50desotocoupe Posted January 29, 2015 Report Share Posted January 29, 2015 Ok, I am sure this has been asked 100000 times and I have looked but didn't find anything on my particular car. I have a 50 Desoto with completely stock engine, trans and rear end. In 4th gear, what is a realistic highway speed that is "safe" engine wise for these cars? I drove it over 100 miles last weekend with a buddy who has a 48 Ford. We kept it between 55 and 60 but as you know there are times when on the highway you can't go below the actual speed limit. I did hit 65 once..the car did perfectly...smooth, stable, no tire shimmy, etc (front end is all basically new with 4 wheels discs). I had a 6volt positive ground tach built from Westach and will be installing that soon. What RPM's are safe but will still not get me rear ended by some idiot on the road that doesn't realize I am not doing their "modern" speeds. Thanks! Quote Link to comment Share on other sites More sharing options...
james curl Posted January 29, 2015 Report Share Posted January 29, 2015 When you get the tach hooked up, the safe sustained rpm is 80 percent of the rated horsepower rpm. Which I believe is 3600 rpm so you should not exceed 2880 rpms for long periods of time. Quote Link to comment Share on other sites More sharing options...
desoto1939 Posted January 29, 2015 Report Share Posted January 29, 2015 Here is another option to help when driving on high speed access roads. I have a 39 Desoto also still stock engine. My car drives and handles the best around 50 MPH becasue it is a 3 speed and has the 4.11 rear end. I have attache a couple of clipon red bicycle flashing light to the license plate that sits inthe middle of the trunk lid. I hae the flashing at a moderate speed of flashing pattenr. I have noticed that when I have th eunit on the peopel seem to give me some more space in the rear and generally do not climb up my buttend. We willnot bebale to stop all of the people that travel like bats out of HxLL and also the people that want to lok at the car when passing you but thais has help and worked for me. I live outside of Philadlephia with all major access roads being 2-3 lane highways. Also you can get some magnitised strip and then add refelective red tape to it and then stick this on your truck lid or some where on the back of the car to warn people. Rich HArtung Desoto1939@aol.com Quote Link to comment Share on other sites More sharing options...
jeffsunzeri Posted January 29, 2015 Report Share Posted January 29, 2015 "What RPM's are safe but will still not get me rear ended by some idiot on the road that doesn't realize I am not doing their "modern" speeds." At least in the Northern California areas where I drive, there is no idiot-proof speed. I will admit that from time to time, when in a bad mood, especially while driving the WD-21 with the 5.11 rear end at its top speed of 48.783 MPH, some bonehead will come rushing up to my tail and stay within 16 inches or so, sometimes with flashing headlights thinking that this will encourage me to hit the afterburners so that they can be the first in line at the next stop sign. At which time I will let off the gas entirely. 6 Quote Link to comment Share on other sites More sharing options...
TodFitch Posted January 29, 2015 Report Share Posted January 29, 2015 . . . I have a 50 Desoto with completely stock engine, trans and rear end. In 4th gear, what is a realistic highway speed that is "safe" engine wise for these cars? . . . I am a little surprised you have a 4th gear. No need for a tach to figure out RPMs at all other than for looking cool or being more precise on shift points when trying to get the most out of the vehicle for things like racing. Engine RPM is based on the mounted diameter of the tires and the gearing. For the non-overdrive equipped people the typical transmission of the era had a 1:1 ratio when in 3rd gear so the rear end ratio is all that usually matters. If your transmission/overdrive setup has something other than a 1:1 "high gear" ratio then you'd multiply that by the rear end ratio to get the overall value. Once you know the overall gear ratio and your mounted tire diameter there are a number of calculators out there to tell you what speed matches up with what engine RPM. Mine is located at http://www.ply33.com/Misc/speed#calc 1 Quote Link to comment Share on other sites More sharing options...
greg g Posted January 29, 2015 Report Share Posted January 29, 2015 Have put many miles on my engine at an indicated 65, which is GPS confirmed 62. With my tires and 4.11 to1 rear that puts rpm at 3280. Lots of 200 mile + trips over the past 12 years. I believe the desotos had a slightly lower numerical rear gear which would lower your rpm Dependant on tire diameter. 1 Quote Link to comment Share on other sites More sharing options...
Tom Skinner Posted January 29, 2015 Report Share Posted January 29, 2015 I cruise between 55-60. I have had her up to 70. Stay in the right Lane on Interstates. Don't give a damn what people behind me want/think. They aren't in my world anymore anyhow, they are just knee jerk re-actioning down the road stressed out. I tend to take older country roads where 45-55 is acceptable, but as mentioned above the pineapple heads abound and crawl up behind you expecting you to floor it. That ain't gonna happen. They usually back off if you slow down some. Then I resume the speed limit. Driving on weekend seems to be the only way to go with the older cars now. Gone is good old 55 mph Law when we could keep up to traffic in the right lane like back in the 70's.(enacted law after the 74 Oil Embargo and maintained until the early 80's, even though it was largely a scoff law). 2 Quote Link to comment Share on other sites More sharing options...
