What motor has the best mileage?

vpearce said:
Does the 455 have the best milage?
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Not much difference, but if anything the 403 has a slight advantage. Things like proper tire pressure, engine maintenance, and an upgraded final drive (to 3.55 or 3.70 vs the factory 3.07) make as much if not more of a difference. The final drive factor is counterintuitive, but with the factory 3.07 gears a lot of time is spent with the accelerator floored which yields low vacuum and therefore poor mileage.

A good target is 8-9 MPG, maybe +1 with the 23' vs the 26'. Some have installed fuel injection which can make MPG a little better but is more for improved starting and more consistent operating performance across a range of conditions.
kidjedi said:
Between the stock 403 and 455, it depends who you ask.

There have been successful diesel swaps that are the most significant increase in mileage.
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However, the conversion will never pay for itself in gas savings.
 
Christo said:
Not much difference, but if anything the 403 has a slight advantage. Things like proper tire pressure, engine maintenance, and an upgraded final drive (to 3.55 or 3.70 vs the factory 3.07) make as much if not more of a difference. The final drive factor is counterintuitive, but with the factory 3.07 gears a lot of time is spent with the accelerator floored which yields low vacuum and therefore poor mileage.

A good target is 8-9 MPG, maybe +1 with the 23' vs the 26'. Some have installed fuel injection which can make MPG a little better but is more for improved starting and more consistent operating performance across a range of conditions.

However, the conversion will never pay for itself in gas savings.
Click to expand...

If you want to track your mileage, try Fuelly. It makes it easy.

I don't even use the App


When you fill up the odometer reading, gallons added are texted to their number and it adds your data to your record. Super easy.

Using this, my honest mpg is 8.5 according to Fuelly, after factoring in a slight correction for the odometer being off slightly, the true honest mpg is 8.8.

This is over 28 fill-ups and 7612 miles.

There's so much variability just in the fill-ups, you really can't get a good mpg reading without several complete fill-ups (not just 10-20 gallon short fills). You need several good sized 30-45 gallon fill ups to get an actual mpg number worth anything.

The confidence limits on a single data point are +/- infinity; in other words: worthless.

I'd love to get up into the teens, the only way that seems to be possible is with a 6.5l diesel. I happen to daily drive one (1997 K1500 suburban) and have repaired it and maintained it for the last 15 years or so. It has 362,000 miles on it currently. If I were to do the conversion and then eventually retire the way my wife and I are considering, it actually does make financial sense to do the conversion. There's a break-even point and it's really not beyond reach especially as fuel costs rise!
 
Not just to pitch my own show, but I had data scatter out the bazoo until I did the High T bit. It was so simple and it only took a year and a 20K$us software package to figure that out. (The software was not actually pirated as I was in the employ of the license holder for a very short time.)
If you missed this, all you have to do is take the fill vent line T out of where it is near the T in the fill pipe, and move it to under the cab floor.

Matt
 
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With my '76 455 Birchaven, was pushing around 58 and got 7.3 MPG - damn! Have to drive slower. Are there any other motor options?
Thank you all for your info. So far, here in OR we haven't seen $5 per gallon yet!
 
Matt Colie said:
Not just to pitch my own show, but I had data scatter out the bazoo until I did the High T bit. It was so simple and it only took a year and a 20K$us software package to figure that out. (The software was not actually pirated as I was in the employ of the license holder for a very short time.)
If you missed this, all you have to do is take the fill vent line T out of where it is near the T in the fill pipe, and move it to under the cab floor.

Matt
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Can you doodle a picture of what you mean by moving the T?
 
The EPA never did MPG testing on the GMC, but they did test the Toronado so you can see that the in the same car the 403 got a whopping 1 MPG better than the 455.

1975 Toronado 455 V8 11MPG
1976 Toronado 455 V8 12MPG
1977 Toronado 403 V8 13MPG
1977 Toronado 403 V8 13MPG
 
JSanford said:
The EPA never did MPG testing on the GMC, but they did test the Toronado so you can see that the in the same car the 403 got a whopping 1 MPG better than the 455.

1975 Toronado 455 V8 11MPG
1976 Toronado 455 V8 12MPG
1977 Toronado 403 V8 13MPG
1977 Toronado 403 V8 13MPG
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Thank you so much. I drove a friend's 1975 Toronado - just didn't remember the MPG, just filled when needed at a ridiculous cost!
 
Lower you right rear as much as possible with some lowerig on the left then slow fill. no burbs !!!455 for me the mileage is an insignificant factor.. Larry 78/455 XPB Pearl
 
tmsnyder said:
Makes sense! The rear one must fill first then puke into the vent tube limiting the air flow out of the front tank.
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That is exactly what happens. Almost without regard to the coach's attitude during fueling, the arrangement of the basic fuel fill means that the rear tank will fill first and then the foam means you can't fill the auxiliary tank. Even if you wait out the foam in the main, the hydraulic head that gets to the main tank can still push fuel to the vent to shut off the auxiliary tank. With this mod I can fuel at the top hook until nearly full. I used to schedule fuel stops as 45 minutes, after this mod I have been able to take on near 50 gallons in less than 10 minutes. It's a good thing we co-drive or that would not be enough of a break.
Matt
 
Applied GMC
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