th350 + highway speed - The 1947 - Present Chevrolet & GMC Truck Message Board Network

enaberif · 02-15-2009, 04:55 PM

Having to do a bit of highway travel this weekend I noticed with my truck it seems to be really screaming about 110km/h (70mph).

Now I have no idea what my rear gears are and will soon be finding this out nor what my rpms are (no tach yet) but I was told I had 3.73 gears.

Does this seem about right?

Trick76 · 02-15-2009, 04:58 PM

28 inch tall tires with 3.73s should give you 3200 rpm give or take at 70 mph

http://www.csgnetwork.com/multirpmcalc.html

enaberif · 02-15-2009, 05:03 PM

Quote:

Originally Posted by Trick76

28 inch tall tires with 3.73s should give you 3200 rpm give or take at 70 mph

http://www.csgnetwork.com/multirpmcalc.html

3200rpm .

Hmm guess that could explain heavy gas usage on highway! haha I can see now why people prefer the TH700r for highway driving.

maxreps · 02-15-2009, 07:37 PM

I feel your pain, mine has 4.10's - 29" tires and definitely needs OD bad!

slugish · 02-15-2009, 09:15 PM

Rather than go thtu all the expense & time of an overdrive tranny , just get you a 3.42 ring & pinion for under 200.00 . With a 3 speed this is aboutthe best of both worlds . with a 3.73 or 4.10 you need overdrive if you do any freeway cruising at all . You must be screaming at 50 mph with a 4.10 - 3 speed .

enaberif · 02-15-2009, 09:41 PM

Quote:

Originally Posted by slugish

Rather than go thtu all the expense & time of an overdrive tranny , just get you a 3.42 ring & pinion for under 200.00 . With a 3 speed this is aboutthe best of both worlds . with a 3.73 or 4.10 you need overdrive if you do any freeway cruising at all . You must be screaming at 50 mph with a 4.10 - 3 speed .

Dunno for the cost of a transmission I'd rather just go that route and have the best of ALL worlds

heavychevy79 · 02-15-2009, 10:23 PM

Put some 31x10.50 tires on the back and see if that doesn't help.

enaberif · 02-16-2009, 12:45 AM

I've been doing some reading and can someone tell me if there is any truth to this.

Some people say that based upon tire height it can actually make your gear ratio seem lower.

For example say you have 28" high tires and you have a 3.73 gear ratio, with the 28" tires the 3.73 may seem more like 4.10 gears?

Any truth?

I currently run 28" tall tires.

willett · 02-16-2009, 01:03 AM

the bigger the tires, the higher the speed, and the lower the revs

Mechanic707 · 02-16-2009, 01:36 AM

My 77 gmc 4 speed 4.11? screams in 4th @ 50mph 15" tires

Dooley · 02-16-2009, 02:27 AM

This question would be easier to answer if more truck info was offered. I've run 3.73's with T350 trannies for 30+ years in several GM trucks with no problem at highway speeds. I do run 33/12.50-tires though,(no lift required on squarebodies). This is a good tire size for those gears and transmission. No problem running 70/75.

enaberif · 02-16-2009, 02:37 AM

Quote:

Originally Posted by Dooley

This question would be easier to answer if more truck info was offered. I've run 3.73's with T350 trannies for 30+ years in several GM trucks with no problem at highway speeds. I do run 33/12.50-tires though,(no lift required on squarebodies). This is a good tire size for those gears and transmission. No problem running 70/75.

I don't have a tach at the moment and I know I'm running 275 60r15 tires.

I can't guarantee I'm running 3.73 so next few days I'll get the rear diff oil changed and find out.

jchav62 · 02-16-2009, 11:06 AM

Sounds to me like you have 3.73's. The best gear ratio for a th350 and lower profile tires to me is 3.08's. I ran 3.08's on my '62 for a while with no o/d. It did 75 on the freeway with no problems. If you plan on running overdrive, I'd go with 3.42's or 3.73's. When I put a gearvendor overdrive in my truck with the 3.08's, it tended to be a little doggie on the hills.

boatpuller · 02-16-2009, 11:49 AM

my th 350 with 3.08's turns 2500 rpm at about 75 mph on the freeway.that's with 29.8 inch tire's.it does good on gas too.it lack's a little off the line.but,i didn't build it for the quarter mile.i like the lower rpm's to freeway cruise.

02-15-2009, 04:55 PM	#1
enaberif Registered User Join Date: Jan 2009 Location: Calgary, AB Posts: 1,181	th350 + highway speed Having to do a bit of highway travel this weekend I noticed with my truck it seems to be really screaming about 110km/h (70mph). Now I have no idea what my rear gears are and will soon be finding this out nor what my rpms are (no tach yet) but I was told I had 3.73 gears. Does this seem about right?

