What is overdrive on a truck?

Yep. Early Chevys had the Powerglide, then came the Turbo 350/400, then the 700R4, then the 4L60 and 4L80. Best I can recollect, anyway.

 

Okay if you pulling reefer 53 loads around 35k.lb or may sometimes more
The truck Cascadia DD15 Detroit
Ratio ..3.42
Transmission DT12 auto
Overdrive /No
HP / 455
It's okay to go with this without overdrive option or you think should I go with one has overdrive option available ?

 

Back to basics, overdrive on a 13 or 18 roadranger means the splitter is direct in low, over in high, that's your RTO, The RTOO (maybe more letters depending on other stuff but whatever) is overdrive on the transmission and the splitter, double over. You can recognise it by the shift pattern which is an inverted u rather than an H. Bull low, right and back, 2, middle forward, 3, middle back,4, left back 5 left forward. Over the years manufacturers have tried different ways to get speed and economy. Really the only thing that matters is whether your engine is running in its "sweet spot", most efficient RPM at the cruise speed you want. One final note you are not going to get the advertised MPG at 75 mph. The physics of wind resistance mean that you get a steep increase in wind resistance over 60 MPH. auto trans makes no difference except it adds another level of complexity, if you do much city driving you will love it though

 

i believe the 200r was before the 700r. nonetheless, the 200/700/4L60(e) are not known for their strength in stock form. with that being said i cut 1.5x 60' on a stock 4L60e in a 3500lb car for a couple years. and a lot of street abuse. roughly 200 short times under 1.65 and 35k street miles. granted it never did want to shift 2-3 without making not so sweet love to the rev limiter.

now they have 6 and 8L90 trans. monsters. very durable and are geared to move anything.