RE ratio/tire size ?

A

A Rookie

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I have a 48 Mercury Town Sedan, survivor, When traveling it gets to be a bit of a white knuckle feel. Only because the RPM starts getting a bit scary sounding approaching 55 MPH, I THINK the car has a 3:78 Ratio RE as that is what is stamped on the bottom of the center section. Finding a tech who can or will do a RE gear change has proven almost impossible. The car has 670 15 tires and I am thinking that if I change the rears to 7.10 or 7.50 15's it will help. I know this was done to a lot of vintage hotrods. I have skirts to cover them to hide the larger tires. Thoughts?
 

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Are you running a Tach? Doubt the RPMs are that high, just sound like it to the ear. And secondly, these cars/trucks were designed to run at these speeds with these rear axle ratios.
 
no tach , 6 volt, so I have never found one in the past for other cars. I'd like to be able to drive to a larger city (MNPLS MN) a few times a year at something like freeway speeds so that is another incentive.
 
My 40 with 3.78s and l78:15s ran well on the open road. Going to a larger tire size will help. You ought to be able to cruise in comfort at 60 and be able to press it a bit faster if necessary but a nice easy cruise at 55-60 should be perfectly comfortable especially in such a big heavy car.

It may be that you are not familiar with older cars and expect it to behave (more like sound) like a modern car at speed. Well it aint a modern car, it's 70 years old you have to drive it at the road speeds of 70 years ago.

It's the journey, not the destination.

Mart.
 
Larger tires will reduce the RPMs, there are formulas to calculate the difference if you are interested in the numbers.
 
That is the size I was thinking of using, Trying to run several 40's Brand X sixes at freeway speeds cost me several motors in the early years of my hobby involvement, So I am more than a little paranoid about blowing my survivor. Tho its probably a given the flathead v8 is a much better design. Thanks,
 
Another possibility is to change to 16" wheels with either 6.70s or a little larger. The 16" wheels from a 47-48 Ford will bolt straight on.
 
One the Fordbarners posted an Excel spread sheet that you plug in the tire size in diameter, and the rear end ratio, it will spit out the RPMS at different speeds. Very simple to use decision model.
 
Keep in mind too that going to a taller tire will throw your speedometer off. It will read slower than you are actually going.
 
I figured that, I have the wife drive that daily and match the speed and use a wax pencil to mark the speedo accordingly. Red slashes at 55 and 65 . Done that many times in my rods and muscle cars. Used to be a bit of a speed freak. lol
 
If you have a smart phone, there are free apps that use the phone's GPS for speed readings. For my iPhone I use an app called "Speedometer - Free - Speed Limit Alert". It is free and very accurate. I used it to check the speedometer accuracy of my '37.
 
How about a Mitchell overdrive? $2,500. An OD is the only thing that will significantly reduce the RPM by 26% or 36%. I have one on my 36 and am ordering one for my 41.

If you go from 670-15 at 28.58 inches in diameter to 890-15 at 31.00 inches. The difference is only an 8.5% reduction in RPM. I got the numbers at cokertires.com
 
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Believe there are apps for RPM as well.

OD is the way to go if you are doing to drive on the highway a lot.
 
I've got a Mitchell and 3:54s in the rear end and can run all day at 70mph while turning 2000rpms in my 41 pickup. Love the Mitchell.
 
A Rookie said:
That is the size I was thinking of using, Trying to run several 40's Brand X sixes at freeway speeds cost me several motors in the early years of my hobby involvement, So I am more than a little paranoid about blowing my survivor. Tho its probably a given the flathead v8 is a much better design. Thanks,
Click to expand...

Your comparing a splash n squirt bottom end oiling system that was out of date when the ark was built, to an exceptional oiling system! Highway speed is a often a killer for them stove bolts. NOT for a flathead!
I'm gonna be totally honest here, the 3.78 is a perfect ratio for that heavy old Merc. It will cruise all day at 70 mph, it won't hurt it, it'll be perfectly happy. It will sound busy sure enough, but not a problem. It's being used to modern stuff with lazy arse gears and a 67 speed trans that makes these old cars sound "like it's gonna blow".
With 3.78 rear, your 6.70's at 70 mph your rpm is 3100. At 55 mph it's 2500 rpm. You really have nothing to worry about, 70 mph is not hurting anything.
Martin.
 
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