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01-14-2010, 10:51 AM | #1 |
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What rear gear ratio is good for highway???
My 72 c10's top speed is about 55...i wanna swap the rear with one with a higher ratio...any suggestions???
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01-14-2010, 11:00 AM | #2 |
Try spinnin 4 rear tars
Join Date: Sep 2003
Location: Salt Lake City, Utah
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Re: What rear gear ratio is good for highway???
Mine is tall, 3.07 ratio. I run 75mph at 3k (255/70R15 tire).
That is about as fast as I hold it for long periods of time (traveled this way for 2 years on the interstate from northern to southern Utah during college years). It would do over 100 if I let it but the motor was singing pretty good. I just found out last week that my motor isn't a 327, it is a 307ci according to the casting # Avg of 75mph was getting me 13-14mpg.
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SWEET7T 1970 C10, 2wd, LB, 307/TH350/3.08. PS, PB Last edited by SWEET7T; 01-14-2010 at 11:02 AM. |
01-14-2010, 11:01 AM | #3 |
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Re: What rear gear ratio is good for highway???
Well, first of all, get all your information first so you know what you're dealing with:
1) Verify that your speedo is correct. 2) If you don't have a tach, install one and see what your actual RPM is at 55. 3) Figure out what rear end ratio you have now, what tire size, etc. Then do the math and decide what to go with. If you feel it's only comfortable at 55, you'll probably want to go with a 3.08 or 3.42 rear end as long as you don't have an OD tranny. |
01-14-2010, 11:09 AM | #4 |
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Re: What rear gear ratio is good for highway???
very good ideas...thank you...the engine sounds like its racing @ 60mph but it would he a good idea to get a factual RPM figure before searching for a replacement rear.
What would I look for on the rear that would tell me what i have? I have 307 CI, 3 on the tree...no OD. Thanks |
01-14-2010, 11:33 AM | #5 |
20' Daredevil (Ret)
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Re: What rear gear ratio is good for highway???
Surest method is count the teeth on the R & P gears and do the math... but you can jack up the rear, turn a tire one rotation, and count the times the driveshaft rotates.
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01-14-2010, 11:41 AM | #6 |
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Re: What rear gear ratio is good for highway???
3:08 gears would be vote for best highway gears....
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01-14-2010, 11:40 AM | #7 |
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Re: What rear gear ratio is good for highway???
thank you very much.
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01-14-2010, 11:50 AM | #8 |
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Re: What rear gear ratio is good for highway???
yeah thats what im gonna keep my eye out for...any estimate on how much that swap would cost?
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01-14-2010, 04:51 PM | #9 |
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Re: What rear gear ratio is good for highway???
I just got my truck back from the shop yesterday. It had a 3.73 open rear. They put in a brand new Eaton Posi, New 3.08 gears, all new bearings inner and outer and did some other odds and ends. It came to around $1500. You could save a lot obviously finding a used one but you never know what youre getting unless you can drive the vehicle it's coming out of. Oh, and I can still turn the tires from a dead stop, only now it's both of them instead of just one. The marks on the pavement are a lot more impressive
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------------------------------------------------- Jim Garage fillers... 85 Chevy C10 Silverado LWB 07 Nissan Altima 2006 Mini Cooper S Last edited by jholley; 01-14-2010 at 05:08 PM. |
01-14-2010, 12:02 PM | #10 |
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Re: What rear gear ratio is good for highway???
3.07 would probably be your best bet for highway gears.
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01-14-2010, 02:36 PM | #11 |
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Re: What rear gear ratio is good for highway???
I have 3:07's in a box ready to go in....be interested in how much people are paying for the install??
I can't see myself doing it.....
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
01-14-2010, 03:10 PM | #12 |
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Re: What rear gear ratio is good for highway???
mine has the 3.07 rear gear with a turbo 350 tranny and 31in tall tires and it cruises down the freeway under 3grand keeping up with traffic
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-steven Alte Seele S.S. "Allison" '72 C/10 LWB 350SB/TH350 - DD PITBULL IS NOT A CRIME |
01-14-2010, 03:30 PM | #13 |
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Re: What rear gear ratio is good for highway???
with the 350 auto 3.73's were a pretty good gear.
I have a 307 and 3 on the tree in the panel. Haven't taken it on the highway yet, so not sure. |
01-14-2010, 03:32 PM | #14 |
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Re: What rear gear ratio is good for highway???
