Year One wheels and gear ratio - Pontiac GTO Forum
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post #1 of 10 (permalink) Old 02-21-2015, 02:09 PM Thread Starter
 
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Year One wheels and gear ratio

I have a 68 with Posi and a 4 speed that I recently put the year one 17 ince wheels on. It is a 3:55 now and it does not have anything off the line. Can anyone suggest a better ratio and where to source the gears? The year end is a BOP.

Thasnks
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post #2 of 10 (permalink) Old 02-21-2015, 04:57 PM
 
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What size tires do you have on your new wheels? Engine and trans?
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post #3 of 10 (permalink) Old 02-21-2015, 06:41 PM
 
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here's a website you can use to figure out the tire/wheel size, then put your rear end ratio in and it will tell you what the tire difference has changed your rear end ratio to....

4Lo.com :: Tire Size Change, New Gear Ratio Calculator
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post #4 of 10 (permalink) Old 02-22-2015, 08:17 AM
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Garage
You've got a stick and "nothing of the line"? Raise the revs before you drop the hammer. Also maybe check and set the timing, check for brake drag etc.

3:55's and your Pontiacs torque should be plenty of oomph of the line, the wheels themselves (unless you are rolling on DUBS lol) shouldn't make much of a difference.
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post #5 of 10 (permalink) Old 02-22-2015, 09:12 AM Thread Starter
 
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More information

I switched out from 14 inch BF Goodrich radial TA's 225/60/14 on stell rally 11's. The new cast Aluminum wheels have BF Goodrich G Force 245/45/17's on all four corners. That was the only change made. With the 14 inch wheels and tires, wheel spinning burnouts were quite easy and second gear was up in smoke as well. The new combo requires much more rpm to smoke the tires and just seems sluggish off the line. I know the aluminum wheels and tires weigh more than the old combo, but I got advice that the circumference was the issue and that a gear ratio change may be helpful.
Hope this extra info helps
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post #6 of 10 (permalink) Old 02-22-2015, 09:29 AM
 
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Your old tire combo measured 24.6 in height.......
Your new tire combo measures 25.7 in height

your new wheels and tires added weight that needs more torque to over come.

Your old rear end ratio 3:55 with added tire height is equivilant to a 3.41 now

so you've added mass, which added weight and tire height which equates to more power to do the same job you did before.

Plug the numbers in the calculator i suggested and you will see numbers don't lie nor do the laws of physics..............
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post #7 of 10 (permalink) Old 02-22-2015, 09:48 AM
 
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Your speedo will be off as well.........the larger tire will be turning less revolutions to cover the same distance.........when your speedo reads 57.5.you will be actually going 60 and as your speed increases, so does the error your speedometer will show.
It's not a huge factor but one to keep in mind. You can always download an app to a smartphone and check your speed using gps which will give you a truer reading of your actual speed than a mechanical speedometer.
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post #8 of 10 (permalink) Old 02-22-2015, 10:10 AM
 
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To have everything go back to status quo with your new tire/wheel combo.
You decreased your gear ratio by 9.6% or say 10%
You would need to increase your gear ratio to 3.70 from 3.55 and also get the correct speedometer gears for the tailshaft of your 4 speed.
This does not take into account the added mass you added to the car as far as weight. Heavier car, needs more leverage to move it thus the change in gears.
Chrome wheels weigh more than polished so you'll rarely see chrome wheels on a race car.
I don't mean to give you too much info but as I have learned....one change in a mechanical equation affects many other things to equalize the change. I was never good at math until a friend of mine who was a mechanic AND engineer showed me the numbers on some changes I wanted to make and saved me from chasing my goal throwing money at different parts to get to where I wanted to go.

Feel free to PM me if you wish....there are some other things like your transmission gear ratios that will also affect your car. I didn't want this to turn in to a physics lesson.

Last edited by azmusclecar; 02-22-2015 at 10:28 AM.
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post #9 of 10 (permalink) Old 02-22-2015, 11:21 AM
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Something else must be wrong besides just the tires/wheels. Granted, I don't care for 17" and up wheels at all, but the rear ratio isn't the issue if it is really a 3.55. I have 28" tall tires on the back of my 4 speed 3.36 geared '65 GTO, with 9 inches of rubber on each tire, and it goes out of the hole like a slingshot. This is with a little 389 engine. Can you install the original wheels for a quick side by side comparison? And verify the rear ratio?
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post #10 of 10 (permalink) Old 02-22-2015, 11:47 AM
 
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Quote:
Originally Posted by geeteeohguy View Post
Something else must be wrong besides just the tires/wheels. Granted, I don't care for 17" and up wheels at all, but the rear ratio isn't the issue if it is really a 3.55. I have 28" tall tires on the back of my 4 speed 3.36 geared '65 GTO, with 9 inches of rubber on each tire, and it goes out of the hole like a slingshot. This is with a little 389 engine. Can you install the original wheels for a quick side by side comparison? And verify the rear ratio?
Trying to compare apples to apples is great when you both have apples.

The OP may have a different clutch, a different compression ratio, what rpm does is the OP using to launch? Maybe the OP has an M21, not an M20, the OP may have PS/PB, engine robbing parasites, his engine may have a different torque band, there are too many variables from car to car to just say what should be is the norm.

I'm not suggesting anything other than owners of cars using their as the benchmark for tests when they really dont' have ALL the facts from the OP seems fruitless.

Not trying to suggest anything other than this, the OP knows all the facts that are in their car. More info is needed to truly ascertain what they have, not what they should have.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Wheels & Tires

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