55 Fargo Posted January 29, 2015 Report Share Posted January 29, 2015 I am a little surprised you have a 4th gear. No need for a tach to figure out RPMs at all other than for looking cool or being more precise on shift points when trying to get the most out of the vehicle for things like racing. Engine RPM is based on the mounted diameter of the tires and the gearing. For the non-overdrive equipped people the typical transmission of the era had a 1:1 ratio when in 3rd gear so the rear end ratio is all that usually matters. If your transmission/overdrive setup has something other than a 1:1 "high gear" ratio then you'd multiply that by the rear end ratio to get the overall value. Once you know the overall gear ratio and your mounted tire diameter there are a number of calculators out there to tell you what speed matches up with what engine RPM. Mine is located at http://www.ply33.com/Misc/speed#calc M6 semi-auto trans with 4 forward gears lo range 1-2, high range 3-4 Quote Link to comment Share on other sites More sharing options...
Phil Martin Posted January 30, 2015 Report Share Posted January 30, 2015 My Dodge with gyromatic likes 50 to 60 mph saw 85 when I first got it and could have done a little more. Quote Link to comment Share on other sites More sharing options...
Merle Coggins Posted January 30, 2015 Report Share Posted January 30, 2015 Without knowing your diff ratio it's hard to advise on what speeds you should run. However I believe that many won't push their engines to what they are truly capable of. Here's proof of what a flattie can do with 3.73 gears and 235/75R-15 tires on I-94 in Michigan. Just trying to NOT get run over. Quote Link to comment Share on other sites More sharing options...
ledfootslim Posted January 30, 2015 Report Share Posted January 30, 2015 Have put many miles on my engine at an indicated 65, which is GPS confirmed 62. With my tires and 4.11 to1 rear that puts rpm at 3280. Lots of 200 mile + trips over the past 12 years. I believe the desotos had a slightly lower numerical rear gear which would lower your rpm Dependant on tire diameter. So what do you think my 1948 desoto's rearend gears are? I saw something along the lines of semi-auto cars getting 3.9 rearends..... Quote Link to comment Share on other sites More sharing options...
meadowbrook Posted January 30, 2015 Report Share Posted January 30, 2015 My 1950 Dodge Meadowbrook with an r7 overdrive will run all day at 70 mph, with no shimmy and straight as an arrow, with my westach tach reading about 2800 rpm. My engine is not only stock, but never rebuilt with 80,000 miles. While not at its prime, with 100 psi on 5 cylinders and 85 psi on the 6th from a compression test, she does not smoke, runs at about 45 psi oil pressure and if I do decide to keep under 55, I even average 22 mpg. It is still 6 volts, has the original radiator and never overheats. Completely useful. I have done 75 to 80 for about 30 minutes and no issues, but I try not to be too hard on her. 1 Quote Link to comment Share on other sites More sharing options...
MarcDeSoto Posted January 30, 2015 Report Share Posted January 30, 2015 So what do you think my 1948 desoto's rearend gears are? I saw something along the lines of semi-auto cars getting 3.9 rearends..... Slim, I also have a 48 DeSoto. Mine is a DeLuxe business coupe. The standard differential ratio on these DeSotos and Chryslers with the semi-automatic transmissions is 3.73 to 1. They did make a higher speed 3.54, and a lower speed 3.91 for the taxis and long wheelbase cars like the Suburbans. I'm not sure what cars would get the 3.54 and what would get the 3.91 differentials. I'm guessing that maybe taxis would get the lower speed 3.91 rear ends for more stop and go driving, and maybe the Suburbans would get the higher speed 3.54 rear ends for more crusing, but that's a guess. 1 Quote Link to comment Share on other sites More sharing options...
soth122003 Posted January 30, 2015 Report Share Posted January 30, 2015 I drive my 48 P-15 coupe to work 3 days a week. I usually drive between 55 and 60 for most of the way. I just filled up the tank today and I am getting 16.5 MPG. I have had it up to 75 for a short time just to see if I could and the ride was smooth and steady, no shimmy or shake in the wheel. Joe Quote Link to comment Share on other sites More sharing options...
DonaldSmith Posted January 30, 2015 Report Share Posted January 30, 2015 My DeSoto Suburban with semi-automatic: 3.91 to 1. 1 Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted January 30, 2015 Report Share Posted January 30, 2015 The 1946-8 chrysler Windsors six cylinder cars with the M-5 were 3.54 to 1. Std 3 speeds-used the 3.90 to 1. 1946-8 NewYorker/ Saratoga straight eight cars used a 3.36 to 1 rear axle... these cars are at ease at 70 MPH. Quote Link to comment Share on other sites More sharing options...
