I have never seen a race team ever measure a tire like that, they
always check the circumference, but I have never compared the measured
circumference to the roll the car one tire revolution method. Yes the
weight of the car compresses the diameter of the tire some but I doubt very
much that the tire circumference can change more than a mm. It does pose an
interesting experiment to compare these 2 methods. Tire wear can change the
diameter by of the tire .5" or more; this changes the circumference the
From: firstname.lastname@example.org [mailto:email@example.com]
On Behalf Of Jim
Sent: Sunday, July 04, 2010 9:41 PM
To: Thomas Witt
Subject: Re: [Tigers] Gas milage with different tires?
Measuring the 'off car' circumferance doesn't give you a very accurate
'tire revs. Per mile' as it's compressed on the bottom making the
rolling diameter slightly smaller. When I sent the numbers to get a
custom speedo gear for my Sprite/B210 5 sp. Conversion they asked that
I mark the tire and ground with a chalk mark and then roll the car one
tire revolution and make another mark on the ground and take a
measurement between them to show 'rolling' circumferance...
Sent from my iPhone
On 2010-07-04, at 4:31 PM, "Thomas Witt" <firstname.lastname@example.org> wrote:
> Back in 2003 Ron Fraiser sent this to the list:
>>>> Good Question about the original tires. I don't know the real
>>>> answer; I can only reverse engineer the size of the tire from the
>>>> printed facts in the
> manual and on hand.
> Original Tire = 5.90 x 13 - not much help here.
> 1-From the Tiger Manual - 23.92mph @ 1000 rpm Calculate tire size
> using 2.88 gears Tire circumference = 72.75" - Diam. = 23.15"
> 2-Let's calculate again using the speedometer gear ratios. Speedo
> cal# = 980 rev/mi, speedo cable gear = 15 teeth,
> trans drive gear # B7C-17285-A = 6 teeth, 2.88 gears
> This gives you 24.49mph @ 1000rpm
> Tire circumference = 74.48" - Diam = 23.71"<<<
> A number of years ago (6 years to be more accurate) Jim Boynton sent
> me the circumference of his original, barely used spare tire. It was
> So, for those that want to be "factory accurate" take your pick.
> 72.25", 74.48", or 74.875 as each has its merits. In closing his
> post Ron wrote, "I hope this helps and doesn't confuse people too
> much." I would agree.
> Tom (Witt)
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