What ratio would give someone an RPM of 4300 in 4th gear at 60 mph?

What options would I have to lower the RPM at highway speeds?

This would be on a 69 442 convertible 4 speed trans.

 

Resurrecting a thread that's been dead for three years probably isn't the best way to get an answer to your question, so here's your own shiny new thread.

Unfortunately, we need to know the tire diameter and trans top gear ratio to answer your question. If this is an overdrive trans, the answer will be different than if the trans has a 1:1 top gear. If that's a stock Muncie, then fourth will be 1:1. Smaller diameter tires will increase the RPMs for a given rear axle ratio.

 

It's more than just the gear ratio you need to provide. Also need the tire size. Joe P should be able to answer this with that info. From the sounds of it you're likely a 4. Something gearing. Doesn't sound like the car was built or modified for highway use.

 

the tires are stock 14 inch, I'd like to get the RPM's down so I can drive the highway without running at 4 plus RPM

 

Well, after running a couple of numbers, either you have wheelbarrow tires on your car, or your tach is wrong. To really hit 4300 RPM at 60 MPH, you'd need 5.00:1 rear gears AND 205/60-14 tires (which are about 23.7" in diameter). My money is on the tach being wrong.

 

I would hook up an aftermarket tach to check it, as the originals can get out of whack after 40+ years. My tach jumped 300 rpm while cruising at a set speed, so it can happen, and oddly enough it was 60 mph/4300 rpm.

 

Even at a 300 jump that puts me at or around 4000, all I know is that it does rev high at 60 mph. I'm looking for a gearing combination to lessen the rpm at cruising speeds.

What information would you need in order to make a suggested change?

 

One more time.

Unless you are driving around on the metal rims without tires (which would explain the high RPMs), 14" is the WHEEL diameter. We need to know the TIRE diameter (or size on the sidewall).

 

One other crazy though comes to mind. If the linkage on your four speed is flopped, you may be in third when you think you are in fourth. That would also explain the RPMs.

 

Tire size is 235-60-14

The tranny gears are correct as there is a definitive difference in the two.

 

Good point about the gears, Joe! I didn't think about that, and was leaning toward the tach being innacurate.

I had two 1970 455 442's, both with factory 4-spd Muncie and 3.42 positraction rear, with stock G7014 Polyglas tires, and both of them ran 3000 rpm's at 60 mph with factory tachs. That much rpm (4300) would indicate a very low rear end ratio (high numeric) to achieve those readings, unless you are not in high gear at 1:1 ratio.

 

Well, if the trans REALLY is in fourth, 5.00:1 gears with those tires would give you about 4,000 RPM at 60 MPH (ASSuming both the tach and speedo are correct).

If you really have a Type O axle with the dealer-installed 5.00:1 gears, that's pretty rare. The car must be a rocket off the line.

 

I meant to say there is a definite difference between 3rd gear and 4th. A significant drop in rpm's. I am able to pull out from a dead stop in 3rd gear.

 

Msarver, how do you know your speedometer is accurate? It's easy to check: using mile markers on the highway, take a 10 - 20 mile drive at exactly 60 mph (as indicated on your speedometer) and time it with your watch. It should be exactly one mile per minute. If it isn't calculate the percentage it's off.


It's not uncommon for modifications over the years to have affected the speedometer accuracy. The speedometer itself might be completely accurate, but a rebuild or replacement of transmission, removal or replacement of the adapter, or change in rear end gears will all affect the accuracy.

 

I do not know if either the speedo or the tach are accurate. My gauge was driving down a highway at 60 (on my speedo) and watching everyone fly by me. Now I'm sure they were doing 65 or 70 but the I could definitely feel the engine was running at high RPM's. There was more there but I really don't think it would like it for a sustained period of time.

I want to thank everyone for the input so far. What I am looking for are recommendations to change to a different gear set

 

Wow! If he can pull from a dead stop in 3rd gear he might have 5.00's.

 

Or a slipping clutch, whichmay also explain the rpm, I doubtthis is right, cause slipping wasnt mentioned and is kind of obvious, I had a dodge power wagon with 4.56 gears it would climb a tree, but on the highway that 318 was screaming....

 

5.6 to 1 with a 26 inch tall tire

 

nah, I did it w/'3:42's frequently before getting the shifter rebuilt.

 

My 1970 with a four speed and (incorrect) 3.08 rear axle can do that (with some slipping of the clutch).

 

Which doesn't exist for a Type O, thus the 5.00:1 and assumptions on speedo/tach accuracy and actual rolling radius of the tire (as opposed to theoretical O.D.). Keep in mind that tires, especially radials, compress under the weight of the car, so the rolling radius is smaller than the theoretical radius. This causes the effective diameter to be smaller, increasing RPMs.

To the O.P.: You'll find that gears for your axle are nearly impossible to find, especially since 5.00:1 gears would have a four-series carrier. It will be easier (and no less expensive) to change the whole axle. Or, consider an O.D. trans, which gives you the best of both worlds.

 

Know this is old, but interesting to me. What was the reason you ran the 3.08? Was it for fuel mileage?

 

A previous owner had installed it. I bought the car that way in 1986.

 

A phone and a speed display using your gps based app tells the speed of 'most anything.

I like the Escort program that alerts you [sometimes] to police presence if others have reported it. I report them every time.

 

Hi Joe.

How does the '70 442 run with the 3.08 gears and a 4-speed?

Does the 442 still have an M-21 trans, or was it switched to an M-20 with the lower first gear? Does it have original 14" size tires, or upsized 15" tires?

Thinking of changing the original 3.42 gears in my '70 442, which still has the original M-21, to either 3.08 or 3.23 gears for drivability reasons so your input would be greatly appreciated.

Thanks,
Anthony

 

If you have a 12 bolt olds...you can not buy NEW 3.08 or 3.23 gearing... YOU have to find good used gears...GETTING harder and harder to find these days.

Jim
JD

 

Thanks. Seems everything has changed since the day, those gears were plentiful back then and the 3.42 up gears were hard to find. Hard to fathom how many these now wanted good gears went into the crusher over the years.

 

Well you need to thank your Government for the crushing of the old cars... Incentives for crushing cars is still going on to this day.

Another reason is these car are now 40-50 year old vehicles with 100+ k mileage and not easy miles since we all think they are ALL Muscle cars...

 

Msarver,

Now there has been lots of discussion about your situation and not much in the way of getting to the bottom of what is going on., Where are you located? Maybe one of us can get with you to help. Some things that you could do to help out to get to the bottom of your issue.

1- Use a smart phone and a speedometer ap to get an accurate speed of your car.

2- hook up an aftermarket tach and get a reading of the RPM's at those speeds. I would create a log to keep track of what you are doing.

3- Identify the transmission. Is it a Muncie? If so is it a M20 wide ratio or a M21 close ratio? These operate differently in the first and second gears and gives you different options for a rearend.

4- Identify the rearend you have by type and ratio. We can help if you take pictures of it and pull the cover and count the teeth of the ring and pinion gears or get the information off the ring gear.

5- If you are running the 235 60 14 tires this should not be an issue as many of us run them.

6- what is your goal? Do you want a car that comes out of the hole really quick and has limited top end or are you looking for a car that performs well but is still very highway usable. I am assuming the second from what you have said so far.

Get all of this info and let us know and maybe we can help you come up with something of a game plan to make the car more like you are wanting.

Hope this helps

Larry

 

Octania,
The Waze app. for Android, & maybe others, is a very good app. for GPS & you can post police sightings, objects in road, stopped car on shoulder, & other items for other users to see. The police notice has saved my rear a few times for sure.