4L80E vs. manual trans - considering a change

[rcrahn]

With recent talk of gas going to $5 per gallon I'm beginning to consider swapping the 4L80E (mated now to 6.0 LQ9) to a manual transmission in our project, a C20 with 14 bolt 3.73 posi rear.

Some questions I have.....
1. Theoretically speaking, how much better gas mileage will be gained by switching to a 5 or 6 speed manual trans?
2. I do know the very cool 67 Holley shop truck used a Tremec trans. What other trans can be used? Note: I'll keep the LQ9 stock and use the truck for towing occasionally, so I want something stout.

Thanks for any feedback

[tonypaul]

I have a 4l80e and 3.73s in my truck and it seem to be the perfect combo for me. I, myself couldnt see that it would be worth it to me to change just for the cost of gas. The expense of another trans, clutch, pressure/plate, ect, drive shaft work,ect... It just seems alot of investment when I could adjust my driving habits to get more miles per gallon.

[jocko]

concur with previous post - if you want a 5-speed for the joy of rowing gears, go for it, but it's probably not going to save you a lot of extra dough at the pump. O/D trans make their fuel-saving money in the o/d final drive ratio. 4L80E final ratio is approx .75:1 whereas Tremecs are available in .64,.68 and .82:1 versions (at different hp ratings).

Note that the NEXT to final gear in a 4-speed automatic and a 5-speed manual are still 1:1 - so the fuel economy associated with both transmissions comes from the final gear ration only.

Anyway, the advantage of a 5 (or 6) speed over a 4-speed (automatic or manual) is in keeping the engine in a tighter/more optimized rpm band (for a perf engine with a limited/peaky torque/hp curve - i.e the same reason 4-speeds became more popular than 3-speeds in the 60's/70's - note that 3 and 4-speeds usually have a final ratio of 1:1, so there really wasn't much gas mileage advantage in a 4-speed over a 3-speed).

Bottom line, your 4L80E should get about the same mileage as a 5-speed. The extra gear is spent below the O/D ratio gear in the 5-speed, so it really doesn't affect fuel economy that much by having it. Both an O/D automatic and a manual O/D trans provide fuel economy improvements over an older trans (auto or manual) with a 1:1 final ratio, but the 5-speed isn't much of an improvement over the 4L80E.

One note - you MIGHT see some MINOR gains in fuel economy if you were to go from the 4L80E to a 5-speed with a final ratio in the .64-.68 range, but probably not worth the heavy investment to do so.

My 2 cents. For the record, I'm switching from a stock 3-speed to a 5-speed in my 66 - and I chose the 5-speed over a 700R4/2004R/4L80E, etc solely because I like to row the gears. All are good fuel economy transmissions.

[jocko]

Sorry - forgot about question 2:

The Holley truck used a Tremec - and they are offered in the final drive ratios mentioned above and also in 6-speed versions. Other options are the T-5 trans that came in mustangs and camaros. Early T-5s were "non-world class" and I'd personally avoid them since they are limited in the amount of hp they can withstand. A "World Class" T-5 can handle in the neighborhood of 350-ish, but don't quote me. If you have a 6.0, I'd recommend the Tremec vice the T-5, but depends how much you want to stomp on it. I'm considering a T-5 but it's to be mated to a mostly stock 66 283, so it should handle the torque easily.

Anyhoo, there you go. Good luck whichever way you decide to go.

[kSStamp0182]

the 4L60E in my K1500 turns about the same RPM's on the hwy as my 6 speed did in my camaro. i would think it would take a LONG time to offset the costs.

[rcrahn]

Appreciate the replies guys.....Good input
Also need to factor living in the highly developed bay area and traffic, which is mostly stop and go. I'll stick with the 4L80E.