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GTO Gear Ratio's

169K views 171 replies 74 participants last post by  DONNY 
#1 · (Edited by Moderator)
Hi. I'm posting a new thread because I didn't want to hijack another thread that this subject came up on Gear ratios. I was bragging about my "racing" days, and gear ratios were mentioned. The following is some information that I have learned over the years, and the following opinions on gear choice are my opinions and nothing else. I am no engineer.

I've run GTOs over the years with ratio's from 2.56:1 to 4.56:1. Stock early GTOs with stick shift usually came through with 3.55 gears, and auto cars had 3.36 gears standard. The 3.55 (4-Series) carrier could accommodate gears from 4.88 or so down to 3.36. The 3 series carriers could accommodate gears from 3.23 down to 2.51. I found that "back in the day", with cheap gas and road noise a fact of life, the 3.55 gears were okay.....the '66 tended to be running about 3200 rpm at 70mph....right in the power band. The speed limit was 55 at the time. The top end was ok. The low-end was great. Good "all around" gear, back then.

Now, 3200 rpm at 70 is too darn high. Too much noise, too much fuel, and with a big block Pontiac, unneeded. I ran a 3.90 gear in a '65 for a while. That car HATED the highway. The top end was 107 mph, at the redline. It would get there really quick, though. I changed out those 3.90s with a 3.36 gear set (the tallest gear set I could run in my 4 series carrier) back in 1990. Changed the car. Now, it loves 70 mph. It feels just about as strong out of the hole (Still can't hook up....tire smoke central), my fuel economy went way up, and my road noise went way down.

I had another '65 with a 3.23 gear and a 4-speed, and the 3.23 is my FAVORITE gear ratio for these cars. It does everything well: accelerate, cruise, and top end. I drove a '65 with a 2.56 gear. GREAT top end, doggy as hell off the line. Too extreme. A friend had a 2.93 gear in his '67. The car would kick down into 2nd gear (turbo 400) at 85-90 mph. We smoked a 427 '65 Corvette on the highway one day....he was running 4.11's and was done at around 105mph. The '67 would top 140 mph, I'm sure.

Another friend had a 4.88 geared, dual quad 455 '66. We broke the wheel studs on the rear wheels getting on the freeway one day. He jumped on it, and there we were. The car would lay rubber when you got on it at 65mph, in 4th gear. Scary. The cruising speed was 55 mph. Got about 8mpg at 55!

My convertible '67 has the stock, 3.36 gearset. I wish it had a 3.23. It is a little busier at speed than I wish it was. I guess I'm getting old...I like running at 75 mph with minimum road noise and fuel consumption.

Today, there are a lot of options with overdrives, lock-up converters, etc. If you want to modify your GTO and add a 700R4 trans or add a 5 or 6-speed stick, you can run a 3.55 or 4.11 gear and STILL cruise at high speeds. If you're like me and don't want to cut up your original old Goat, you have to choose what gear ratio best suits YOUR needs.

For this guy, it's 3.23s.

Jeff
 
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#37 ·
Having the 390's taken out and the 323's put in my 66 next Tuesday.Looking forward after almost forty years of driving this car to cruising the highway. Thanks for all the great info!!! Rick
 
#41 ·
OK I don't get it..... I pick up my 66 yesterday, I dropped it off on Tuesday, a few hrs later I got a call from the shop saying that I did not have 390's but 355's instead. I was having 323's installed. He said that there would not be a big difference, but I said do it anyway. The car ran 3500 @ 65 MPH on the way to the shop, and on the way home after the change to the 323's it ran 3500 @ 65 mph. No change at all!! I can not figure it out, what are you guys revving at 65, with 323's??
$789.00 and no difference.....Thanks Rick
 
#42 · (Edited)
I`m running about 2800rpm @ 65 with 15" tires on the back and 3.55 gears, manual tranny.
Remember too, you did not change the tranny speedo gear, so more then likely you were driving faster then the 65 indicated on the speedo.(or you use to drive slower then 65 when the speedo said 65) Without changeing the speedo gear it will show the same speed and RPMs you were running at before as the change you made was after the speedo gear, in order for the speedometer to be acurate you need to put the right speedo gear in the tranny too.
Here is a gear ratio calculator, if you messure your tire size you can see what speed you should be running at what rpm.
Differential Gear Ratio Calculator
To verrify your gear ratio you could jack up the car and spin the wheels over one revolution and count the # of times the drive shaft turns, it should be 3 and about a quarter turns for every time the tires turn over once.
 
