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RPM and gear ratio ?s

diablo1

diablo1

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I am trying to figure out which direction to go in with the transmission, tires and gearing.

I would like to run a 37" tire with the Th350 and 203 I currently have or upgrade to a th400. I have a line on some 1 tons with 5:13 and a doubler.

Would this be to much gear to run at least 65-70 MPH.

1) What RPM should a chevy 350 with a TH350 or a Th400 be running at down the highway.

2)Does anyone know the gearing in the Th350 and Th400 so I can try to work on the conversion chart?

3) What do you guys recommend.

Thanks,
B
 
Yep.....too much axle gear.

Or not enough overdrive, if you want to think of it the opposite way. A 700R4 or 4L80E could make those 5.13s a lot more liveable.

It would be interesting to do a poll:

How many people with 5.13s actually still daily-drive their rigs, or run them on the highway for more than an hour at a time?

My guess is that 90% of the 5.13 equipped rigs are trailered and/or not daily driven anymore.


:usaflag:
 
well, my m1008 cucv has the 6.2, th400 and np208 with 4.56s in the axles. she screams at 50-55. i couldnt tell you about the th350, but 5.13s are alot deeper... maybe around town but no highways for me if i ran that. i hardly like taking my cucv on the highway now. feel like im gonna blow it up
 
HEre is my setup, 1984 K5 Silverado

305cid, 700r tranny, stock TC (not sure what it is), 33 x 12.5 x 15", 4:10 gears and 2" lift.

I run 65mph @ 2100rpm.
 
you would be runnin at 2600 rpm with that setup.without overdrive i wouldnt run any lower than 4.10.that would give you a a rpm of 2400.optimum mpg is achieved at 2200 rpm.so you will be close.5.13 and an overdrive unit will keep you in the 2270 rpm range.
 
Calculations

Go to randys ring & pinion there web site has calculator.you put in the specs on what you want.
 
chulisohombre said:
you would be runnin at 2600 rpm with that setup.without overdrive i wouldnt run any lower than 4.10.that would give you a a rpm of 2400.optimum mpg is achieved at 2200 rpm.so you will be close.5.13 and an overdrive unit will keep you in the 2270 rpm range.
Click to expand...

Nope, more like 3500 RPM. With 5.13's in the diff, 1:1 in the trans and 36" tires (most 37's measure a little smaller than their advertised size) you would be spinning 3352 RPM. Since TH350's and TH400's don't have lockup torque converters, they will slip and the RPM will be even higher, more like 3500 RPM.

That is way too high for sustained highway travel IMO.
 
38377k5 said:
Nope, more like 3500 RPM. With 5.13's in the diff, 1:1 in the trans and 36" tires (most 37's measure a little smaller than their advertised size) you would be spinning 3352 RPM. Since TH350's and TH400's don't have lockup torque converters, they will slip and the RPM will be even higher, more like 3500 RPM.

That is way too high for sustained highway travel IMO.
Click to expand...

Actually depending on what year the TH350 is it may have a lockup torque converter. But I do agree that running over 2600 at highway speeds is going to kill your mpg. I personally run a TH350 NP205 GM 12 Bolt 3.42:1 with 35" tires and at about 60 mph I am turning some where in the neighborhood of 2300 RPM.
 
38377k5 said:
Nope, more like 3500 RPM. With 5.13's in the diff, 1:1 in the trans and 36" tires (most 37's measure a little smaller than their advertised size) you would be spinning 3352 RPM. Since TH350's and TH400's don't have lockup torque converters, they will slip and the RPM will be even higher, more like 3500 RPM.

That is way too high for sustained highway travel IMO.
Click to expand...

i was just going by the calculated numbers actually.every setup will run a lil diffwerent.slippage in tranny can give you up to anether 500 rpm or more depending on how worn it is.i was doing best case scenario with actual final drive numbers. and eas said already.some do have lockup converters.so depending on what is installed,we are both correct:wink1:
 
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