02-15-2009, 04:58 PM	#2
Trick76 Living la vida broka Join Date: Jan 2003 Location: P'ville California Posts: 221	Re: th350 + highway speed 28 inch tall tires with 3.73s should give you 3200 rpm give or take at 70 mph http://www.csgnetwork.com/multirpmcalc.html __________________ 1976 C10 Longbed lowered 5/7, hot 355/TH350. ~ SOLD 1974 C10 Shortbed Stepside 350/Saginaw ~ SOLD 1984 C10 Longbed 6.2l diesel 700r4. ~SOLD 1985 GMC C20 350 700r4, ~under the knife Last edited by Trick76; 02-15-2009 at 04:59 PM. Reason: linky

02-15-2009, 07:37 PM	#4
maxreps Registered User Join Date: Jan 2009 Location: greensboro, nc Posts: 174	Re: th350 + highway speed I feel your pain, mine has 4.10's - 29" tires and definitely needs OD bad!

02-15-2009, 09:15 PM	#5
slugish Slugish Join Date: Jan 2005 Location: Earlysville, Va. Posts: 1,024	Re: th350 + highway speed Rather than go thtu all the expense & time of an overdrive tranny , just get you a 3.42 ring & pinion for under 200.00 . With a 3 speed this is aboutthe best of both worlds . with a 3.73 or 4.10 you need overdrive if you do any freeway cruising at all . You must be screaming at 50 mph with a 4.10 - 3 speed .

02-15-2009, 10:23 PM	#7
heavychevy79 Cheyenne Super Join Date: Nov 2008 Location: Dallas Texas Posts: 545	Re: th350 + highway speed Put some 31x10.50 tires on the back and see if that doesn't help.

02-16-2009, 12:45 AM	#8
enaberif Registered User Join Date: Jan 2009 Location: Calgary, AB Posts: 1,181	Re: th350 + highway speed I've been doing some reading and can someone tell me if there is any truth to this. Some people say that based upon tire height it can actually make your gear ratio seem lower. For example say you have 28" high tires and you have a 3.73 gear ratio, with the 28" tires the 3.73 may seem more like 4.10 gears? Any truth? I currently run 28" tall tires. Last edited by enaberif; 02-16-2009 at 01:00 AM.

02-16-2009, 01:03 AM	#9
willett Another Day, Another Dollar Join Date: Nov 2008 Location: North Vancouver, BC Posts: 1,316	Re: th350 + highway speed the bigger the tires, the higher the speed, and the lower the revs __________________ -78 K20 Cheyenne 454 long box, 4" lift, 35" MT's, '84 cab, '80 box "its hip to be square"

02-16-2009, 01:36 AM	#10
Mechanic707 Registered User Join Date: Feb 2009 Location: NorCal Humboldt County Posts: 14	Re: th350 + highway speed My 77 gmc 4 speed 4.11? screams in 4th @ 50mph 15" tires __________________ 1977 GMC C2500 5.7L V8 Long Bed 4-Speed Stick 12/08 ~12/08 Sanded/Primed/Rhino Lined inside bed 1988 Chevy C3500 5.7L 1-Ton w/utility bed10/08 ~1/09 Tie Rods, Ball joints, spindles, wheel bearings, steering gearbox ~11/08 distributor, cap, rotor, wires, plugs, thermostat 180' & chrome water inlet, radiator flush, hoses, Trans filter & pcv, air & oil. ~11/08 Sanded & painted blue. new brake booster & master cylinder w/flush & adjust. ~Bought 285/70/16 orig. chevy rims w/tires on C/L$100

02-16-2009, 02:27 AM	#11
Dooley Registered User Join Date: Mar 2005 Location: North Coast Posts: 1,012	Re: th350 + highway speed This question would be easier to answer if more truck info was offered. I've run 3.73's with T350 trannies for 30+ years in several GM trucks with no problem at highway speeds. I do run 33/12.50-tires though,(no lift required on squarebodies). This is a good tire size for those gears and transmission. No problem running 70/75. Last edited by Dooley; 02-16-2009 at 02:29 AM.

02-16-2009, 11:06 AM	#13
jchav62 1962 C-10 Join Date: Apr 2005 Location: Las Vegas, New Mexico Posts: 4,978	Re: th350 + highway speed Sounds to me like you have 3.73's. The best gear ratio for a th350 and lower profile tires to me is 3.08's. I ran 3.08's on my '62 for a while with no o/d. It did 75 on the freeway with no problems. If you plan on running overdrive, I'd go with 3.42's or 3.73's. When I put a gearvendor overdrive in my truck with the 3.08's, it tended to be a little doggie on the hills. __________________ Joseph Chavez "See the USA in a Chevrolet!!" ......................_____ .........________//__{\_____ ,,,,,,,/__(O)___//___/__(O)_/ 1962 C10 Shortbed Fleetside 327/th350 GV overdrive 1974 C10 Longbed Fleetside 350/th350 "The Trash Truck" 1999 Chevy 2500 OBS, Longbed 5.7/4L80E 4x4 2015 GMC Sierra SLE 1500 4x4

02-16-2009, 11:49 AM	#14
boatpuller Registered User Join Date: Sep 2005 Location: Sun City West, Az Posts: 3,822	Re: th350 + highway speed my th 350 with 3.08's turns 2500 rpm at about 75 mph on the freeway.that's with 29.8 inch tire's.it does good on gas too.it lack's a little off the line.but,i didn't build it for the quarter mile.i like the lower rpm's to freeway cruise. __________________ " Never look up a dead dogs butt."



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