Hi I have just joined this forum , I’ve always admired theses Motorhomes. I have been researching on here and other places to better understand the vehicle before I buy one . Something that keeps coming up is that you can’t drive theses over 55 or 60 that the motors can’t take it ? I then read other claims it the hot rod of rvs . Which is it . I currently have a Ford e350 chinook and fully loaded I can do 80 ... not the best on mileage but no problem for the motor and rides fine.
Top speeds
- Thread starter Papillon
- Start date
Welcome Papillon! Consistently running the engine at high speeds (say, over 3250-3500 RPMs) will shorten its life; it wasn't really designed for that. It is also not good to "lug" it; if you're going up a steep hill and your speed drops below 40 mph in 3rd, manually downshifting into 2nd is a good idea.
With all that said, the speed at which you'll hit those RPMs varies depending on whether you have the factory final drive ratio (3.07), widely acknowledged to be too low numerically, or a replacement such as 3.55 (many agree this is ideal) or 3.70 (if towing heavy loads frequently in the mountains). I have a 3.50 ratio, achieved by changing the chain drive sprocket between the engine and transmission, and I typically cruise at 65 mph which puts the RPMs at about 2800 IIRC.
Finally, as you go faster your MPG will drop significantly, and it's not very good to begin with. All the weight and frontal area of the coach means that at 55 mph you'll be lucky to hit 10 MPG. At 75 mph you might get more like 7 MPG (guessing). And if you're always 'putting your foot in it' (accelerating heavily) such that the massive secondaries are opening up on the Quadrajet carburetor, don't even ask about MPG!
Here's an article with more detailed info: http://gmcws.org/Tech/Final_Drive_Ratios.pdf
Every year we leave for the south between Christmas and the New Year. Wanting to get out of the cold as quick as possible, I hook up the towd, get on the interstate, set the cruise at 70mph (or what ever the speed limit is) and drive a couple of 12hr days. If you've done your homework relative to preventive maintenance, there is no reason why you shouldn't be able to drive the legal speed limit. But let's be reasonable about it. No need to keep up that pace driving through the mountains on long climbs. Once we get into the warmer weather, I tick the pace down a bit to bring the gas milage into more reasonable range. IMO it is also important to use fluids and oils that rank high in wear protection, especially if you are going to push it. So do your research and use the best you can find. Good oils/fluids/greases are cheap when compared to cost of break-downs and rebuilds. It is also important to be running tires that are no more than 6yrs old in good condition and set at the proper pressure. Speed increases heat and if not in good condition, you run the risk of failure. JMHO
OH....and welcome to the group.
OH....and welcome to the group.
With the stock 3.07 final drive which I have, I find the engine feels like its lugging bit at 50-55mph (which is the speed limit on many secondary highways in Canada). It starts to get into its torque range at 60mph and suddenly I find myself at 70+.
I drive at 55-65mph depending on the speed limit and road conditions. When I'm driving the Murray we are on vacation and its not a race. We like to shunpike and stop & smell the roses.
I drive at 55-65mph depending on the speed limit and road conditions. When I'm driving the Murray we are on vacation and its not a race. We like to shunpike and stop & smell the roses.
The GMC is a "hot rod" compared to other Class A motorhomes of its day, but it is not a actual hot rod. 0-60 time is leisurely, and interstate merges, particularly uphill, can be a bit nerve wracking.
I cruise at 70-75 on the interstate. I could go faster but don't feel the need to put the undue stress on the 43 year old engine and transmission. The faster you go the louder the wind noise gets and the lower the fuel economy goes. I have a 455, but I understand the 403 is smoother and and has longer legs at high RPM, so if your goal is long periods of 80 MPH driving you may want to get a later '77 or '78.
I have literally never heard a GMC owner say you can't drive over 60 so I am really curious where you are getting this bad info?
I cruise at 70-75 on the interstate. I could go faster but don't feel the need to put the undue stress on the 43 year old engine and transmission. The faster you go the louder the wind noise gets and the lower the fuel economy goes. I have a 455, but I understand the 403 is smoother and and has longer legs at high RPM, so if your goal is long periods of 80 MPH driving you may want to get a later '77 or '78.
I have literally never heard a GMC owner say you can't drive over 60 so I am really curious where you are getting this bad info?
Papillon,
You are clearly laboring under the weight of a preconceived misconception.
When first introduced, the published upper speed was 90+ (that memory is foggy). Remember that was nearly fifty years ago.
Data will confirm that it was probable. When we first went out with Chaumière, we were north west of Chicago and Mary was driving. She is a good driver and very sensitive to the machinery. I was not paying a lot of attention and had no cause for concern, but then I looked at the speed the GPS put on my screen. It said we were doing 83mph and slowing down. We were still in the left lane. At that point I suggested that maybe we could slow down. She remarked that everything felt just fine, and I told here that we just don't need to beat the horses like this. I did not yet have a lot of fuel rate data to work with, but the next fuel stop came up at just over 7MPG. When you consider that all the subsequent data gave a fuel rate at 9+ (9.2 was typical) and that speed was only maintained for a few miles, the incremental fuel used in that run must have been serious.
