The Stovebolt Site Forums The Parking Lot Big Bolts REAREND QUESTION

I've got a 6.14 ratio single speed rear end in my truck (ex firetruck 13,000 miles) now, and am looking at a another rear end out of the same year of truck (1972)ex fire truck 30,000 miles which is a 7.17 with a 2 speed rear. My question is will this keep my rpm's down and give me a bit better fuel mileage?? For all the driving I'll be doing is this a waste of time and effort?? Let me know what you guys think.
Thanks
Steve

Hy topkick, the lower the numeric number, the lower the engine r.p.m. in high gear. So a 6.14 to 1 ratio would take less engine r.p.m. to attain 60 m.p.h. in high gear than a 7.17 to 1 ratio. Do you know if the 7.17 ratio is the high speed or low speed ratio of that two speed rear axle?

The 7.17 will be the "high" gear and it will have a 9.75 "low" gear ratio. You don't want that rear, it will be slower than what you have now.

I took a 7.17 out of my Fire Engine and installed a 2-speed with 6.14/8.36 ratios and gained about 10 mph. The truck will run in the low 60's all day long.

Mike B

Thanks Mike,
I'll stay away from that one, it was going to be $300.00 taken out of the truck. Mike what should I be looking for in terms of gear ratio's to get a 2 speed rear end??

Thanks for any info.
Steve

When I did my swap I didn't know much about rears so I drew a picture of my rear center pumpkin. I measured bolt hole spacing and any dimention I thought would help be locate a replacenemt while walking the junk yard.

We have a BIG truck junk yard about 2 hours from me. On my first trip there the yard man knew exactly what rear I had and what I needed. He even told me what rack to look at as he was sure he had what I needed already pulled, and he did.

I ended up with a 6.14/8.36 2-speed pumpkin with the shift motor for $250.00. I installed it in one weekend and never even jacked the truck up. I pulled the axles, dropped the drive shaft, removed the old pumpkin, installed (2) 12" long guide studs (one on etther side of the pumpkin), put the new pumpkin on a floor jack and rolled it under the truck, jacked it up and lined it up with the guide studs, slid it in and bolted it in place, removed the guide studs, installed the axles, had the drive shaft shortened (new pumpkin is longer than the original one), wired the 2-speed switch on the gear shift lever, checked the operation and went for a drive! It was that easy and I never had to touch the springs or brakes.

Hope this helps!

Mike B

Steve, I dont think your going to find a better gear ratio to get you more speed/economy. I think thats about the best you can hope for from a larger truck.

Jeff

Yep, you ain't going to get a much better ratio for a big truck. The only thing you may try is an over drive unit you mount before the rear-end. There's some postings already on here about them but there expensive. You might try rigging something up to. I'm putting a NP 205 transfer case in backwards in my driveline. Will give me a .5 over drive in high gear and about a .86 in 3rd. I can't say if it will actually work well though. I got the unit in the truck but haven't got my drive shafts cut or my rear-end back together. If you did this it would effectively drop your rear end to a 3.07. And as far as speeds...

With engine Running at 2900rpm in...
4th & OD my truck will run 98mph.
3rd & OD my truck will run 84.28mph

With Engine Running at 1750rpm in...
4th & OD my truck will run 59.1mph.
3rd & OD my truck will run 50.8mph