Quick Gear Ratio Question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

BigRed · 12-11-2002, 02:08 PM

79BIG10 tried to explain to me how to do a quick-and-dirty assesment of my gear ratio. However, don't think I understood it right. Anyway, I counted 3 full turns of the rear right tire and about 4 1/2 - 4 3/4 turns of the driveshaft. Anybody have a clue what ratio that is? I'm thinking it's a 3:73.

Also, what would be the best all-around ratio to have for a TH350 driving mostly street miles but highway miles every once in awhile?

bigblock73 · 12-11-2002, 02:21 PM

jack up the truck, mark a spot on the driveshaft, rotate your wheels one full revolution while counting the number of times the driveshaft goes around...this will be your estimated rear gear ratio.

3.50 to 3.73 would be a good street gear, great off the line acceleration while decent hwy RPM's.

-Jeff

BigRed · 12-11-2002, 02:39 PM

Jeff, that doesn't make sense to me. You can tell by my numbers above that if I turned it three times and the drive shaft turned 4 1/2 times, then I could just divide those number by three and get that the drive shaft turns 1 1/2 turns for every rotation of the tire. What does that relate to? Sorry, I can sometimes make things harder than they need to be.

bigblock73 · 12-11-2002, 02:51 PM

The way you did it makes no sense. You need to turn one of the two (DS or tire, can't remember which one) one full revolution to get the estimate. Where did you get ditections to turn the tire 3 times or the driveshat 4 1/2 times...there has to be a base line somewhere. If that was the case...what made you stop at 3 turns rather than 4, 6, 10, or whatever.

You can also guess based on your RPM at 60MPH. If you have 3.73 then your RPMs should be a little below 3000 RPM (depending on tire size) at 60 MPH.

If all else fails, pull off you cover and start counting teeth or look for the ratio stamped on the gear.

Hope this makes some sense.

-Jeff

BigRed · 12-11-2002, 02:53 PM

Jeff, I didn't explain this well and I don't think I understood the directions well either.

I did jack up the truck and I turned the Driver's side wheel. I counted how many times the driveshaft rotated. For one turn of the wheel, the driveshaft rotated 1 1/2 times. Does that not sound right? If not, I can go do it again.

Sorry for the confusion!

bigblock73 · 12-11-2002, 03:00 PM

it sounds right. Do this, put a piece of masking tape on the DS and repeat the prevoius steps. It sounds like you might have miscounted the # of rotations the driveshaft made (easy to do whan you are watching the tire at the same time). If you still get the same number, try the opposite...rotate the DS once and count the number of times the tire rotates. It is one of these two ways...I just can't remember...it has been a while and i am at work...no where near my own truck.

-Jeff

**swervin ervin** · 12-11-2002, 05:53 PM

You need to turn the passenger side wheel. The drivers wheel is free floating on a one legger.

BigRed · 12-11-2002, 06:50 PM

Quote:

Originally posted by swervin ervin
You need to turn the passenger side wheel. The drivers wheel is free floating on a one legger.

Well Mike, I did turn the passenger side wheel, just explained it wrong earlier. Man, I'm having problems today! :p

Anyway, I'll probably do it again and see what I get.

gchemist · 12-11-2002, 08:59 PM

Turn both wheels at the same time at the same rate in the same direction. This will lock the spider gears and give you a really close clue on the ratio.

**swervin ervin** · 12-11-2002, 09:50 PM

Quote:

Originally posted by gchemist
Turn both wheels at the same time at the same rate in the same direction. This will lock the spider gears and give you a really close clue on the ratio.

Huh??

bigblock73 · 12-11-2002, 10:05 PM

double-HUH?!

ElGracho · 12-11-2002, 10:17 PM

If you really want to know your gear ratio, take the hour to remove the rear cover, read the numbers off the ring gear, replace the gasket and refill with oil. It probably needs to be changed anyhow and it will give you a chance to look for any other surprising nastiness in your rear axle housing. As a side benefit, no guessing!

BigRed · 12-11-2002, 11:56 PM

Quote:

Originally posted by ElGracho
If you really want to know your gear ratio, take the hour to remove the rear cover, read the numbers off the ring gear, replace the gasket and refill with oil. It probably needs to be changed anyhow and it will give you a chance to look for any other surprising nastiness in your rear axle housing. As a side benefit, no guessing!

You know, I could have done that alot faster than asking all these stupid questions but was trying to go the easy route! Ha!

Just a little lazy I guess.

Anyway, I'll probably do that prior to swapping out the tranny. You know the oil capacity off hand?

Oh, and thanks for everyone's inputs!!

Txstruck · 12-13-2002, 01:12 PM

If your talking about the axle oil, Just start filling it until it reaches the bottom lip of the fill plug and your done.

gchemist · 12-15-2002, 08:24 PM

Quote:

Originally posted by swervin ervin

Huh??