Most of these coaches seem to have a "sweat spot" in that 60~65 region. Where this actually is will be specific to that coach. I can assure you that it exists.
Bruce is very correct, the 3.07 was wrong when the coaches were made and it is still wrong.
To Bruce:
Thanks for the link about LiFePo4 and all that it entails. I am in the process of quoting an upgrade for an owner.
Matt
You are clearly laboring under the weight of a preconceived misconception.
When first introduced, the published upper speed was 90+ (that memory is foggy). Remember that was nearly fifty years ago.
Data will confirm that it was probable. When we first went out with Chaumière, we were north west of Chicago and Mary was driving. She is a good driver and very sensitive to the machinery. I was not paying a lot of attention and had no cause for concern, but then I looked at the speed the GPS put on my screen. It said we were doing 83mph and slowing down. We were still in the left lane. At that point I suggested that maybe we could slow down. She remarked that everything felt just fine, and I told here that we just don't need to beat the horses like this. I did not yet have a lot of fuel rate data to work with, but the next fuel stop came up at just over 7MPG. When you consider that all the subsequent data gave a fuel rate at 9+ (9.2 was typical) and that speed was only maintained for a few miles, the incremental fuel used in that run must have been serious.
Most of these coaches seem to have a "sweat spot" in that 60~65 region. Where this actually is will be specific to that coach. I can assure you that it exists.
Bruce is very correct, the 3.07 was wrong when the coaches were made and it is still wrong.
To Bruce:
Thanks for the link about LiFePo4 and all that it entails. I am in the process of quoting an upgrade for an owner.
Matt
I have to watch it or I'll look down and find out it's doing 75mph. I try to keep it around 65mph but it keeps creeping up, everything seems really happy at 75 except me! They just want to roll.
Compare that with my 31' class A Holiday Rambler and it was a fight to get it and keep it at 70mph. We just did 60mph everywhere and had to get used to the steady stream of traffic passing us on the left.
OK, I am a new guy here too... just got my coach last month and only driven it for 100 miles or so. Not new to GMC's. Owned a 77 Kingsley for 12 years in the 80's. This 75 Eleganza II has original engine and tranny and final drive. I have good reason to believe it has 55,000 or so actual miles on it. I live in Montana... very small town 100 miles from ANYWHERE. I am thinking I would like to upgrade the final drive but, at present have no idea where to turn to get that done.... Any advice? Thanks, Rod
If you were to order the FD from Applied, have it shipped to your local trusted mechanic. I don't think it requires any special mechanic skills. I think it's the type of work someone could do in their driveway.
Rodknu,
Welcome to the group, family, cult, asylum.....
You are far from the first repeat owner here. I know of several that had TZEs, went off with SOBs for a while and came back.
Do us and you both a favor before long, build a sigfile so we know who you really are, what the coach is and where without you having to repeat it everytime.
There are really more good reasons than that, but I am tired.
To get the plan about swapping final drives, the weight is a lot like a solid iron bowling ball. In spite of this singular fact, many of done the exchange in their driveway and with not much more than readily available jacks. Over at bird feeder, there are many in this crowd.
Matt
Welcome to the group, family, cult, asylum.....
You are far from the first repeat owner here. I know of several that had TZEs, went off with SOBs for a while and came back.
Do us and you both a favor before long, build a sigfile so we know who you really are, what the coach is and where without you having to repeat it everytime.
There are really more good reasons than that, but I am tired.
To get the plan about swapping final drives, the weight is a lot like a solid iron bowling ball. In spite of this singular fact, many of done the exchange in their driveway and with not much more than readily available jacks. Over at bird feeder, there are many in this crowd.
Matt
If it were me, I'd order the FD (with LSD) from Applied, have it delivered to my house. Then put the front end on jack stands and lay on my back and put it in. In about 25 years from now that might no longer be an option, I would probably call my local mechanic (or maybe my son! 
) and sit comfortably in a chair in the shade and 'direct' them putting it in.
As far as something like a custom motorhome repair facility specializing in replacing FDs in the GMC, there are a handful of those across the country. Coop, Applied, Cinnabar.... they might not be near you but you could drive to them if you wanted.
But that trip to them is not needed. Any small town or shade tree mechanic will recognize the FD replacement if they ever worked on a Cad El Dorado or Olds Toronado from the 70s . (NOT a chain like PepBoys or Goodwrench, find an old school mechanic or a shade tree mechanic) Any decent mechanic can follow the directions illustrated in the Maintenance Manual. They might even be provided with the FD from Applied.
