how much mpg increase if I add overdrive transmission?

[Eric F]

The guy who pulled the 454 engine on my 1976 argosy MH tells me that its TH400 tranny is in bad shape. He's suggesting that I either rebuild/replace the TH400, or upgrade to a TH700 (overdrive + cable shift) for $700 more.

Have others done this? How did it work out? What sort of mileage improvement did you see?

Thanks,
Eric

[Waipio Rim]

We have one coach with the stock th475 non-od transmission and one with the same trans but an added Gear Vendors overdrive unit. Both get about the same mileage, but the overdrive is great to have. Just being able to keep up with traffic when I have to be on a freeway makes it worth it. The non-overdrive coach is a traffic hazard on a freeway.

[2012FB]

I personally would not change the original gearing on a motor home. Go back just the way it was engineered. If you are putting in a new HD diesel then I would still run some calculations before changing.

[Can of beans]

I wouldn't think a 700r4 would be a good idea. That's the forebear of the 4l60(e) box of neutrals.

a more modern 4l80E would be better with a standalone controller

[Oreg52]

Agree with the above. The 700 and 4L60 are derived from the TH350, good, but light duty transmissions. I would stay with the TH400/475. If you want that extra 2-3 MPG of the OD, insist on the 400's overdrive descendants, the electronic 4L80e/6L80e which would need a separate controller. And if you go that route, go with the 6L80e.

But the money you save by rebuilding the 400 will buy a lot of gas.

[Icy]

I am a retired GM mechanic, do yourself a favour and stay with the 400 transmission.
Pete

[hairyclive]

>The guy who pulled the 454 engine on my 1976 argosy MH tells me that its >TH400 tranny is in bad shape. He's suggesting that I either rebuild/replace t>he TH400, or upgrade to a TH700 (overdrive + cable shift) for $700 more.

>Have others done this? How did it work out? What sort of mileage >improvement did you see?

https://www.motortrend.com/how-to/11...-transmission/

The 4L80e or 4L85e is more reliable, You will have to install a speed sensor and control box, and alter the driveshaft length and trans mounts to use one in your application.
The 6L80e has a lower low and 2 overdrive gears.

https://www.holley.com/blog/post/eve...transmissions/

https://gearstar.com/overview-4l85e-transmission/

There are other articles on some of the hotrod, 4x4 and other sites regarding installing electronic transmissions and controllers in older vehicles.

The other option is installing a 2 speed rear or a separate HD overdrive unit like a Hone-O-Drive or similar

https://www.irv2.com/forums/f115/hel...le-110895.html

https://www.rv.net/forum/index.cfm/f...print/true.cfm

https://www.youtube.com/watch?v=mEhgAIVEWX4

Don't try to go with too low a gear that you lug the engine. It will take a certain amount of power to cruise the vehicle at a certain speed. Try to estimate from the torque curve of the engine where the best RPM range for regular cruising is and adjust the gear ratios to match the overdrive ratio. Then downshift when required for heavy loads or climbing.

https://www.engineeringtoolbox.com/c...ue-d_1784.html

https://www.diyelectriccar.com/threa...-i-need.15508/

https://www.researchgate.net/publica...EL_CONSUMPTION

https://www.mdpi.com/1996-1073/9/8/575/pdf

[Scott S]

Keep it original, less flashy, but generally better reliability.

[n2916s]

It takes “x” btu’s to move your rig down the road. All of the btu’s available are in the fuel so, while a different transmission or final gearing may be a smidgeon more efficient, fuel consumption will be pretty much the same in the end.

[martin300662]

It depends on the weight you are dragging around, and the type of roads you travel. We use Bella (a 20ft, weighed at 7000lbs) mainly on Interstates/motorways, and travel at around 70mph, so get 25% better gas with the GearVendors, just because 70mph without it was rev'ing the guts out of the non-OD box.

I would do the same swap in a heart-beat, unless I was changing to TBI and then I would use a 4L85E as the aftermarket controllers drive both TBI and transmission.

[SYC2Vette]

As mentioned the TH400 is stronger than the 700. The 700 was used mostly in 1/2 tons, and a few "light duty" 3/4 ton. For that reason I personally would not change it.

That said, the 700R4/4L60e can be modified to handle more power, but I don't think I have ever seen one behind a 454 V8, that engine is almost always combined with the TH400 & 4L80e

[Life is a Highway]

agree stay with the 400. built right it’s a good transmission. Jasper transmission has figured out the unit an has a good warranty program.

Originally Posted by Icy

I am a retired GM mechanic, do yourself a favour and stay with the 400 transmission.
Pete

[Eric F]

Thanks all for your advice. I'm gonna stick with a th400.

[tjdonahoe]

Originally Posted by Eric F

Thanks all for your advice. I'm gonna stick with a th400.

…yes….you pull in overdrive ..more transmission problems..

[martin300662]

Originally Posted by tjdonahoe

…yes….you pull in overdrive ..more transmission problems..

Don't remember anyone talking about 'pulling'. The OP as a 24ft Argosy motorhome!

[martin300662]

In a 24ft your would have a TH475 (only a few early 20fts had TH400), so straight cut teeth. Make sure your mechanic knows the difference.

[PeterH-350LE]

The answer to your questions depends on your intended use. If you are planning on doing a lot of mountain driving, up down 10k passes, the TH475 is the better choice. If you plan on going cross country using the interstate system, you will be a road hazard @55. Any speed above that will turn your rig into a a roaring beast running at 3200rpm or above.
The 4l80E is a fantastic transmission, but does require throttle position, by either tapping into the existing TPS of a fuel injection system or an aftermarket cable mounted sensor, which in most cases surrenders your cruise control useless.
Other than that, some rear mounting bracket, driveshaft length, shift gear linkage and trans line changes will be needed.

I personally would always upgrade to a 4l80e along with a throttle body injection system.

[Icy]

Have work and driven mid 70’s engine’s, I don’t think 1976 454 has enough power to utilize an overdrive transmission like an 4L80e. If you were to drop in a GM 454 crate engine with fuel injection and 4L80e you would be golden. Then you would have the PCM to operate the electronic controlled transmission and engine functions. Also an large increase of power and fuel economy. 454 engine in 1976 was only about 230hp and a little of 300 ft/lb of torque. Most modern day V6 engines done better than that today.
Pete

[PeterH-350LE]

Originally Posted by Icy

Have work and driven mid 70’s engine’s, I don’t think 1976 454 has enough power to utilize an overdrive transmission like an 4L80e. If you were to drop in a GM 454 crate engine with fuel injection and 4L80e you would be golden. Then you would have the PCM to operate the electronic controlled transmission and engine functions. Also an large increase of power and fuel economy. 454 engine in 1976 was only about 230hp and a little of 300 ft/lb of torque. Most modern day V6 engines done better than that today.
Pete

Always open to good info, since i was under the impression that 70's 454 was used all the way up to 1996. I do not think my 1992 350LE/4L80E has more than 230 HP



>>>>>Chevrolet developed three 454-cubic-inch engines for the 1970 model year: the LS5, the LS6 and the LS7, though the last one was not produced for the public in any vehicles. These engines were used in trucks from 1975 until 1996.<<<<<

[PeterH-350LE]

Real life data: Stock 1983 454, carbureted with 4l80e and standalone controller.