Mph vs Rpm = rear end gears ??? - Pontiac GTO Forum
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post #1 of 11 (permalink) Old 09-16-2015, 11:18 PM Thread Starter
 
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Mph vs Rpm = rear end gears ???

Just got my first GTO last week ...35 years to late...but better late than Nevah. So Iam going down the Interstate...the speedometer says 70..providing it's correct...and Iam at 3,000 RPM !!! Running a 67 gto 400 and turbo 400. They ain't Hi-Way gears... Could they be 3:55 ???? Thanks
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post #2 of 11 (permalink) Old 09-17-2015, 07:41 AM
 
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Could be 3:37. Need exact tire height.

Differential/Speed/Axle Ratio Calculator

BTW PHS shows 8 gear rations for 1967. As I understand it, a/c cars got 3:23 as standard, and non a/c cars 3:55. Other ratios needed to be ordered or dealer-installed.
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post #3 of 11 (permalink) Old 09-17-2015, 08:48 AM Thread Starter
 
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Could be 3:37. Need exact tire height.

Differential/Speed/Axle Ratio Calculator

BTW PHS shows 8 gear rations for 1967. As I understand it, a/c cars got 3:23 as standard, and non a/c cars 3:55. Other ratios needed to be ordered or dealer-installed.
It has AC......so 3:23 ...??
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post #4 of 11 (permalink) Old 09-17-2015, 03:09 PM
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70 mph at 3,000 rpm is 3.36 gears....with stock tires. Totally normal. And not really screaming. But we got spoiled by overdrives. Back in the '70's when I was driving these cars, the speed limit was 55 and buzzing along at 3k wasn't really an issue. Nowadays, it is an issue to some of us older guys who like things nice and quiet. That's why I pulled the 3.36 out of my '67 and installed the 2.56!!
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post #5 of 11 (permalink) Old 09-17-2015, 10:06 PM
 
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Depends on tire height and transmission slippage (auto). Not all new same size tires are the exact same height, if you assume that and dont meaure actual tire height mounted on your wheels, usually will be off. An example... In the 80's I ran 255/60R15 Goodyears on 8" wheels on my T/A, both on the street and for autocrossing. The sticky Wingfoots and Eagle NCTs did not have the same section width or same height as the same size Eagle GT's (which were over a half inch taller). All were the same numerical size.

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post #6 of 11 (permalink) Old 09-17-2015, 10:10 PM Thread Starter
 
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So being that this 67 GTO with a 400 cid is my first Pontiac ever.....if you were to take it to the Drag Way for s...& giggles and make a pass .....what RPM would you shift at ?? And it has a turbo 400. Thanks
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post #7 of 11 (permalink) Old 09-18-2015, 07:14 PM
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Stock '67 400, I'd leave it in drive and let it shift itself. As long as the modulator was calibrated properly for 5200 rpm shifts. Over 5200 rpm, the power level drops off and you're wasting time.
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post #8 of 11 (permalink) Old 09-18-2015, 07:40 PM Thread Starter
 
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Then 5200 it is ......thanks !!!
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post #9 of 11 (permalink) Old 09-19-2015, 03:24 PM
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Irish, if you are planning to regularly buzz your engine up to 5200 rpm, you had better know its condition, inside and out! Just saying....a Pontiac 400 will rev like a small block Chevy, but can't take it like a small block Chevy. They like to spit rods out of the block if revved to the moon too many times. If your engine is fresh, you can abuse it. If it's old and loose, not so much. Take it easy and enjoy the car!!
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post #10 of 11 (permalink) Old 09-20-2015, 01:45 AM Thread Starter
 
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Yep ...she was freshened up about 5k so Iam told....it's all painted up nice...so someone at least had it out. Back in the late 70's when I drag raced...I would shift my 454 Big Block Chevy at 6200 all day long..never had a problem...but I know pontiacs power curve was much lower...just short shift it at 4500 and get it into high gear and let it rip.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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