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Good Day All

Have a '66 GTO with 14" Rally 1 and a 3:55 posi.

Looking to decrease cruising RPM, so I'm considering a 3:36 and taller tires.

I really don't want to change the axle assembly, so a 3:36 gear set is the limit.

I'm leaning towards 15" Crager (Hurst style) 6" width.

The question is: How tall of a tire will work, and what should the wheel back spacing be?

The suspension is stock.

All comments welcomed.

Thanks to all who reply.

Omni:cheers
 

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I've got zero clearance issues with my 235 60 14's right now. Backspacing is 4.5 all around on 8" rims. Are you doing 6" width on all four corners?
 

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Don't know about the spacing, but I changed the 3.55 gears in my '65 to 3.36 and am running 235/70/15 tires on repro Rally 1's out back. These tires are 28" in diameter, as opposed to the stock 26.5" tires. Noticeable difference.....it cruises 70 mph at the same RPM's it used to turn at 60. Well worth it.
 

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Tire questions too.

I have 215/70r/14's on my 65. I know very little about tires or what sizes are best. I guess my question is what's the difference between running 14 inch tires or 15 inch tires ..
 

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The bigger the wheel and tire height (meaning the bigger wheel in general if you imagine a cave mans stone wheel) the less times it has to spin around to go the same distance. Imagine if you could put skateboard wheels on your car, one full turn would move your car a few inches instead of a few feet your motor would have to be at full throttle just for the car to be moving at a very low speed. That said if you have bigger wheels on an older 3 speed car the motor is not racing just for the car to go at highway speed.
It also has the same effect as a ten speed bike shifting gears, so if you put on bigger tires the motor will have to work harder to get the car moving like you would have to pedal harder to move a bike trying to start off in tenth gear. Guys who want to race 1/4 miles at the track want lower rear gears (same theory as smaller wheels) to take off faster like 411s or to a lesser extent 3:55, they don't mind the motor having to race at highway speeds as much cause they don't do long trips. As some of us car guys get older we cruise more and race less, so the low gears and 14' tires that some GTOs came with to be factory quick off the line is no longer a good thing to us as it uses a lot of gas and runs the heck out of the motor on the highway. Newer cars with 6 speed transmissions don't suffer from this problem as much as the older 2 and 3 speeds.
 

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That really helped me understand...so moving up to 15's with my stock 3:23 rearend will lower my rpm's at hi way speeds and burn less fuel.
There will be a marginal reduction in acceleration as the trade off. I can live with that as I gave up street racing back in the early 70's. The insurance companies saw to that😜
 
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