# Thoughts on Tire Size



## Omni (Jul 7, 2016)

Good Day All

Looking at purchasing some taller tires for a '66 GTO (389 tripower /4 speed).

For 14" rims I can find a 26.7 diameter tire (215/75R14)

For 15" rims there is a 28.8 (225/75R15)
29.0 (235/75R15)

The end game is to slow down the engine rpm's.

My question is fitment. Any one out there with knowledge as to any clearance issues with these selections?

All comments welcomed.

Omni:cheers


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## GTO JUDGE (May 14, 2005)

You want to reduce the engines RPM by increasing tire size?
Why not address your rear end? What gearing are you running? 
Seems to me if you increase your tire size you'll throw off your mileage calculator and overwork the engine trying to maintain the speed you travel?
Unless it's minimal figure in a re-calibration of your speedometer. My '70 was raced and had 4.33's in it when I got it, I installed the 3.55 and the RPM at 60-65 dropped from 3500 to about 28-2900. Running 14x65's. Was over by 20 mph and finding the right the speedo gear am now off 2 mph at 60 which accounts for the 65 series tire.


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## 64GTOConvertible (Aug 28, 2016)

Changing tire size does the same thing as adjusting ratio, so why not. The taller the tire, the further the car goes with each revolution. Same thing as reducing the gearing. But a speedo adjustment is going to be necessary. I'm sure there are plenty of formulas out there to help you figger out what you need at the end.

That said, I just bought tires for my '64. The 66 supposedly has more clearance, so you should be OK with 275/60 on the rear and 235/60 on the front.


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## Pinion head (Jan 3, 2015)

On listed tire dimensions, be careful trusting height numbers, as even same size very similar tire designs by the same manufacturer can have different section widths, different contact width, and different tire height when mated to the same size wheel. Ran into this over 30 years ago when auto crossing. Desiring tallest possible rear tires on a '66 cruiser, I'd look into 15" wheels, many more tire choices.


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## 1968gto421 (Mar 28, 2014)

Whether or not one likes TCI Transmission, they have some great tech stuff. Here is a link to their page on figuring out speedo gear ratios and lists gears for GM tranny's.I've used it alot over the years with different setups.

Speedometer Gear Calculator and Charts - TCI® Auto

Luck with all this.


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