) and sit comfortably in a chair in the shade and 'direct' them putting it in. As far as something like a custom motorhome repair facility specializing in replacing FDs in the GMC, there are a handful of those across the country. Coop, Applied, Cinnabar.... they might not be near you but you could drive to them if you wanted.
But that trip to them is not needed. Any small town or shade tree mechanic will recognize the FD replacement if they ever worked on a Cad El Dorado or Olds Toronado from the 70s . (NOT a chain like PepBoys or Goodwrench, find an old school mechanic or a shade tree mechanic) Any decent mechanic can follow the directions illustrated in the Maintenance Manual. They might even be provided with the FD from Applied.
Todd and Matt, thanks for the advice and I will look into exactly what you recommend. Sigfile updated....
Todd,
When I hear of someplace I don't know (as I grew up living on a boat on the east coast, this is not that hard to do), I like to learn where it is. So, I fire up Street Atlas and found Thompson Falls. Years ago a friend from Maine used to describe where he lived as being 50 miles from Nowhere. I think you have him beat.
I was curious, so I got Google Earth to give me a look at the area. Not much going on there. There is the power pole factory, but not much else that could help you maintain the coach.
Too bad Lame Massoula is gone. Up until 12000 years ago, that would have been a great attraction to the area.
If you put your name on the GMC Assist (aka Black's) list, you are never going to get called because there will always be someone that is closer.
When I hear of someplace I don't know (as I grew up living on a boat on the east coast, this is not that hard to do), I like to learn where it is. So, I fire up Street Atlas and found Thompson Falls. Years ago a friend from Maine used to describe where he lived as being 50 miles from Nowhere. I think you have him beat.
I was curious, so I got Google Earth to give me a look at the area. Not much going on there. There is the power pole factory, but not much else that could help you maintain the coach.
Too bad Lame Massoula is gone. Up until 12000 years ago, that would have been a great attraction to the area.
If you put your name on the GMC Assist (aka Black's) list, you are never going to get called because there will always be someone that is closer.
Matt, you are right... not much here... official census 1,300 with a thousand or two in outlying area. We do have some shade tree mechanics. I am a retired Navy Fighter Pilot but after retirement lived full time and cruised on a 40' Passport with wife and young daughter for 7.5 years mostly in Sea of Cortez and Puerto Vallarta, MX
So Rod,
After ricocheting around the sky at M1+ and landing on a postage stamp, you still survived years at 7~9 knots???
Navy wanted me to fly for them and I thought I might until I got a ride in the back of an F4. I had 40+ hours in my student book and landing without being able to see the deck at a sink rate that is faster than a Cessna can dive was just not for me. As a lifelong waterman, I kind of like being closer to it then you often were.
Your case, you might try to locate some one a state or two away and just plan to take the coach there and have a good long punch list.
I do hope we get to meet some day.
Matt
After ricocheting around the sky at M1+ and landing on a postage stamp, you still survived years at 7~9 knots???
Navy wanted me to fly for them and I thought I might until I got a ride in the back of an F4. I had 40+ hours in my student book and landing without being able to see the deck at a sink rate that is faster than a Cessna can dive was just not for me. As a lifelong waterman, I kind of like being closer to it then you often were.
Your case, you might try to locate some one a state or two away and just plan to take the coach there and have a good long punch list.
I do hope we get to meet some day.
Matt
As far as the capabilities of the drivetrain goes, I "cruise" at 62-65, seems comfortable there. I also don't try to pull any "wheelies" when starting from a dead stop. A lot to ask the only drive wheel / axle doing all the work. I believe I have the 3:55 already in when I bought it. On my ole Palm Beach I had a 3:42 (IIRC) that Cinnabar introduced. (yes, I was in the family way back when, then left the family to explore old GM buses, and then came back). My Samurai dinnghy, when I am driving it, never achieves the speeds it reaches (periodically) behind the EII.
In the 30+ years I have owned my coach (78 Royale, 403, 3.07) I have driven it about 100K. I routinely cruise 72-75 MPH on the flatter roads and stay right around 3000 RPM. I'm not in a rush, the entire coach just feels better at that speed. I let the speed drop on hills of course and seldom open the secondaries. I have an auxiliary down shift switch so I keep the trans pressures up at lower RPM. I don't track extremely closely but I average somewhere in the 7-8 mpg range. This is just my experience, your results may vary. Remember, I'm also the guy who cared for and maintained the power train for most of it's life.
If you have a tach and a GPS you can figure it from that.
There's a table of speed vs rpm here: https://www.gmcrvparts.com/product-p/gm1-1.001.1.htm
It's the 2nd photo
There's a table of speed vs rpm here: https://www.gmcrvparts.com/product-p/gm1-1.001.1.htm
It's the 2nd photo
