9 inch ratio - 3.25 or 3.55?

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roadwarrior

Well-known member
Joined
Nov 8, 2013
Messages
53
Reaction score
8
Location
Corona (Beer), California
My Car
1971 Sportroof, original V8 gage a/c 3spd car that was very unloved

1971 convertible purchased from original owner
Hi All,
I have an opportunity to get the spare 9 inch rear axle i have been hanging onto for my Sportsroof rebuilt at a good price, and need to decide on a gear ratio for it.

The car is not a racer, it's a mild 351W/4 speed currently with a 3.00 ratio in it now, which has been fine, but as long as I am going to be in the axle I am looking into a ratio upgrade.

For comparison, I have a '70 El Camino with a close ratio (M21) 4 speed and 3.31 which moves out nicely and doesn't rev too much on the freeway, so was looking at the 3.25 gear set. However most of the 71-73 Mustangs and Cougars cars I see discussed seem to be factory equipped or have been upgraded with the 3.55 ratio, maybe because of trans first gear ratio? I know I can go to an overdrive, but the current Toploader is in good condition, so that would not happen for a while if I find i need it.

Does anyone here have any experience with going from 3.25 up to 3.55, or the reverse, and what you feelings were on this are? Going up revved too much at higher speeds, or the 3.25 made the car a slug?

Also, I don't know the Toploader's ratios, or if that is even relevant like with M20 and M21 Muncies, but it is originally from a 69 Mercury Cougar with a 351W if that would tell someone what it has for first gear, which may influence the decision on the rear ratio.

Thank you for any input, it is appreciated.

Best regards,
Tom
 
Wide ratio toploader has a 2.78 first, close ratio is 2.32. Both were available on the '69 351 cars, so you can check the tag on the passenger side by the bellhousing. Most likely it's a wide ratio.

http://www.davidkeetoploaders.com/idchart1.htm
I ran a 3.40 rear in my 71 for many years with 225/70R14 tires. I don't do a lot of highway, so it was fine. If you're traveling to shows at longer distances, I'd stick with the 3.25.

My 71 Cougar has a 3.5 rear with 245/60R14 tires and an AOD. At 75mph it's turning 2250. If I didn't have the overdrive, it'd be at 3500rpm.
 
Hemikiller makes an important point, that is to consider tire diameter when considering gear ratio. Back in the day my 73 sports roof was running a 28 inch tall tire with a 3.55 gear. That works out to pretty close to what a 3.30 gear is with a 26 inch tire as I recall
 
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I say 3.25 for cruising and 3.55 for extra acceleration. With a stock transmission it will be revving high at highway speed. I have 3.5 and it's perfect but I also have a 5-speed.
 
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I say 3.25 for cruising and 3.55 for extra acceleration. With a stock transmission it will be revving high and highway speed. I have 3.5 and it's perfect but I also have a 5-speed.
Hey Tony, that’s perfect in my opinion. What are your RPMs in fourth and in fifth and what is your tire diameter? Mark.
 
Hey Tony, that’s perfect in my opinion. What are your RPMs in fourth and in fifth and what is your tire diameter? Mark.
Cruising is between 2,000 and 2,500 rpms in 5th depending if you go at the speed limit or above it. I have not checked in 4th gear. My street tires are 27" in diameter.
 
Cruising is between 2,000 and 2,500 rpms in 5th depending if you go at the speed limit or above it. I have not checked in 4th gear. My street tires are 27" in diameter.
Which five-speed you have Tony?
 
All, thank you for your input and check points.

I'm going to look for an ID tag on the trans, in absence of that I will see if I can figure first gear ratio by rotate crank, count trans output rotation.

Based on I-net info, looks like the trans from that '69 Cougar should be a 2.78 first, so wide ratio. This would make sense as the car can get out of its own way with a 3.00 rear gear.

Also, I am running 15" tires/wheels, but don't remember the size, I'll get that info as well.

Thank you again, and best regards,
Tom
 
To see what RPM you will be turning with the 3.25 rear axle gears at 60 MPH, take your current engine RPM at 60 MPH using the 3.0 gears. Divide that RPM by 3.0, then multiply that result by 3.25. The final result is the engine RPM using 3.25 gears.

If you are turning 2,300 RPM at 60 MPH (depending your tire circumference that RPM will likely be different on your vehicle) using the 3.0 gears, here is how that formula lays out to see the RPM with 3.25, 3.5, and 4.11 gears at 60 MPH:

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 3.25 = 2,491 RPM at 60 MPH

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 3.5 = 2,683 RPM at 60 MPH

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 4.11 = 3,151 RPM at 60 MPH

IF you were to move to an AOD, the OverDrive ratio (4th gear) is 0.6667:1. You can multiply the above final RPM using the respective rear axle ratio by 0.6667 to get the RPM in 4th (AOD) gear at 60 MPH for each of the rear axle ratios.

