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New tires for the 69 GTX

Quintin

Well-Known Member
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Have 225/70/14 BFG TAs on it now. They're old. 4.10 rear end. Thinking about going to 225 or 235/15. Of course that means new wheels. Basically to get the cruising rpms down. There must be a chart to show mph vs rpm. Not sure if it would be enough of a change to be worth new wheels. Looking for opinions and advice.
 
15" will help a small amount, but it's still 4.10's so probably not enough.

Wheather the cost justifies it depends on your wallet and wife and we can't help you there.
 
1. Circumference of your tire = 3.14 x diameter (height). You can get tire height (diameter) from
tirerack.com
2. Old tire circumstance divided by new tire circumference = relative rpm.
Example: 4.10’s and current tire (28” tall x 3.14 = 87.92 inch circumference)spin 3500 rom at 70 mph. New tire (30” - 94.2” circumference) so……(87.93/94.2) x 3500 rpm = 3267 rpm at 70 mph with the taller tires
 
15" will help a small amount, but it's still 4.10's so probably not enough.

Wheather the cost justifies it depends on your wallet and wife and we can't help you there.
Wife gave up on motorcycles over 50 years ago so she isn't a factor.
 
I have 15" wheels on a '69 GTX, super track pac, 4.1 gear.


I like this website, very helpful in choosing aftermarket OD transmissions combined with tire diameter.
 
The rpm change is simply the ratio of the two tire diameters to rpm.

If you cruising at 3000 rpm now, a 225/70/15 tire will cruise at 3000 rpm x 26”/27” = 2890 rpm.

If you want to make a meaningful rpm change, tire size is not the way to do it.
 
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