With an older engine would it be better to detune an engine for better MPG rather than to improve the specs?

The gas engines used in RV service were already "detuned" truck versions, your best mileage will be obtained with it in the best original condition possible, mileage will be terrible no matter how much money you waste on it.

Rule of nature.. more power = more gas.

If you're just looking for some extra to pull the hills or drag a toad, then the extra power would be worth the consumption when needed. The rest of the time it might not pay off.

As Madrone said you're probably best to go for well tuned stock rig, driven carefully. I think speed and driver habits will have the greatest effects on mileage.

A 10% improvement is less than 1 mpg better anyway! (Look ma! We went from 6.5 mpg to 7.2 wow!)

On a 500 mile trip to and from Tennessee to Georgia, towing a '96 Cavalier on a tow dolly, we got 8.8 mpg on the 85 Regal, 454. Driving speed was 60/65 mph. Everything set up to standard factory specs. The kicker? Nology hot wires and Beru plugs installed by the previous owner 10000 miles ago. The other kicker, Gear Vendors over/underdrive. I'm guessing rpms were around 2500 @ 60mph. No tach.

quote:

As Madrone said you're probably best to go for well tuned stock rig, driven carefully. I think speed and driver habits will have the greatest effects on mileage.

A 10% improvement is less than 1 mpg better anyway! (Look ma! We went from 6.5 mpg to 7.2 wow!)

Absolutely, driving habits and speed will have the greatest effect on MPG.

I am en-route to Texas, to do the Texas Mile again. this time I have made an extreme effort to keep the speed at 60 MPH. Very difficult as my Breakaway wants to run 70+. This is the first time that I have had back to back fills that were over 11 MPG. I have averaged 10.2 MPG for 110,000 miles. Keeping the speed at 60 MPH, with occasional bursts to pass some very slow trucks, easy acceleration, has really improved the MPG.

What are the savings? I will save $150 this trip (4000 miles $4.05/gal average) and the driving time will be extended by 9 hours. Probably a lot less wear and tear on the drive train also. RPM at 60 MPH is about 1750 +/-, @ 70 MPH 2000 +/-

My last fill from, El Paso to Ft. Stockton TX I got 12.8 MPG, best I have ever done. Yes going down hill slightly on this leg and I had just a little bit of a tail wind.

NOW, if I can just continue this effort for the rest of the trip.

I can only dream of 8+ mpg.

My heavy 33 ft tag axle rig runs about 3000 rpm at 60 mph. (@ about 7.2 mpg, I got the 10% improvement already when I changed the distributor and played with timing)

The Gear Vendor OD sounds really interesting. It would give me about 2500 rpm, but I'm not sure the old 454 has enough torque at this rpm. It would be a cool experiment... but then I would want Bill h's torque monster 502... maybe a blower..

I have a coach similiar to Steve's. Mine is a 35ft Royale (Barth calls it a 35 foot but it's 34 bumper to bumper) on a P30 chassis with 454. It weighs 18,900 lbs with 2 people on board. Probably close to 20,000 with family and supplies. I also have a GearVendor. I had a 20 yr old, aftermarket, orphan fuel injection system that tanked on our trip to south Texas. I ended up removing the FI system and converting to an Edelbrock carb system while sitting in an RV park...took me 5 days to figure out the problems..thank goodness for the internet. Before the conversion I got 7.0 mpg running the genny for the A/C. After the conversion, I got 7.5 mpg with the Edelbrock while running the genny and A/C. It's so hot here, that we were running the genny from 8am until we parked and hooked up to shore power.

I wondered what the final chapter was on that fuel injection system! Sounds like you got it sorted out well. Glad you made it home OK.

So you're seeing 7.5 with the carb setup and Gear Vendor, too. What rpm do you see at 60 mph?

That's better than I see, the gear vendor must be helping. If I could get one cheap enough the payback might be acceptable.

It's hard to tell....I messed up the tach trying to tighten up one of the terminals. I think I remember it was around 2500 before I messed it up. The 7.5 is with running the genny....so it's slightly better than that. The nice thing about the GearVendor is how quiet it makes the cab area.

I was so whipped after that episode, it's taken a while to recover!