2,300 RPM with 3.0 rear axle ratio with OverDrive becomes 1,533 RPM at 60 MPH

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 3.25 = 2,491 RPM at 60 MPH ===> 2,491 * 0.6667 = 1,660

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 3.5 = 2,683 RPM at 60 MPH ===> 2,683* 0.6667 = 1.788

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 4.11 = 3,153 RPM at 60 MPH ===> 3,153 * 0.6667 = 2,102

Anyway, using that approach you can get a really accurate expectation for your final RPM at 60 MPH using different rear axle ratios, and even extend the calculations to see what the impact of an AOD would have.

What? Me bored? With stuff like that rattling around inside my head? Never! heh heh... I live for that kind of stuff... Well, at least I enjoy it, luckily

Gil
 
To see what RPM you will be turning with the 3.25 rear axle gears at 60 MPH, take your current engine RPM at 60 MPH using the 3.0 gears. Divide that RPM by 3.0, then multiply that result by 3.25. The final result is the engine RPM using 3.25 gears.

If you are turning 2,300 RPM at 60 MPH (depending your tire circumference that RPM will likely be different on your vehicle) using the 3.0 gears, here is how that formula lays out to see the RPM with 3.25, 3.5, and 4.11 gears at 60 MPH:

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 3.25 = 2,491 RPM at 60 MPH

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 3.5 = 2,683 RPM at 60 MPH

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 4.11 = 3,151 RPM at 60 MPH

IF you were to move to an AOD, the OverDrive ratio (4th gear) is 0.6667:1. You can multiply the above final RPM using the respective rear axle ratio by 0.6667 to get the RPM in 4th (AOD) gear at 60 MPH for each of the rear axle ratios.

2,300 RPM with 3.0 rear axle ratio with OverDrive becomes 1,533 RPM at 60 MPH

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 3.25 = 2,491 RPM at 60 MPH ===> 2,491 * 0.6667 = 1,660

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 3.5 = 2,683 RPM at 60 MPH ===> 2,683* 0.6667 = 1.788

2,300 / 3.0 = 766.6667 Then---> 766.6667 * 4.11 = 3,153 RPM at 60 MPH ===> 3,153 * 0.6667 = 2,102

Anyway, using that approach you can get a really accurate expectation for your final RPM at 60 MPH using different rear axle ratios, and even extend the calculations to see what the impact of an AOD would have.

What? Me bored? With stuff like that rattling around inside my head? Never! heh heh... I live for that kind of stuff... Well, at least I enjoy it, luckily

Gil
Gil, you are worth your weight in entertainment value!
 
I have done a switch from 3.25 open, which was stock for my car, to a 3.50 Truetrac. I have 15” wheels with p23560r bfg’s tires and run about 32k ish at 70 mph on the freeway. I don’t drive the on the freeway much, mostly around town. It’s very fun being able to spin the tires at 45-50! I probably should get some wider tires I guess.
I do have a nicely hopped up 351 Q Code.
 
I have done a switch from 3.25 open, which was stock for my car, to a 3.50 Truetrac. I have 15” wheels with p23560r bfg’s tires and run about 32k ish at 70 mph on the freeway. I don’t drive the on the freeway much, mostly around town. It’s very fun being able to spin the tires at 45-50! I probably should get some wider tires I guess.
I do have a nicely hopped up 351 Q Code.
I have long felt 3.5:1 is a decent compromise for performance and practicality on the public roads.
 
With a manual transmission, I find myself using the clutch pedal to coast any chance I get. Gives the engine and me a break. My 2001 Cougar was so aerodynamic that on the highway I could keep up with traffic down a slight incline, with the engine at idle.
My 73 Cougar is unfortunately an automatic, with 3.25 gears, the highway rpm is a bit higher than I would like. Obviously a 5 speed manual has been my dream.
 
I would go with the 3.55, it‘s not that much difference in RPM to the 3.25
Instead of 2500 it‘s turning 2800. But, it will be a much higher difference to the 3.0
 
Hi All,
I have an opportunity to get the spare 9 inch rear axle i have been hanging onto for my Sportsroof rebuilt at a good price, and need to decide on a gear ratio for it.

The car is not a racer, it's a mild 351W/4 speed currently with a 3.00 ratio in it now, which has been fine, but as long as I am going to be in the axle I am looking into a ratio upgrade.

For comparison, I have a '70 El Camino with a close ratio (M21) 4 speed and 3.31 which moves out nicely and doesn't rev too much on the freeway, so was looking at the 3.25 gear set. However most of the 71-73 Mustangs and Cougars cars I see discussed seem to be factory equipped or have been upgraded with the 3.55 ratio, maybe because of trans first gear ratio? I know I can go to an overdrive, but the current Toploader is in good condition, so that would not happen for a while if I find i need it.

Does anyone here have any experience with going from 3.25 up to 3.55, or the reverse, and what you feelings were on this are? Going up revved too much at higher speeds, or the 3.25 made the car a slug?

Also, I don't know the Toploader's ratios, or if that is even relevant like with M20 and M21 Muncies, but it is originally from a 69 Mercury Cougar with a 351W if that would tell someone what it has for first gear, which may influence the decision on the rear ratio.

Thank you for any input, it is appreciated.

Best regards,
Tom
I went from four 410s to 3:25 and I'm very happy with this decision.
 

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