Flemming Larsen writes >
>I apologize if this is something that I ought'a know, or if it is a
>FASQ (Frequently Asked Stupid Question), but what is the nominal
>circumference of a 165-13 tire (tyre)?
Generally, when a size is given with no aspect ratio designation, the
aspect ratio is 80%, or perhaps 78%. I think that 165-13 implies a ratio
of 80%.
Here are specs for various 165 tires via the formulae pirated from BFG:
Size Overall Dia Circ. Rev / mile Rev / mile
(inches) (inches) math BFG form
165-80-13 23.39 73.49 862.12 889.13
165-78-13 23.13 72.68 871.80 899.12
165-75-13 22.74 71.45 886.74 914.52
165-70-13 22.09 69.41 912.81 941.41
165-60-13 20.80 65.33 969.84 1000.23
You will notice that the mathematically-derived revolutions-per-mile
figure is lower than the one gotten by the BFG formula. I can only assume
that BFG and other tire industry testing confirms a "loss" of effective
radius do to the flat spot at the bottom of the tire of abut 3.1%. My
primitive tests with 175-70-13 Yokohama A008R tires, a '66 Midget, and a
stopwatch on highway odometer checks conformed to the BFG formulae.
A tape measure around the tire checked out OK, too.
It may be of some use to you to compare the "standard" radial tires on
various British cars. They were 80-series.
Size Wheel W. Overall Dia Circ Rev / mile Rev / mile
(inches) (inches) (inches) math BFG form
145-80-13 4.00 22.13 69.54 911.19 939.74
155-80-13 4.27 22.76 71.51 885.97 913.73
165-80-13 4.55 23.39 73.49 862.12 889.13
>How much does this dimension vary from one manufacturer to another?
Ah, that's the rub. There are some variations, but I don't expect to see
major ones in 165 tires.
>My rims are marked: "4 1/2J-13" (standard Lotus Elan steel wheels)
>Are 165-13 tires the correct size for those wheels?
Yes, the BFG formula gives the industry measuring rim for 165 tires as
4.55", so I think you ought to be OK with 165 or 155 section tires. I
can't imagine you want 145's.
I have put all the BFG formulae into an Excel spreadsheet which shows gear
ratios, road speeds and other stuff not included in this message. Rather
than ship the spreadsheet itself out, I can Email instructions to set them
up in any spreadsheet program you like. Just let me know if you want it.
Phil Ethier, THE RIGHT LINE, 672 Orleans St, Saint Paul, MN 55107-2676
h (612) 224-3105 w (612) 266-6244 phile@stpaul.gov
"That's why I'm here." - James Taylor
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