Is changing rear ratios worth it?

[jagarra]

My 62 has a 3:54 rear end with a stock 4 speed up front. I have been toying with getting a set of 3:07 gears and having them installed in my pumpkin. Has anyone made this swap, if so , how much did it improve mileage?
I figure the cost to convert will be about $700.00 if I pay to have it done.

[patrickk1]

I would think that for the same price you could swap to a 5spd.

[jagarra]

I was looking at some tyranny's for that too. I am finding Chevy flavors, but I heard the Dodge was preferred. Can anyone advise why.

thanks,

[Clarke]

This doesn't answer your question, but I thought it would be appropriate to share. With my original 4.56 I was getting 9 mpg (mostly highway). After switching to 3.54 it improved to 11.3 mpg (same highway driving). The only reason I made the gear swap was due to the amount of highway driving I do and didn't want to beat the engine up keeping up with traffic.

[Ed Snyder]

Originally Posted by Clarke

This doesn't answer your question, but I thought it would be appropriate to share. With my original 4.56 I was getting 9 mpg (mostly highway). After switching to 3.54 it improved to 11.3 mpg (same highway driving). The only reason I made the gear swap was due to the amount of highway driving I do and didn't want to beat the engine up keeping up with traffic.

David's reply reminded me of an experience I had when I first bought my '67 K model. I traveled from Medford, OR, down to LA (about 700 miles) to buy it. With the 4.56 rear gears it was a long, slow trip home burning lots of gas. I have a long (2/3 mile) steep gravel driveway, so I always use 4WD going up to keep from tearing it up. I got a big surprise when I shifted into 4WD and the truck wouldn't move. Having had the same experience a few years earlier though, I knew right away what the problem was. After shifting back into 2WD and proceeding up the driveway, I discovered that some previous owner had switched out the front gears to 3.73. Needless to say, I had the rear gears switched to 3.73 to match the front, and now enjoy better gas mileage and quieter freeway driving.

Coincidentally, the '62 Suburban I bought several years before that (from Jimjaz, sometimes on our forum) in the Phoenix area had the same problem. It still had the original 3.92 gears in the rear end. When I tried to go up my driveway in 4WD after the 1100 mile trip, it wouldn't go. The front gears had been swapped for 3.54 gears. So I had the rear changed to 3.54 and added a Powr-Lok while I was at it.

So, to answer the original question "Is it worth it?", for me it may never pencil out as a cost savings since I don't put a lot of miles on any of my 4 trucks. But to me it's worth it for peace of mind knowing that it's easier on the engines not to turn so high rpm's. Quieter running and saving gas are just added bonuses.

[jagarra]

Ed, you have some good points there. I figured the change in RPM at 65mph would be 346 RPM lower with the higher gearing 2320 vs 2666 now. Best part it would raise the cruising speed by about 10 MPH.
Still bouncing the idea around. Most of my driving is in town or the surrounding area. I rarely go on the freeway for any distance. I keep going back to the 85% rule once told to me by an industrial engineer. You build for 85% of your activity. So the gearing I have falls into that usage.

Seems I would have to change the yoke too as most all the ring and pinions have the fine spline on the pinion. From what I have heard the original GMs had a coarse spline and the Chevys had a fine spline.

[TJ's GMC]

I have 4.10s or 4.65s on in my 64 GMC 1500. 55 is around 2500. I have found that the best cruising RPM for my 305E is 2000. 2500 sounds like its screaming. I tow and haul loads with my GMC so I'm gonna leave the ratio as is for ease of taking off and ease of load on the engine. But if I ever get tired of the mileage I'll see about converting to an NV4500 if possible.

[Clarke]

Originally Posted by jagarra

I keep going back to the 85% rule once told to me by an industrial engineer. You build for 85% of your activity. So the gearing I have falls into that usage.

Seems I would have to change the yoke too as most all the ring and pinions have the fine spline on the pinion. From what I have heard the original GMs had a coarse spline and the Chevys had a fine spline.

Funny, I'm an IE also and when I was in school 20+ years ago, it was the 90% Rule. Once I got into the real world, it didn't take me long to realize and adopt the 80% Rule. And, still using it today.

Yes, I had to buy a different yoke when I changed my gears over. It was crazy; I checked with several local salvage yards and nobody had one, so I had to order a new one.

[jimjaz]

YEARS ago, like over 40 years ago... I took my 63 GMC Sub with 305/SM420 (3.73 rear gear) to Midas for a dual exhaust setup. I was getting 11 mpg before with the single exhaust setup as they all came from the factory this way. The dual exhaust setup increased my gas mileage to over 13 mpg without doing anything else! Gears and transmission swaps are expensive- going to a dual exh setup gets you the most bang for your buck, in my opinion. NOTE: I had my pipes run between the frame rail and body,, and dump out at a 45 degree angle before the rear wheels rather than up over the rear axle and out the back, so I am sure this helped also with having less pipe. The mufflers I had Midas install were just a simple "turbo" style muffler (non-restrictive in nature) and sounded good without being too loud. Pipe size was a bit bigger than original stock size at 2.25 inch, but nothing crazy.
For what it's worth............

[AZKen]

Don't know how many miles you drive per year but it could take 10 years to recoup the cost vs gas mileage gained. The motor is made to run well on all OE ratios. The rule I go by is: what am I gaining and how much will it cost and what is the risk. Known as: If it ain't broke, don't fix it. I would never pay $700 to have someone mess with all that shimming and etc.
If you really must change ratios, I would buy a good rear end for $150 from an era correct donor (and try to get a posi diff if I could). Easy to install DIY. Then sell my 3.54 diff for $150 to a guy who wants to increase acceleration.

The std rear end for 62 Chevy was 3.90. Options were 3.38 and 4.11. 1T and up had more options. 3.73 and 3.07 came in 1963.(enough about Chevy).

Disclaimer:
I don't know the options for GMC and if there would actually be a 3.07 era donor. A 1963 with 3.07 may fit with no difficulty. Not sure.
Standard 1962 Chevy came with coil rear. Standard 1962 GMC came with leaf rear. You could special order to switch either.
Leaf rear would be super easy DIY swap.