Tach over 3K at 65MPH - The 1947 - Present Chevrolet & GMC Truck Message Board Network

sigurd · 07-06-2004, 05:13 AM

Ok, I have my new motor, tires,gears in the rear end,new tested tach and the speedo dynoed. However, I'm running a tad over 3000 rpm at 65mph. I have 32inch tires with 3.73 gears. How much do you guys and gals think I will drop in rpms if I go with 33 inch tires? My poor new motor needs a break.

thanks for any suggestions,

James

JimKshortstep4x4 · 07-06-2004, 06:48 AM

If you look at the distance traveled per rev, (pi x D), the 32" tire travels 100.53 inches and the 33" tire travels 103.67". Percentage wise this works out to 3 percent. If you recalibrate your speedometer, at 3,000 rpm you will be going 66.95 MPH with the 33" tires on your truck.

At 65 MPH, your engine will be turning 2910 RPM.

It appears that going to 33" tires will not reduce RPM by very much.

Jim

sigurd · 07-06-2004, 06:55 AM

thanks Jim, I guess I should have kept my 308 gears.....

67chevemall · 07-06-2004, 09:07 AM

Man I need the 3:08 cure too!
Any one want to trade? 3:73 posi for 3:08 posi?

cdowns · 07-06-2004, 09:36 AM

overdrive tranny :the best of both worlds

67chevemall · 07-06-2004, 10:40 AM

Couldn't give up the Muncie!

kickinasphalt · 07-06-2004, 11:01 AM

Install a lift and 35's

I have 3.73 and about 2500 at 65MPH.

**JIMs70GMC** · 07-06-2004, 11:15 AM

Maybe you tach is set for a 6 cyl?

71Chev · 07-06-2004, 04:16 PM

Changing tire size will calculate to roughly 100rpms for every 1" change in height.

busterwivell · 07-06-2004, 04:22 PM

My 1 Ton has 4:11 gears. I went from truck 4 speed to a 700R4 and helped it SOME. Now 75 MPH is about 3300 RPM in overdrive. In Drive, 55 MPH is about the same. I have the formula for gear ratios, RPM, etc, all taped on the wall in the shop at home. If you want them, let me know and I'll post them later.

toddtheodd · 07-06-2004, 04:41 PM

Busterwivell, I would love to see that list.
67chevemall, I don't have posi but I have 3.08's. I would love some 3.73's and I don't care about the raise in rpm's. Too bad you're so far away. If not I'd say we could just trade ring and pinion's.

jorgensensc · 07-06-2004, 05:55 PM

Shoot, I got 4.11's in my truck and it's a daily driver, approx 60 miles a day, mostly highway. Talk about screamin engine all the time. I really need to get some better gears!

sigurd · 07-06-2004, 06:33 PM

thanks for all of the replies. Guess I better start towing something....

Bowtie67 · 07-06-2004, 06:52 PM

I was running 26.5 in tires with a 4sp and 3.73 keeping up with traffic my engine was screaming 3800rpm, dropped in a 700r4 and now my cruise speed is around 2200rpm. the OD really is sweet with the super low first, I havent really been able to check out my new gears 4.10 with the 700r4 on the highway, but I dont figure it really will change a whole lot.

Here is a calculator to figure RPMS with tire size and gear ratio, etc.

http://www.angelfire.com/fl/procrastination/rear.html

busterwivell · 07-07-2004, 09:16 AM

Todd, and others, here's the formulas:

MPH = RPM x tire diameter
divided by rear gear ratio x 336

RPM = MPH x gear ratiox336
divided by tire diameter

gear ratio = RPMxtire diameter
divided by MPH x 336

tire diameter = MPH x gear ratio x 336
divided by RPM

Hope this helps, I keep this posted on shop wall (along with SBC and BBC firing order)

07-06-2004, 05:13 AM	#1
sigurd Registered User Join Date: Mar 2000 Location: San Diego, CA USA Posts: 344	Tach over 3K at 65MPH Ok, I have my new motor, tires,gears in the rear end,new tested tach and the speedo dynoed. However, I'm running a tad over 3000 rpm at 65mph. I have 32inch tires with 3.73 gears. How much do you guys and gals think I will drop in rpms if I go with 33 inch tires? My poor new motor needs a break. thanks for any suggestions, James __________________ --------- 72 SWB K10 San Diego CA

07-06-2004, 06:55 AM	#3
sigurd Registered User Join Date: Mar 2000 Location: San Diego, CA USA Posts: 344	thanks Jim, I guess I should have kept my 308 gears..... __________________ --------- 72 SWB K10 San Diego CA

07-06-2004, 09:07 AM	#4
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	Man I need the 3:08 cure too! Any one want to trade? 3:73 posi for 3:08 posi? __________________ 1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD

07-06-2004, 09:36 AM	#5
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	overdrive tranny :the best of both worlds __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

07-06-2004, 10:40 AM	#6
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	Couldn't give up the Muncie! __________________ 1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD

