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04-09-2018, 05:47 PM | #1 |
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Location: Seguin, TX
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Need help figuring out rear end gear
Ok guys I currently have a 250 mated to a Saginaw from a car. In fourth gear (no overdrive) going 60 mph my rpm is about 3200. I'm running a 28 inch tall tire but from what I'm seeing online I have a 4.56 in the rear. Does that sound right? It's a lwb 67 c10. I checked the numbers on the rear end and it days it came with a 3.73. I hope I can put a lower gear in and it's a 4.56 because highway driving is terrible right now.
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04-09-2018, 06:15 PM | #2 |
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Re: Need help figuring out rear end gear
Using the Summit Racing calculator I came up with 4.44, a 29" tire put it at 4.60 so I agree, you have 4.56 gears. 3.73's would be a lot nicer.
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04-09-2018, 06:16 PM | #3 |
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Re: Need help figuring out rear end gear
IMO, pull the cover off the differential and check for sure. On the ring gear it will tell you how many teeth are on the ring and pinion. If not, you can count the teeth yourself on both gears and divide the two....for an example, 41 teeth on ring and 11 teeth on the pinion gear 41/11=3.7272 3.73 gear ratio. maybe a 3.08 gear ratio would help you get the RPM down a little for hiway driving...
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04-09-2018, 07:00 PM | #4 |
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Re: Need help figuring out rear end gear
here's a rpm calculator to help you out....http://www.csgnetwork.com/multirpmcalc.html
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Mongo...aka Greg RIP Dad RIP Jesse 1981 C30 LQ9 NV4500..http://67-72chevytrucks.com/vboard/s...d.php?t=753598 Mongos AD- LS3 TR6060...http://67-72chevytrucks.com/vboard/s...34#post8522334 Columbus..the 1957 IH 4x4...http://67-72chevytrucks.com/vboard/s...63#post8082563 2023 Chevy Z71..daily driver |
04-10-2018, 12:03 AM | #5 |
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Re: Need help figuring out rear end gear
I agree on pulling the cover and having a look....there is no voodoo involved in opening up a differential. More than likely the gear oil needs changed anyway. It'll only cost you a couple quarts of gear out and a cover gasket. I'd be willing to bet your differential still has the 3.73 gears.
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04-10-2018, 12:12 AM | #6 |
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Re: Need help figuring out rear end gear
When I took it to get the oil changed I had them check the rear the diff and the oil looked pretty new. Either they changed just the oil or updated the gearing too. Either way I might change to a 3.08 or 3.42 for better highway driving
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67 C10 "Big Red" |
04-10-2018, 08:03 AM | #7 |
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Re: Need help figuring out rear end gear
I think ol Big Red will be much happier....
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04-10-2018, 11:29 AM | #8 |
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Re: Need help figuring out rear end gear
With the six I think you'd be happiest with 3.42/3.55, or if you wanted to lean toward more pep 3.73's and bump up to a 235/75 or equivalent height tire. 3.08's might be nice on the highway but gobble up too much horsepower to have any oompf.
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
04-10-2018, 02:01 PM | #9 | |
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Re: Need help figuring out rear end gear
Quote:
However, if you're strictly looking for gas mileage the 3.08 would help but the 250 will struggle somewhat.
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68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive... |
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04-10-2018, 02:08 PM | #10 |
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Re: Need help figuring out rear end gear
I checked the rear end by putting it on stands and counting the number of times the driveshaft turned when turning the rear wheel. I chocked the other to get and accurate reading. By my math when I turned the wheel 10 times the driveshaft turned about 18.75 times which came out to a 3.73. my question then is why does the tach let so high? I realized the car saginaw had a 19 tooth nylon drive gear so I am swapping the 21 tooth red gear. I know that will make a huge difference in my calculations but why would the rpms still be that high at a slower speed?
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04-10-2018, 03:06 PM | #11 |
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Re: Need help figuring out rear end gear
you sure your tach is right? also use a gps to check mph
pull the cover and count the teeth to absolutely know the ratio... if you did have 3.73s ...and a 1:1 final in trans...and a 28" tire...at 60 you should only be seeing 2686rpm same numbers but with 4.56s=3283rpm 3.73s at 70 mph=3133rpm...............3.73s at 72mph=3223rpm
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Mongo...aka Greg RIP Dad RIP Jesse 1981 C30 LQ9 NV4500..http://67-72chevytrucks.com/vboard/s...d.php?t=753598 Mongos AD- LS3 TR6060...http://67-72chevytrucks.com/vboard/s...34#post8522334 Columbus..the 1957 IH 4x4...http://67-72chevytrucks.com/vboard/s...63#post8082563 2023 Chevy Z71..daily driver Last edited by mongocanfly; 04-10-2018 at 03:15 PM. |
04-10-2018, 04:29 PM | #12 |
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Re: Need help figuring out rear end gear
I'm starting to second guess the tach at this point. At idle it reads 700 rpm but it is also an intellitronix dash. I will test with a GPS speedometer tomorrow and see what it reads.
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67 C10 "Big Red" |
04-10-2018, 04:37 PM | #13 | |
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Re: Need help figuring out rear end gear
Quote:
Drive gear in transmission is for speedometer. If you suspect it's off try to locate a GPS that measures speed to check as you drive down the road. I have a Garmin and used it to check the speedometer on my 68 with it. I also have a dwell meter with extra long leads I attach to the distributor to check engine rpms.
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68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive... |
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04-10-2018, 05:48 PM | #14 |
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Re: Need help figuring out rear end gear
Just keep it the way it is....since these old trucks only get 15 miles to the gallon on a good day as someone already confirmed....put the coins in your piggy bank....
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04-10-2018, 08:14 PM | #15 |
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Re: Need help figuring out rear end gear
4.56's gear is undrivable in phoenix. You'd be at red line to keep up on the freeway.
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04-10-2018, 08:36 PM | #16 |
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Re: Need help figuring out rear end gear
If you're confident your ratio check on the rear end is correct and it has 3.73's, and the tach is wrong, ignore the tach and enjoy the truck. I don't have, or need, a tach on either of my trucks - 60 is right around 2,600 on both, 75 is more like 3,300, and neither engine will be bothered by short runs to quite a bit faster than that (4,000+?) so I don't have to worry about over-revving it.
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
04-10-2018, 08:40 PM | #17 |
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Re: Need help figuring out rear end gear
math on my end confms your 4.56 with 28" tire.
if you go 29" and 3.73 @ 60 mph = 2600 rpm or 29" tire and 3.42 @ 60 mph = 2370 rpm stay your 28" tire add 100 rpm .
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