79 K30 Mileage & Transfer case question?

[72 GMC K1500]

OK guys,
I am looking at a 79 K30, 454/400, SRW, Full-Time four wheel drive crew cab with about 45,000 miles on it. My question is what type of mileage do you think I should expect? 10 MPG? Second, any suggestions on how to test to see if the transfer case is working in the "lock" mode? I am also not really sure what the lock mode means. What is the difference between when I am driving arounf in regular 4HI and 4Hi locked? Any help would be appreciated.

[abebummy26]

8 if if you are lucky

[Cyclone]

I put the mile marker kit in my NP203, still only got 10mpg downhill. What abe said, you are looking at an 8-10 mpg truck. NP203 info

[72 GMC K1500]

Wow, I am shocked it is so low. Can either of you guys tell me what the difference is when the transfer case is in lock mode?

If you had to do it again would you put the mile marker kit in?

[scotto0609]

I think you should pass on it and give me the info!!!!! My commute is really short!!!

[79Silv4x4]

I could very well be wrong but I thought the K-30 came with a NP205 transfer case (which is a a part time 4wd unit). Maybe that was just the manual tranny versions.

Anyways, if it were me, I would set my mpg expectations at 8-12. . . .

just my opinion/experience

[72 GMC K1500]

This one definately has a full-time transfer case. Believe those are the 203s. Do you know what happens in 4 HI Lock versus 4 HI?

[72lb4x4]

Lock is for 4-wheeling, not driving on the road if I remember correctly. Since it seems to be full time, its only got high, high lock, and low lock right?

[K30_dump]

Originally Posted by 79Silv4x4

I could very well be wrong but I thought the K-30 came with a NP205 transfer case (which is a a part time 4wd unit). Maybe that was just the manual tranny versions.

Anyways, if it were me, I would set my mpg expectations at 8-12. . . .

just my opinion/experience

Absolutely right. If that is indeed a true K30, then that 203 was swapped in at some point. They only came with the 205, reguardless of the transmission. As far as what happens in the different ranges, the NP203 has a differential inside of the rear section. In full time high or low ranges, the differential will shift power to the front or rear, much like a differential in a rear axle allows one wheel to spin faster when turning. In hi or low lock, the front and rear are always 50/50 split, like with the NP205. The 203 is not a bad case, but I would recommend a Milemarker kit and a set of locking hubs (as the others have suggested). I agree on 10 mpg or so, but it also depends slightly on the gearing and how you drive. Overall, a genuine K30 is always a good purchase...

Good luck,

Joe

[Dooley]

Originally Posted by K30_dump

Absolutely right. If that is indeed a true K30, then that 203 was swapped in at some point. They only came with the 205, reguardless of the transmission. As far as what happens in the different ranges, the NP203 has a differential inside of the rear section. In full time high or low ranges, the differential will shift power to the front or rear, much like a differential in a rear axle allows one wheel to spin faster when turning. In hi or low lock, the front and rear are always 50/50 split, like with the NP205. The 203 is not a bad case, but I would recommend a Milemarker kit and a set of locking hubs (as the others have suggested). I agree on 10 mpg or so, but it also depends slightly on the gearing and how you drive. Overall, a genuine K30 is always a good purchase...

Good luck,

Joe

Wrong! The 1979 K30's with auto transmission were equipped with the full time 203 transfer case. They did not come with a 454 as the 400 smallblock was the biggest engine available in the 4x4x's from GM through the 1980 model year. The 454's became available in K30 series in 1982. I bought a new 79 K30 dually. I mounted Husky Selectro hubs and installed a SAE Engineering part time kit purchased from Midwest 4x4 in mine. The 205's came with them starting in 1980. You would have to accept 7-10 mpg if you want it.

[72 GMC K1500]

Joe,
I am not an expert. But this truck was 100% stock, and I find it hard to imagine that the P.O. would go to the trouble to switch hubs, transfer case etc. I would have to image that some 79 K30 Crew Cabs SRW 454/400 models must have had another option. I also read that with the milemarker kits you have to put it in 4wd every 200 miles or so to allow for proper lubrication of the bearings in the case. Have you heard the same?

[Dooley]

Originally Posted by 72 GMC K1500

This one definately has a full-time transfer case. Believe those are the 203s. Do you know what happens in 4 HI Lock versus 4 HI?

The 203 units have a planetary arrangement in the case that allows for the front output and rear output to spin at slightly different speeds. This prevents torque windup in the driveline caused by the front wheels which often would turn faster/further than the rear due to cornering. When you pull the lever to Lock mode it engages the planetary front side and keeps the outputs spinning the same speed. Back in the 70's when most of the full time units got part timed we would put a pipe nipple with an ell (pointed up) into the case oil fill hole. This would allow some over-filling of the case with straight weight 30 motor oil which allowed splash lubrication. With free wheeling lockouts installed and the lever in high, not locked, the gears/chain for the front output did not turn. I never had a problem with any that I converted. Part way through the 78 model year the chains and gears were narrowed a half inch. The teeth pitch was increased which negated slippage after some chain stretch and were more reliable than the earlier units. No 454 engines in 1979 GM K series trucks. I would have ordered one if it had been available! I did replace my 400 SB with a 454 also.