Hello everybody!
Last week, my girlfriend and I bought a 1979 E-150 camper. It's a 3 speed automatic with a 351, raised roof. It's well equipped and we are very happy about it!

I was not expecting great gas mileage but I was also not expecting such bad mileage: 12mpg going at 55mph on the highway (although part of the trip was city). There are a few things I will do make it less worst, but I'm seeking your opinion on one specific aspect.

The tires are overdue. The front ones are P235/75R15, cracked and worn out. The rear ones are LT235/75R15, with an aggressive tread more suited to mud than highway. I suppose 235/75R15 is what I should get to replace all of them.

Questions: Does the rear ones have to be LT or P is fine? I would say P is fine and assume that the previous owner put the LT one to go in the woods (which he was doing) Any models that will be specifically good on mileage than the other?

Without changing the tire height, can I get a narrower tire for less friction? If so, what would be the tire specifications?

I know that 75 on my tire is the height relative to the width, but which way does it go? For exemple is P235/75R15 higher than P235/85R15 or is it the other way around?

This truck is a 3 speed, at 55mph, it revs like hell (3000 rmp maybe?). 99% of the mileage will be highway. Is this a case where slowing the engine down with a larger tire diameter will help mileage? If not, what about changing gears? Or is it possible to put an AOD in there? I would say that at 55mph the 351 is not exactly fighting to keep up, so it could rev slower easily...

I know this is a lot of questions, but you get the idea! I will be searching in the forum for other dos and donts for better mileage, but let yourself go if you have any ideas!

Thanks!

 

I would use no less than the LT235/75 or 85 R15. P rated tires are not stiff enough for good stability and for the load of a camper. The 85 is slightly taller than the 75. You need to make sure the tire is rated for the weight of the rig.

12 mpg sounds about right. The 351 never has been a high mpg motor. An OD transmission would possibly give you a little more mileage but the cost of changing it will buy a lot of gas. Keeping the motor in good tune and keeping the filters clean might help mileage. A light foot on the gas will get you the most improvement in gas mileage.

 

> I suppose 235/75R15 is what I should get to replace all of them.

You should run the LT235/75R15.

An upgrade would be the LT 31x10.50r15. Slightly taller and slightly wider. You are basically driving a shed down the highway, 12 mpg is about right for the 5.8/C-6.

 

If you decide to go with the AOD make sure it's the pre electronic one they're around and if you are going to be driving mostly highway the AOD tranny might be cheaper than replacing the engine from running to hard because you will need to go faster than 55 or you'll get ran over. I have the AOD behind the 4.9 and it makes a big difference in the RPM's and gas mileage.

 

Thank you everyone for your informations!

I'm aware that some investment could buy a lot of gas, but I'm doing it for ride comfort (noise) and environmental considerations.

Your comment is encouraging!
An additional question: I know it's not *that* complicated to swap the transmission to an AOD, but I wonder what are the challenges when it comes to the steering column mounted speed selector? Obviously something has to be done there....

So, I will be looking for LT235/75(85)R15, as recommended. It will certainly get me a tad more mileage considering the carp-o-tires that are currently installed!

I'll give some feedback when things move!

 

The AOD has the same three posotion selector as the C6, to put it in overdrive place the selector in "D" and it will shift 1-2-3-4 and if you don't need overdrive leave it in second and it will still shift 1-2-3- just like the C-6. Keep it in 2 for slow or town driving or it will shift in and out of overdrive which will harm it after awhile. When you get the AOD make sure you get all the linkage and grommets from the donor. Check the spline on the drive lines to make sure they match. Have fun

 

12 mpg for a camper seems pretty good to me. I only get 10.5 mpg in my E350 work van. It has just the standard fullsize body. It has the 5.4 litter motor and it has AOD. I do weigh about 7000 lbs on average with all the tools I have in it.

 

fwiw: Tires I usually see that are 85r are LT235/85r -16-

 

The van most likely came with P235/75R15. Upgrading to an LT235/75R15-C 6PLY would give more stability and handling. 235 is the width in MM. 75 is the height in % and 15 is the wheel in Inches... No such thing as a 235/85r15. and a "31-10.50r15 is much bigger/wider.And more expensive. And usually a Mud tire. I run, "Toyo Open Country HT"
Les Schwab Tire Centers - Your Performance Headquarters For Tires and Wheels

 

If you want another option on the OD, you might consider the Gear Vendors overdrive unit. It would work with the C-6 and not require any of the headaches associated with adapting another transmission in. They are pricey, but are durable and can handle the application.

Gear Vendors under/overdrive, Application Guide 2 Wheel Drive 3-Speed Automatic Models.

By the way, 12 mpg is great! My Dodge motorhome with a similar setup only got 10 at best. It too would pull that many rpms at highway speed.

 

A quick update on this, I wanted to say thanks to all posters.

I ended staying with the original LT tire size and changed all 4 tires. The ride got stiffer, but that's better because they were P rated tires at the front. It' a little quieter too. I have to change all 4 shocks and then I should not feel like the road is made of jello anymore

So far I couldn't notice a change in millage. I have a friend who is a mechanic that is willing to help me convert the camper to manual 5 speed, this should help more and it is a challenging project! It will come alive in many months as winter is coming here... It's already too cold to work on it outside and I have to think about storage...

Thanks all for your ideas and comments!

 

I don't think you will ever get a payback for what it will cost to put a manual trans in that van. It is huge with a raised roof. I have had 4 big vans including a couple Turtle Top conversions over the years and the older ones all got 12 mpg. And that was only a 4" tall roof when folded, and half ton vans with smaller engines (a Ford 302 and Chevy 305). In fact, the GMC van with the 305 had a 2.73 gear from the factory and it sucked SO bad that I installed a 3.42 and a posi and the mileage went UP. This was a Turbo 350 transmission but with a lockup converter.

A manual conversion might be able to get it up to, say, 12.2 mpg in my opinion. Like I said, it will never give you a return for the cost and hassle of conversion. A manual trans with an OD gear would tend to lug when you hit headwinds and hills. I have had some overgeared trucks in my time and they are no fun. I'm guessing your transmission/gear setup is not that far off the mark for efficiency, with maybe a lockup torque converter being the only thing lacking. If you could convert to a newer 4 speed OD automatic, I would say that would work much better than a stick.

I'd keep it as is. Good luck,

George

 

I'm with George on this one.

It sounds cool to have a manual in your camper, but just keep in mind all the extra work you'll have to drive your van - traffic can kill you and if you have to start on a hill.....

I'm not against the idea, but you need to think it through completely before doing it. Consider the impact on the resale value in the future. There's a lot of potential buyers out there that will walk away from a camper van with a manual because fewer people out there know how to drive a manual properly. The AOD on the other hand will attract buyers because of its more modern design.

In the end, its your decision. I've made mods that sounded cool and worked well for me, but killed the resale value of the vehicle. No one plans on selling their vehicle, but eventually it happens.

 

I understand your point but this is something that specific to the USA. Here in Canada, and even more in the province of Québec, most people know how to drive a manual transmission.

Back to the idea of switching to a 5 speed manual, I am getting totally opposite commentaries. I am still debating. Spring is comming and I will soon be able to proceed with a change, whatever it is.

Thanks

 

Good luck finding a clutch/brake pedal assembly for that van.

Unless you fab one up.