#43 ·
Hey Rukee, How would I change the speedo gear, and where to purchase? I was also listening to the motor at 65 seemed like 3500 rpm's to me.???
 
#44 · (Edited)
The gear is on the end of a holder that goes into the tranny at the tail housing. The speedo cable screws into it. Remove the cable, then remove the attaching nut (or bolt) that secures the gear holder to the tranny. The holder unit will pull straight out, though it may be tough to get out, try twisting or turning it to get it to let go. Once out there will be a removable plastic gear on the end, checking with your GM dealer and giving them your tire size and rear axle ratio they should be able to fix you up with a gear that will be closer then you have now. If the dealer can`t help you, check with some tranny shops in the area. You need an RPM gauge to be sure what you`re running at.
 
#54 ·
Your engine can be a factor in your gear ratio. My current 65 has 4.10's and they good match for engine I have if wanted to race it, but I have decided not to put on the track. The car has a 12 bolt so I am go to change either to a 3.42 or 3.55 so that I can cruise with out the high an rpm an less noise.
 
#55 ·
Gear ratios with a 2 speed Trans in a 64

Hey everybody!! I'm new to the forum and wanted to add a question to this discussion. I have a 64 with Tri-Power and a 2 speed tranny. Did the GTO only come with a powerglide or somebody told me I may have a Super Turbine 400. So the reason for the question is I have read the posts and I want to know if the transmission I have will affect the rear end gearing? I run my car on the highway just to get around town and it is reving about 3500 when I am keeping up with traffic. My speedo hasn't worked since I got the car, but I just installed an MSD so I know that the tach is correct. I am having going to check to see what I have now in the car. It seems like most guys like the mid 3's for gearing, will that still give me good performance with my 2 speed and not blow my engine on I-540??? Thanks for any help, I'll post some pics soon
 
#57 ·
The Super Turbine 300 is also a heavier duty unit than the standard Powerglide. Bulletproof trans, if not too inspiring. It won't affect your final drive ratio. Like all other standard and automatic non-overdrive trannies, it has a 1:1 final drive ratio in 2nd (top) gear. Your car should have come with a 3.23 gear standard. 3500 rpm at 70-75 mph sounds like it has 3.55's in it. You can easily verify it.
 
#58 ·
So the car does have a 3.55, I want to lower the gear so I can drive on the highway and run my aftermarket air. So the big question where can I get a lower gear? The 8.2" rear end is difficult to find gears. I want to run a 3.08 so I can stay cool and still go down the highway. Where could I find one? The other thought is to switch to a 8.5" rear end, obviously save all the old parts. Would a 8.5" rear end have any issues (drive shaft, suspension, anything?). Has anybody done this and how did that go?
 
#59 ·
I just read through this post and I don't recall anyone mentioning how tire diameter effects the rear gear ratio.
If you're running a wheel and tire combo that's the same overall diameter as what came stock from the factory, then 3.55's or higher (numerically) might be too much on the freeway.
My '69 GTO has the stock 350HP/400 with a TH400 trans and 3.55's, but I run a set of 15X8 Drag Lites with 255/60-15's. If I remember right (the car has been torn down for paint), the tach read 2200RPM at 60MPH.
For me, that's acceptable.
But then again, the car I owned before this GTO was a '69 Dart with a built 340 and 4.88's. That thing screamed at 60MPH.
 
#60 ·
Good point, Silver. Going from a stock 25 inch tall tire to a 28 or 29 incher will make a world of difference. That's why 4x4 guys have to gear lower when going with the tall tires....to keep the revs in the powerband. 3.55's with a tall tire sounds like it's a good combo for you.....2200 at 60 puts you right at the beginning of your powerband.....right where you wanna be!
 
#61 ·
#64 ·
I've got a question about the two rear ends I have currently for the 66. I have the original 3.55 rear end stored away right now and it was replaced, not by me, with a 2.56 rear end for another year, don't know which year or model. I'm wanting to go to 3.23 so should I use the original 66 rear end or change out the current rear end that is on her?
 
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