I've found it difficult to acuratly count the teeth on the pinion gear with the differential installed.
Get a tach and do some calculations. |
01-14-2010, 03:42 PM | #15 |
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Re: What rear gear ratio is good for highway???
I have 3:73's in conjunction with a 402BB and 400 auto, I am still going to be getting 10-12 mpg.
(I Hope)
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A husband can be right...or...A husband can be happy. 67-72 Chevy and GMC Trucks...The Classic Truck for the Classic Folk. 1970 CST Two tone green, 402BB, 400 Automatic, Tach, Buckets, AC, AM-FM, Tilt, GM CB, GM 8 Tract, LWB, etc JOHN 17:3...The better side of "LIFE" Remember: Everyday is a good day...Some are just gooder! Last edited by 70cst; 01-14-2010 at 03:43 PM. |
01-14-2010, 04:42 PM | #16 |
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Re: What rear gear ratio is good for highway???
To check the ratio, I jack up only 1 rear tire and have a friend spin it 20 times. I get under the truck and use a paint marker to mark the driveshaft, then count the number of times it turns. Divide that number by 10 for your gear ratio.
This method is accurate enough to tell the difference between a 3.50 and 3.55 ratio. Edit: I know our trucks don't have 3.50 or 3.55 ratios... I was just using those as examples to show the accuracy of this method. . Last edited by 1972; 01-14-2010 at 04:43 PM. |
01-14-2010, 09:54 PM | #17 |
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Re: What rear gear ratio is good for highway???
But they can have a 3.54. Some of the Dana 60s came with them.
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01-14-2010, 10:17 PM | #18 |
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Re: What rear gear ratio is good for highway???
if you want to use your truck on the highway but still want to be able to accellerate, pull loads, etc, there are two words to consider....OVER & DRIVE
i would not want anything less than a 3.42, even in a 1/2 ton. the 3.08 makes for lousy get up and go. a 3.73 with a .7 overdrive comes out to 2.61.....way better than a straight 3.08 in top gear. 3.73 gear isn't bad when you are towing and staying under 60mph. gas mileage is best if you keep the speed low too, regardless of the gear ratio....trucks displace a lot of air and those forces add up when you get moving at modern highway speeds. my 72 burb 3/4 ton has the 4.57 geared ho52 and i have a 4.10 3rd member to go in with plans to install the factory OD tail shaft on the original 3 speed saginaw (3OT). that will equate to a 2.89 final drive when overdrive is engaged. right now i figure the 307 turns about 3100rpm at 60mph, so i keep it under 50 for long drives and only go higher for short jaunts. |
01-14-2010, 04:47 PM | #19 |
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Re: What rear gear ratio is good for highway???
Gear ratio can be figured out by doing this:
(RPM x Tire Diameter) divided by (MPH x 336) |
01-14-2010, 05:07 PM | #20 |
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Re: What rear gear ratio is good for highway???
Makes me remember how BAD I am at math. So simple but it fries my brain
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
01-14-2010, 05:25 PM | #21 |
its all about the +6 inches
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Re: What rear gear ratio is good for highway???
your 307 with a 3 speed manual should have a 3.73.
If you can only do 55, you have bigger problems. You should be able to do 100+ MPH. |
01-14-2010, 05:30 PM | #22 |
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Re: What rear gear ratio is good for highway???
for real? i dont have a tach hooked up but id think that 3rd gear is 1:1 but im far from an expert...actually i dont know much at all about these trucks but thats what my Early Bronco's original 3 spd 3rd gear is...
first gear is high on my c10 but the engine sounds like im racing it anything over 55mph to me...i dunno...im gonna hook up a tach and get some real stats. |
01-14-2010, 05:50 PM | #23 |
its all about the +6 inches
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Re: What rear gear ratio is good for highway???
its one to one.
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01-14-2010, 05:55 PM | #24 |
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Re: What rear gear ratio is good for highway???
these old trucks make alot noise usually, do hook up a tach to make sure youre not over revving it but i wouldnt be surprised to see that youre only doin about 2500-3000k rpms
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-steven Alte Seele S.S. "Allison" '72 C/10 LWB 350SB/TH350 - DD PITBULL IS NOT A CRIME |
01-14-2010, 09:06 PM | #25 |
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Re: What rear gear ratio is good for highway???
As far as RPM, will a 327 rev higher than a 350? I am wanting to drive my truck on the highway, but it sounds like the engine is revving really high. Maybe I need a tach.
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