50desotocoupe Posted January 30, 2015 Author Report Share Posted January 30, 2015 Thanks everyone, this has been very helpful! One thing that might be throwing me off is that I put a 6 blade fan on from a like year Dodge truck. Boy does it pull the air through the radiator. It makes a lot of noise.... I will continue to get used to it (basically have 200 miles on it since all the work) and find out what speeds are comfortable. I think 55-60 is the ideal speed but unfortunately there are times when even 60 is too slow... Now to deal with the smoke on decal... The car hasn't really been driven in quite a few years...so most likely have valve guide issues. Quote Link to comment Share on other sites More sharing options...
TodFitch Posted January 30, 2015 Report Share Posted January 30, 2015 . . . Now to deal with the smoke on decal... The car hasn't really been driven in quite a few years...so most likely have valve guide issues. Decal or decel? Not sure what a decal would have to do with smoke or valve guide issues. Rings could be stuck from sitting. Have you done a compression check? Have you hooked up a vacuum gauge looked at the charts for what different issues show up as on vacuum? Quote Link to comment Share on other sites More sharing options...
50desotocoupe Posted January 30, 2015 Author Report Share Posted January 30, 2015 I clearly needed more coffee before getting behind the keyboard this morning. The smoking is on decel...and from highway speeds. I really don't see smoke on decel on neighborhood streets. I haven't done a compression check yet. I figured I would get a few hundred miles on her and see if it improves...if not I will look at the compression. The car runs great, starts beautifully and has great acceleration, so I would bet either stuck rings or valve guides....it just turned 90500 miles so very well could be time for freshening...although I doubt I will do anything until it gets worse. Quote Link to comment Share on other sites More sharing options...
Niel Hoback Posted January 30, 2015 Report Share Posted January 30, 2015 Altho it sounds like a mileage related symptom, I had the same problem that was about 90% cured by using a quart of marvel Mystery oil. I figured it needed rings but I would only be out the cost of a quart to try it and I was very pleasantly surprised when it actually worked. I suppose any high detergent low viscosity cleaner would have worked to loosen up the rings. Try it, whats the worst that can happen? 1 Quote Link to comment Share on other sites More sharing options...
50desotocoupe Posted January 30, 2015 Author Report Share Posted January 30, 2015 I actually put a few ounces in the tank earlier this week. The tank was 3/4 full. Should I put more than a few ounces in? Quote Link to comment Share on other sites More sharing options...
Niel Hoback Posted January 30, 2015 Report Share Posted January 30, 2015 Put a quart in the crankcase so it can clean out the ring grooves and most important, the holes in the piston behind the oil rings. It'll take a few hundred miles, but for me, it worked. Quote Link to comment Share on other sites More sharing options...
MarcDeSoto Posted January 30, 2015 Report Share Posted January 30, 2015 The 1946-8 chrysler Windsors six cylinder cars with the M-5 were 3.54 to 1. Std 3 speeds-used the 3.90 to 1. 1946-8 NewYorker/ Saratoga straight eight cars used a 3.36 to 1 rear axle... these cars are at ease at 70 MPH. Why would my 1948 DeSoto have a 3.73 rear end and a 1948 Chrysler Windsor have a 3.54? That just doesn't sound right to me. I think the Chrysler 6's have a 3.73 just like my DeSoto. Is there a reliable source of differential specs on the Internet to check this out? 1 Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted January 31, 2015 Report Share Posted January 31, 2015 Why would my 1948 DeSoto have a 3.73 rear end and a 1948 Chrysler Windsor have a 3.54? That just doesn't sound right to me. I think the Chrysler 6's have a 3.73 just like my DeSoto. Is there a reliable source of differential specs on the Internet to check this out? The ratio's are correct. The chryslers are just a faster car!. DeSoto's in the dust I guess 1 Quote Link to comment Share on other sites More sharing options...
MarcDeSoto Posted January 31, 2015 Report Share Posted January 31, 2015 (edited) If you are right about all Chrysler Windsors having a 3.54 rear end that would also make them a slower car from a stop. What is your source for your information about the Chrysler differential ratios? Mine came from a DeSoto shop manual covering the years 1946-1950. I just went to get my DeSoto manual and looked up rear axle specs. For the DeSotos with Tip-Toe Shift transmission, which is about 99% of them, I was surprised to see that regular wheel base cars have two possible differential ratios, 3.73 and 3.54. And I'm sure that most DeSotos and most Chrysler sixes have the 3.73 to 1 rear end ratio. Some had the 3.54 ratio, and it would be interesting to find out why some had that ratio, and if it was picked out for a certain purpose back then. The difference between the 3.73 and the 3.54 is the former has 41 ring gear teeth and the latter has 39 teeth. If you are not sure what ratio your differential is, you can crawl under your car and find the boss on the side of your differential housing where the ratio is stamped into the steel. Edited January 31, 2015 by MarcDeSoto Quote Link to comment Share on other sites More sharing options...
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