gear ratio to tire size question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

MITCH1914 · 07-09-2002, 03:18 PM

i have 373 gears in my 12 bolt. i am also using hoosier 275/60/15 radial tires(27 OD). when i put my rpm/tire size and speed into the gear ratio formula i get a ratio close to 410.

1st question. what tire size or OD do i need to have so that i can have a 373 ratio?

2nd i am going to buy some bfg drag radials 275/50/15. these tires have a OD of 26. what gear ratio will this cause my truck to have?

currently i am running 2950 rpm @ 55 mph on 27inch tires.

what i am trying to do is get a tall enough tire that will drop rpm on the highway. i also want to get a drag tire that i can change to at the track that will get me a good low gear so that i can leave hard and have a stout trap rpm (i will switch to the short tires only when i am at the track)

anyone have any feed back 0n this?

Longhorn Man · 07-09-2002, 07:05 PM

You sure you have a 3.73? Have you counted the teeth on it? Almost 3 grand at 55....maaaannnnnnnnn.
I'm doing 2500-ish at 60 with a 3.07 and 27 - 28 inch tires.

Sierra · 07-09-2002, 08:18 PM

That sounds like 4.33:1 gears. Are you sure of the ratio? There are formulas to plug into in order to determine gear ratio, but the rpms you report sound really high for a 3.73 rearend.

*Fast68Chevy* · 07-09-2002, 11:09 PM

http://www.randysringandpinion.com/c-rpm.html

Super Chevy · 07-10-2002, 12:02 AM

Welcome Back Randy, I knew You couldnt live without us

68 Suburban · 07-10-2002, 01:30 PM

Welcome Back. Longhornmail, you are back in second place. You need to try harder

Stroker · 07-10-2002, 04:57 PM

MITCH1914, are you using a high stall converter by chance. If so it's possible the difference between the calculated RPM and actual RPM is the loss in your converter. If your running an automatic with anything more than stock stall. A 4-7% slip is typical with aftermarket converters and about 3% with a stock converter. The more stall the more the converter will slip unless you're using a lockup converter.

Longhorn Man · 07-10-2002, 05:08 PM

LOl 68...it took me a second to figure what you were talking about.

bigjimzlll · 07-10-2002, 05:15 PM

Your speed is not accurate...the 27 inch tires are giving you a false reading...my 3:73 reared truck had 29 inch tires stock, when i went to 27 inch tall tires, when my speedo says 55..im really doing 50...and at 60 im turning 3100 rpm

Stepside · 07-10-2002, 06:54 PM

According to Fast's link, at 55 I'll be doing 1777 rpm's. At 70...2261. Sounds good to me

.
255-50-17 (27")
3.73 gear
.696 OD ratio (4L60-E)

MITCH1914, 275/60/15 is bigger than 27" I think. I'm thinking more around 28/281/2. Not sure, but an idea.

MITCH1914 · 07-12-2002, 11:10 AM

i do have a 2200 stall converter in my 400 turbo. maybe thats why i am running so high.

thanks

07-09-2002, 03:18 PM	#1
MITCH1914 Registered User Join Date: Mar 2001 Location: indianapolis,indiana,usa Posts: 506	gear ratio to tire size question i have 373 gears in my 12 bolt. i am also using hoosier 275/60/15 radial tires(27 OD). when i put my rpm/tire size and speed into the gear ratio formula i get a ratio close to 410. 1st question. what tire size or OD do i need to have so that i can have a 373 ratio? 2nd i am going to buy some bfg drag radials 275/50/15. these tires have a OD of 26. what gear ratio will this cause my truck to have? currently i am running 2950 rpm @ 55 mph on 27inch tires. what i am trying to do is get a tall enough tire that will drop rpm on the highway. i also want to get a drag tire that i can change to at the track that will get me a good low gear so that i can leave hard and have a stout trap rpm (i will switch to the short tires only when i am at the track) anyone have any feed back 0n this?

07-10-2002, 01:30 PM	#6
68 Suburban Registered User Join Date: Oct 2000 Location: From Chicago, Live in Phoenix, AZ Posts: 6,802	Welcome Back. Longhornmail, you are back in second place. You need to try harder __________________ Just sitting here contemplating contemplation.

07-10-2002, 04:57 PM	#7
Stroker Registered User Join Date: Mar 2001 Location: Moore, Ok Posts: 1,149	MITCH1914, are you using a high stall converter by chance. If so it's possible the difference between the calculated RPM and actual RPM is the loss in your converter. If your running an automatic with anything more than stock stall. A 4-7% slip is typical with aftermarket converters and about 3% with a stock converter. The more stall the more the converter will slip unless you're using a lockup converter. __________________ 72 SWB parts and pieces Oklahoma

07-10-2002, 05:15 PM	#9
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	Your speed is not accurate...the 27 inch tires are giving you a false reading...my 3:73 reared truck had 29 inch tires stock, when i went to 27 inch tall tires, when my speedo says 55..im really doing 50...and at 60 im turning 3100 rpm __________________ It's called "drag racing" if they called it "tic..tic..WHAM!..BANG! F*&K!!!", they'd have to keep the magazines under the counter with the other men's publications click the clicky to join the site.... http://67-72chevytrucks.com/vboard/payments.php 67 lwb..first hotrod in 25 years..540 best ET is 9.45 @ 141.44 Anderson,CA

07-10-2002, 06:54 PM	#10
Stepside Gasser Join Date: May 2000 Location: Thomasville,NC USA Posts: 456	According to Fast's link, at 55 I'll be doing 1777 rpm's. At 70...2261. Sounds good to me . 255-50-17 (27") 3.73 gear .696 OD ratio (4L60-E) MITCH1914, 275/60/15 is bigger than 27" I think. I'm thinking more around 28/281/2. Not sure, but an idea. __________________ :thud 70 sstep tubbed, Muncie, double hump, snowflake, Last edited by Stepside; 07-10-2002 at 08:30 PM.

07-09-2002, 07:05 PM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	You sure you have a 3.73? Have you counted the teeth on it? Almost 3 grand at 55....maaaannnnnnnnn. I'm doing 2500-ish at 60 with a 3.07 and 27 - 28 inch tires.

07-09-2002, 08:18 PM	#3
Sierra Registered User Join Date: Nov 2000 Location: Victoria, Texas Posts: 71	That sounds like 4.33:1 gears. Are you sure of the ratio? There are formulas to plug into in order to determine gear ratio, but the rpms you report sound really high for a 3.73 rearend.

07-09-2002, 11:09 PM	#4
Fast68Chevy Account Suspended Join Date: May 2000 Location: IL Posts: 0	http://www.randysringandpinion.com/c-rpm.html

07-10-2002, 12:02 AM	#5
Super Chevy Registered User Join Date: Feb 2001 Location: Ohio Posts: 1,025	Welcome Back Randy, I knew You couldnt live without us

07-10-2002, 05:08 PM	#8
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	LOl 68...it took me a second to figure what you were talking about.

07-12-2002, 11:10 AM	#11
MITCH1914 Registered User Join Date: Mar 2001 Location: indianapolis,indiana,usa Posts: 506	reply to stroker i do have a 2200 stall converter in my 400 turbo. maybe thats why i am running so high. thanks



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