07-06-2004, 06:48 AM	#2
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	If you look at the distance traveled per rev, (pi x D), the 32" tire travels 100.53 inches and the 33" tire travels 103.67". Percentage wise this works out to 3 percent. If you recalibrate your speedometer, at 3,000 rpm you will be going 66.95 MPH with the 33" tires on your truck. At 65 MPH, your engine will be turning 2910 RPM. It appears that going to 33" tires will not reduce RPM by very much. Jim

07-06-2004, 11:01 AM	#7
kickinasphalt Registered User Join Date: Jan 2000 Location: North Dakota Posts: 351	Install a lift and 35's I have 3.73 and about 2500 at 65MPH. __________________ 1972 K-10 Longbox,6" lift w/35's/350/SM465/NP205/3.73 1972 C-10 Longbox,350/350 1998 Pontiac Grand Prix GTP FOR SALE: 1971 K-10 Longbox,350/SM465/NP205

07-06-2004, 11:15 AM	#8
JIMs70GMC user # 2756 Join Date: Jun 2001 Location: Chesapeake, Virginia Posts: 4,612	Maybe you tach is set for a 6 cyl? __________________ 1970 K25, 8' stepside bed 350/465/205 44 up front, 60 in the rear 4.10s rolling on 33" Dunlop MTs 1986 K5, 350/465/208 Dana 60/14 bolt from a cucv 36" Super Swampers TSL/SX 1983 K20 w/ CUCV axles, 350/700R4/208 sitting on 37" Goodyears 1986 M1031 6.2 diesel, TH400/NP205 locker in the rear and a LS in the front, all stock for now..... 1986 K30, 350/400/205 dana 60 and 14 bolt. I kept the drivetrain. Body/bad and chassis are gone. 1981 K30, 350/465/205 dana 60 and dually 14 bolt. Has a G80, and a flat bed. Going to replace the flat bed. 1985 K20, 350/400/208 10 bolt and SF 14 bolt. I wonder where I can find some 1 tons. Hmmmmm

07-06-2004, 04:16 PM	#9
71Chev Senior Member Join Date: Dec 2000 Location: Indiana Posts: 3,074	Changing tire size will calculate to roughly 100rpms for every 1" change in height. __________________ 71 Chevy CST Long Fleet 350/350/3.07 leaf rear.

07-06-2004, 04:22 PM	#10
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	My 1 Ton has 4:11 gears. I went from truck 4 speed to a 700R4 and helped it SOME. Now 75 MPH is about 3300 RPM in overdrive. In Drive, 55 MPH is about the same. I have the formula for gear ratios, RPM, etc, all taped on the wall in the shop at home. If you want them, let me know and I'll post them later. __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

07-06-2004, 04:41 PM	#11
toddtheodd The oddest Todd around Join Date: May 2004 Location: Miami, Florida Posts: 1,418	Busterwivell, I would love to see that list. 67chevemall, I don't have posi but I have 3.08's. I would love some 3.73's and I don't care about the raise in rpm's. Too bad you're so far away. If not I'd say we could just trade ring and pinion's.

07-06-2004, 05:55 PM	#12
jorgensensc Registered User Join Date: Jul 2003 Location: Cypress, Tx Posts: 4,005	Shoot, I got 4.11's in my truck and it's a daily driver, approx 60 miles a day, mostly highway. Talk about screamin engine all the time. I really need to get some better gears! __________________ 1972 C20 Suburban- Big Blue Betty '56 Chevy Bel Air Sedan- Frame up Restoration -What would you attempt to achieve if you knew you could not fail?- -I Refuse To Tiptoe Through Life, Only To Arrive Safely At Death's Door- R.I.P. EAST SIDE LOW LIFE

07-06-2004, 06:33 PM	#13
sigurd Registered User Join Date: Mar 2000 Location: San Diego, CA USA Posts: 344	thanks for all of the replies. Guess I better start towing something.... __________________ --------- 72 SWB K10 San Diego CA

07-06-2004, 06:52 PM	#14
Bowtie67 Registered User Join Date: Jan 2004 Location: NH Posts: 2,266	I was running 26.5 in tires with a 4sp and 3.73 keeping up with traffic my engine was screaming 3800rpm, dropped in a 700r4 and now my cruise speed is around 2200rpm. the OD really is sweet with the super low first, I havent really been able to check out my new gears 4.10 with the 700r4 on the highway, but I dont figure it really will change a whole lot. Here is a calculator to figure RPMS with tire size and gear ratio, etc. http://www.angelfire.com/fl/procrastination/rear.html __________________ 1967 C10 - SWBSS Pro Street 427sbc, 700R4 & 4.10 Gears 1948 Chevy FleetMaster Coupe, LS1, (almost done) 1950 Chevy StyleLine Coupe 250/6 3x2 Dueces 2009 Toyota Tundra 1996 Harley RoadKing "I maybe getting old, but I can still burn rubber with the best of them"

07-07-2004, 09:16 AM	#15
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	Todd, and others, here's the formulas: MPH = RPM x tire diameter divided by rear gear ratio x 336 RPM = MPH x gear ratiox336 divided by tire diameter gear ratio = RPMxtire diameter divided by MPH x 336 tire diameter = MPH x gear ratio x 336 divided by RPM Hope this helps, I keep this posted on shop wall (along with SBC and BBC firing order) __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com



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