I hope he figured out his tranny needs to be in neutral. :p

12-11-2002, 02:08 PM	#1
BigRed Registered User Join Date: Jun 2002 Location: DFW, TX Posts: 1,680	Quick Gear Ratio Question 79BIG10 tried to explain to me how to do a quick-and-dirty assesment of my gear ratio. However, don't think I understood it right. Anyway, I counted 3 full turns of the rear right tire and about 4 1/2 - 4 3/4 turns of the driveshaft. Anybody have a clue what ratio that is? I'm thinking it's a 3:73. Also, what would be the best all-around ratio to have for a TH350 driving mostly street miles but highway miles every once in awhile? __________________ Scott Shelbourne

12-11-2002, 02:39 PM	#3
BigRed Registered User Join Date: Jun 2002 Location: DFW, TX Posts: 1,680	Jeff, that doesn't make sense to me. You can tell by my numbers above that if I turned it three times and the drive shaft turned 4 1/2 times, then I could just divide those number by three and get that the drive shaft turns 1 1/2 turns for every rotation of the tire. What does that relate to? Sorry, I can sometimes make things harder than they need to be. __________________ Scott Shelbourne

12-11-2002, 02:53 PM	#5
BigRed Registered User Join Date: Jun 2002 Location: DFW, TX Posts: 1,680	Jeff, I didn't explain this well and I don't think I understood the directions well either. I did jack up the truck and I turned the Driver's side wheel. I counted how many times the driveshaft rotated. For one turn of the wheel, the driveshaft rotated 1 1/2 times. Does that not sound right? If not, I can go do it again. Sorry for the confusion! __________________ Scott Shelbourne

12-11-2002, 05:53 PM	#7
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	You need to turn the passenger side wheel. The drivers wheel is free floating on a one legger. __________________ Mike 1985 Chevy C-10

12-11-2002, 08:59 PM	#9
gchemist BAD BOW-Silverado XST Join Date: May 2000 Location: Senior Member from Austin, TX Posts: 6,431	Turn both wheels at the same time at the same rate in the same direction. This will lock the spider gears and give you a really close clue on the ratio. __________________ Gerardo a.k.a. Mad Chemist Silverado XST videos

12-11-2002, 02:21 PM	#2
bigblock73 yeller Join Date: Oct 2001 Location: Twin Cities Posts: 13,824	jack up the truck, mark a spot on the driveshaft, rotate your wheels one full revolution while counting the number of times the driveshaft goes around...this will be your estimated rear gear ratio. 3.50 to 3.73 would be a good street gear, great off the line acceleration while decent hwy RPM's. -Jeff

12-11-2002, 02:51 PM	#4
bigblock73 yeller Join Date: Oct 2001 Location: Twin Cities Posts: 13,824	The way you did it makes no sense. You need to turn one of the two (DS or tire, can't remember which one) one full revolution to get the estimate. Where did you get ditections to turn the tire 3 times or the driveshat 4 1/2 times...there has to be a base line somewhere. If that was the case...what made you stop at 3 turns rather than 4, 6, 10, or whatever. You can also guess based on your RPM at 60MPH. If you have 3.73 then your RPMs should be a little below 3000 RPM (depending on tire size) at 60 MPH. If all else fails, pull off you cover and start counting teeth or look for the ratio stamped on the gear. Hope this makes some sense. -Jeff

12-11-2002, 03:00 PM	#6
bigblock73 yeller Join Date: Oct 2001 Location: Twin Cities Posts: 13,824	it sounds right. Do this, put a piece of masking tape on the DS and repeat the prevoius steps. It sounds like you might have miscounted the # of rotations the driveshaft made (easy to do whan you are watching the tire at the same time). If you still get the same number, try the opposite...rotate the DS once and count the number of times the tire rotates. It is one of these two ways...I just can't remember...it has been a while and i am at work...no where near my own truck. -Jeff

12-11-2002, 10:05 PM	#11
bigblock73 yeller Join Date: Oct 2001 Location: Twin Cities Posts: 13,824	double-HUH?!

12-11-2002, 10:17 PM	#12
ElGracho Gentleman Jim Driver Join Date: Nov 2002 Location: Poulsbo, WA Posts: 1,553	If you really want to know your gear ratio, take the hour to remove the rear cover, read the numbers off the ring gear, replace the gasket and refill with oil. It probably needs to be changed anyhow and it will give you a chance to look for any other surprising nastiness in your rear axle housing. As a side benefit, no guessing! __________________ Joe '75 GMC Gentleman Jim '84 Chev C10 Short Wide - Super duper plain (manual steering, manual brakes, no dome light, no cig lighter) '85 Chev C10 Short Wide - Super plain Vortec 4.8 4L60E trans also: '81 K30, '83 C30 Crew Dually, '84 M1028 CUCV, '85 M1009 CUCV, another '85 C10 SWB, '89 R3500 Flatbed

12-13-2002, 01:12 PM	#14
Txstruck Registered User Join Date: Nov 2002 Location: Dallas Posts: 459	Oil Capacity If your talking about the axle oil, Just start filling it until it reaches the bottom lip of the fill plug and your done. __________________ Steve 86 K30 4x4 6.2L Diesel\TH400\NP208 - 14bolt FF rear w/detroit locker and Dana60 front w/detroit locker- both with 4.56 gears "Runs on Mud"

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