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Old 04-09-2018, 05:47 PM   #1
Sleipnir
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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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Old 04-09-2018, 06:15 PM   #2
B. W.
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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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Old 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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Old 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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Old 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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Old 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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Old 04-10-2018, 08:03 AM   #7
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Re: Need help figuring out rear end gear

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Originally Posted by Sleipnir View Post
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
I think ol Big Red will be much happier....
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Old 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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Old 04-10-2018, 02:01 PM   #9
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Re: Need help figuring out rear end gear

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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.
I agree here. I would check before spending money because gas mileage won't be really good on these old trucks. My 68 GMC has 3.54 and does well with the 250 and 3 speed saginaw power wise. I have a 3.08 in my 69 and with the 350/350 combo has plenty of pull power but neither one gets good gas mileage. On a good day, 15 mpg on the highway is max on either one.

However, if you're strictly looking for gas mileage the 3.08 would help but the 250 will struggle somewhat.
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Old 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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Old 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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Last edited by mongocanfly; 04-10-2018 at 03:15 PM.
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Old 04-10-2018, 04:29 PM   #12
Sleipnir
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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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Old 04-10-2018, 04:37 PM   #13
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Re: Need help figuring out rear end gear

Quote:
Originally Posted by Sleipnir View Post
... 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?...

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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Old 04-10-2018, 05:48 PM   #14
YoungPup1977
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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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Old 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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Old 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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Old 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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