Hot Rods Rear End - Gear Ratio??

Happy New Year, HAMB -
So here's my setup:
'63 Mercury Comet, originally a 260 car with a Merc-o-matic 2-speed. I put a 289 out of a '66 Mustang, and a C4 from around '74 (dude I got it from wasn't sure...he thought maybe a Maverick).
Car shifts fine, but cruising in 3rd at 55mph it's pushing 3000 rpms. If I wanna go 75mph, forget it. It'll redline. My buddy told me that it's a gear ratio thing. That is, if I switch out the rear end, then I'll be able to take her on the freeway. I've had one mechanic tell me I could do a whole axle swap out of a 90s Mustang, and another guy say pull the shaft and gears outta the pumpkin and switch with a different ratio. I'm not smart on this stuff, so any help is much appreciated!
Thanks,
RNY

 

You could do either, but first it would help to know what rearend is in the car now. It's kind of likely that it's an 8" type, but could be something else. Pictures would be a big help, as always.

If it has bolts around the front of the housing, then you can remove the center and replace it, or just replace the gears. If it has a rear cover plate with bolts around it, then you might be better off replacing the whole rear end.

But you might also find that the engine won't explode if you just drive fast. Those little engines with a 3" stroke were built to run high RPM, and they can take it.

 

I'd say it probably has an 8" (drop out the front) if it has a deep gear set. A 3.25 gear isn't hard to find with a 3.00 being common, both will work good with a 26" tire.

 

***uming it is a 5 lug car, it is likely an 8" axle with 28 splines. As @squirrel said, pictures would help. I would change just the center section with a 2.75 to 3.00 ratio if it is an 8". Both were common ratios and came in millions of Ford and Mercury intermediate cars. An integral carrier axle (has a rear cover) would be harder to change ratios, and would probably be best to replace the rear axle. An 8" out of a 74-77 Maverick would be close in width.

 

A 90s mustang will probably have close to the gears you have now or even lower, overdrive transmissions wether automatic or manuel were normal then. A pumpkin swap would be the easiest.

 

I'd say it has a non stock axle ratio, either ordered that way as an option or changed out at some point. My mom got a new Fairlane wagon in 63. 260 and Fordomatic. That car would loaf along at 70-75 mph all day long on the interstate and never show any sign of high rpm's. I'm thinking it came with a 3.0 axle ratio but it might have been a 3.25. ***uming the OP's Comet has 14" wheels, 3000 @ 55 mph sounds like a 3.70 or higher axle ratio, more like what might have been stock in a car with the 6 cyl or 221 V8 engine and a manual trans. AFAIK, the 8" axles all had the ratio tag under one of the third member nuts like the 9 inch ones.

 

According to what I can dig up, a 63 Comet came with a 170 cu in, 6 cylinder. The rear axle ratio was about 3.5:1 with the automatic. A 3.0:1 ratio or higher (numerically lower) would certainly help your cruising rpm. If you choose to replace just the ring and pinion, you need the help of someone who knows how to properly set up rear axle gears.

 

A 2.80 which was the highest ratio available is about 2400 at reasonable freeway speeds. Using a C4 it won’t be the same street jumper you have now but nicer and quieter on the highway. I went from a 3.21 to 2.74 in a similar weight and horsepower car and will never go back.

 

Adding a pic. Looks like 8 bolts (if that means anything), with bolts facing drive shaft.

8-bolt Rear End by RednekYoga posted Jan 6, 2023 at 9:50 AM

 

8-bolt Rear End by RednekYoga posted Jan 6, 2023 at 9:50 AM

 

Hope this helps.

 

Pics don't work for me.

 

I think you need to make your page public, IIRC

 

We need more info namely, tire height. Do you have a stall converter? Are you getting vibrations?

The ****er Calculator with 4.6 rear gears shows roughly 4500K at 75mph with a 26" tire and straight through auto transmission. I doubt that you have 4.56 rear gears.

The stock SBF upper power band is to 5-55K. 4500K is well below the upper end of a sbf.

Honestly this could be a perception issue because in this day in time we are not used to running an engine in its upper power band.
Also, it could be a mechanical issue or multiple mechanical issues. Things out of balance, something slipping....things like that.

 

8 inch rear, which should be an easy find in a desirable ratio. @Moriarity has a good plan, verify the existing ratio and report back.

 

that sure looks like an 8" rear. (not 8 bolts....it's 8 inches, the ring gear diameter)

 

There may also be a tag still attached to the rear, as mentioned earlier, where the center section bolts to the housing, and the warranty tag, if original, will show what the ratio was originally.

 

Yep, your basic Ford 8". On the '64 comet I used to own a 3.25 rear axle got me 25 mph per 1000 rpm in high gear with a 27" tall tire (about the biggest you could stuff inside the wheelwell). That was 3K rpm @ 75 mph and was a bit buzzy but was livable. Drop down to a 2.8 and the rpm is now about 2600. If your running smaller diameter tires the rpm will go up. According to this... Tire Size, RPM, Speed, and Differential Ratio Calculator (advanced-ev.com) you probably have a 4.11 rear which will make for a very lively stoplight bandit but not so hot for freeway cruising. I'd go with either a 3.0 or 2.8 ratio, it won't be a rocket between lights anymore but that will drop your freeway rpm to or below 3K at 75 mph.

Ford 8" rears aren't so common anymore, but look under 289/302 powered small/intermediates (mustang, falcon, fairlane, maverick, etc) from the mid/late '60s through the 70s. There will be some in some six-cylinder cars too but those will tend to be steeper ratios. Stay away from the early versions ('62 to about '65) as those had issues with pinion bearing failures. Ford upgraded the bearings on the later rears (and the early rears can be upgraded with the later bearings). New gear sets are available, but ratio choices are limited.

 

I own a 40 ford with a chevy small block ,with a 3.24 rear ,with 26” tall tires ,that engine spins at 3000 at 70 mph and will do it all day long , and don’t hurt it at all .it just loves stretching it’s hind legs !!!

 

How does it run before the swap? It should’ve been the same rpm at hwy speed

 

Did you pull the dipstick on the trans to see if the fluid is burnt or starting to get burnt? This doesn't sound like a gear ratio problem. The trans might be slipping in high gear.

 

As mentioned, verify the gear ratio you have.
Is the 3k RPMs via a good tachometer, or your ear?
Someone made a similar thread a while back. Seems he had the tach set for 6 or 4 cylinders, not 8.

 

.......... or not shifting into 3rd..........

 

Open your driver's door, and look about where the latch mechanism is:


If nobody swapped the gears, or the axle:

 

Leave the rear end alone and swap in an AOD for the C4 and have the best of both worlds.

 

AOD to a 260 would involve an expensive adapter.

The 260 and early 289 have a 5-bolt bellhousing pattern that does not readily attach later transmissions.

I am all about overdrive transmissions, but it would need to be cost-effective.

 

he said the engine it has now is a 289 from 66 and the C4 he now has is from 74. It's the modern 6 bolt pattern that an AOD would bolt to.