Slant Six Forum :: new tires speedo accuracy?

Quote:

If you were calculating overall ratios and speeds would you figure the theoretical tire size or the squished down size?

What you're concerned with is the distance the vehicle travels with one revolution of the tire, in other words the circimference. Even though the tire "squishes" with weight on it, the distance around the tread (circumference) does not change; therefore you would use the diameter of the properly inflated but unloaded tire. The circumference, by the way, is calculated using the formula C=pi * D where C is the unknown circumference, "pi" is approximately 3.14, and D is the measured diameter of the properly inflated but unloaded tire.

Author:	64'4$peed [ Wed May 17, 2006 10:25 am ]
Post subject:	new tires speedo accuracy?
My Valiant came with 205/75r14's will this affect the speedo accuracy? it seems to be fairly accurate

Author:	SlantSixDan [ Wed May 17, 2006 10:35 am ]
Post subject:
That's a bigger tire than the originals, which were very nearly identical in size to a P175/80R13 (overall diameter 24.02"). If whoever put on the 205/75R14 tires (overall diameter 26.11") also changed the speedometer drive pinion to one with fewer teeth, to compensate, then the speedometer will remain accurate. If not...that's something you'll want to do.

Author:	69valiant [ Wed May 17, 2006 12:57 pm ]
Post subject:	Speedo Drive Pinion
How do you know which drive pinion to get to compensate for changes such as larger tires, or different rear end gearing?

Author:	emsvitil [ Wed May 17, 2006 3:09 pm ]
Post subject:
Actually the old bias ply tires (6.50 13) were 24.7" in diameter which is a 185/80 13 tire.......... (whatever.. ) As for the pinion, it's ratios: (old pinion tooth count) * (old tire dia)/(new tire dia) = (new pinion tooth count) That's assuming that the speedo was accurate to start with..........

Author:	64'4$peed [ Wed May 17, 2006 5:58 pm ]
Post subject:
thanks guys. I'll have to check and find out

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Author:	Hyper'72Valiant [ Tue May 23, 2006 12:58 am ]
Post subject:
What about if your changing your rearend gear size? How do you figure out which one you need now? Thanks, Cory

Author:	emsvitil [ Tue May 23, 2006 1:20 am ]
Post subject:
Quote: What about if your changing your rearend gear size? How do you figure out which one you need now? Thanks, Cory Same basic idea as tire diameter...... (old pinion tooth count) * (new rear end)/(old rear end) = (new pinion tooth count)

Author:	RossKinder [ Tue May 23, 2006 11:21 am ]
Post subject:
Quote: Actually the old bias ply tires (6.50 13) were 24.7" in diameter which is a 185/80 13 tire.......... (whatever.. ) Are these calculated sizes or measured sizes? If measured I assume not from center to the ground X 2, but across? I have half tread 185/80 13's and they are as near exactly 24" as I can measure. Would half a tread equal .70"? If you were calculating overall ratios and speeds would you figure the theoretical tire size or the squished down size?

Author:	dakight [ Tue May 23, 2006 12:23 pm ]
Post subject:
Quote: If you were calculating overall ratios and speeds would you figure the theoretical tire size or the squished down size? What you're concerned with is the distance the vehicle travels with one revolution of the tire, in other words the circimference. Even though the tire "squishes" with weight on it, the distance around the tread (circumference) does not change; therefore you would use the diameter of the properly inflated but unloaded tire. The circumference, by the way, is calculated using the formula C=pi * D where C is the unknown circumference, "pi" is approximately 3.14, and D is the measured diameter of the properly inflated but unloaded tire.

Author:	emsvitil [ Tue May 23, 2006 12:24 pm ]
Post subject:
The 24.7 was a published diameter of the 6.50 13 (forgot where I found it) New tread depth is about 8/32, so half of that is 4/32, then both sides would equal 8/32 again or 1/4" less diameter. But there's some variance around the mathematical size so .7" is possible. You should probably use rolling radius or the squished down size, but using the published diameter (or mathematical size if you can't find the spec) is probably close enough. I would compare squished to squished and published to published though. Then there's centrifugal expansion to contend with when you're going really fast.............

Author:	emsvitil [ Tue May 23, 2006 12:28 pm ]
Post subject:
Quote: Quote: If you were calculating overall ratios and speeds would you figure the theoretical tire size or the squished down size? What you're concerned with is the distance the vehicle travels with one revolution of the tire, in other words the circimference. Even though the tire "squishes" with weight on it, the distance around the tread (circumference) does not change; therefore you would use the diameter of the properly inflated but unloaded tire. The circumference, by the way, is calculated using the formula C=pi * D where C is the unknown circumference, "pi" is approximately 3.14, and D is the measured diameter of the properly inflated but unloaded tire. Actually there is some squirm in the tread, so the rolling circumference comes out less than the calculated circumference. If you look at specs that have diameter, rolling radius, and revolutions per mile; then do some math to make them all the same measurement (take your pick: rolling radius,diameter,revs/mile) you'll come out with 3 different answers..........

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