the calculations, but it would
appear at first glance that going from a 23 inch diameter tire to a 24 inch
diameter tire isn't going to save you a lot of gas.
Tom
' 67 Mark 2
On Sun, Jul 4, 2010 at 7:28 AM, Ron Fraser <rfraser@bluefrog.com> wrote:
> Duke
> I'm not sure your numbers are that accurate. I prefer to run the
> numbers using the measured circumference of the tire. There is also the
> question of how accurate is your speedometer and tachometer. Measure your
> tire circumference and re-calculate. The stock Tiger tire I believe had a
> diameter around 23.7" so unless your buying a tire close to that size I
> don't think you will see much MPG improvement from just the tires.
>
> I'm not sure where you can drive 78 mph all the time but here in NY
> State you would be getting tickets all day long. This state is short on
> money, just like many others, and one way to generate more cash is to catch
> more speeders. The Police have reduced their max speed tolerance to match
> the State's need for income. One speeding ticket would overshadow any MPG
> savings not to mention putting a damper on the trip. Slowing down a bit
> should increase your MPG.
>
> Ron Fraser
>
> -----Original Message-----
> From: tigers-bounces@autox.team.net [mailto:tigers-bounces@autox.team.net]
> On Behalf Of wsamouce
> Sent: Saturday, July 03, 2010 11:49 PM
> To: tigers@autox.team.net
> Subject: [Tigers] Gas milage with different tires?
>
>
> Hey guys,
>
> I am thinking about a bigger rear tire to save me some $$ when driving 1600
> miles to TE/AE United 2010. The Sumitomo's I have (195/60R13) are 22.4 in
> diameter and 935 revs per mile. If I go to the 205/60R13, they are 22.8
> in
> diameter and 914 revs per mile. I am trying to figure out if the gas
> savings would offset the $170 cost of the tires with mounting.
>
> I just can't get my head around the math to figure it out. I am currently
> running 3600 RPM at 78 MPH (3.07 gear). I plan on running the same speed,
> with cruise control, all the way to Maine. The larger tire would allow me
> to run at 3535 RPM for the same speed.
>
> TIA!
>
> Duke
> B382002037
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