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Jul 21st, 2025 | | | | Joined: Oct 2011 Posts: 172 Wrench Fetcher | | Wrench Fetcher Joined: Oct 2011 Posts: 172 | hi,
my 1951 gmc 451 road tractor is fitted with a 270 engine ,a clark 5 speed DD trans and an eaton two speed rear axle with 6.50 and 9,04 gears and 900x20 tyres
what sort of road speed should she be capable of ?
cheers,,
Last edited by spanners; 11/11/2013 1:19 PM.
live or die do it with spanner in hand
| | | | | Joined: May 2005 Posts: 8,877 . | | . Joined: May 2005 Posts: 8,877 | You can calculate it here http://www.onlineconversion.com/bigger_tires.htmWith 6.5 gears, tire diameter of 38" (a guess), and engine speed of 2,500 rpm you get 43.5 MPH. Take it up to 3,000 rpm and you get 52 MPH You can play with the combinations and adjust your tire size to what they actually measure. Grigg | | | | | Joined: Apr 2001 Posts: 268 Shop Shark | | Shop Shark Joined: Apr 2001 Posts: 268 | My 454-30 (302) with Clark Speed 205V and same rear axle does 55 MPH. It has a stop under the throttle pedal and a governor on the carburetor.
And 9.00x20 tires.
Pedal to the metal, flat out on flat ground 55MPH clocked with radar gun.
Last edited by GMCPic; 11/12/2013 2:43 AM.
Dance like no one is watching,
Sing like no one is listening,
Love like you've never been hurt.
1948 GMC FC101 1/2t Pickup w/270 and SM420
1948 GMC FC253 1t Factory 80"x9' Flatbed Dually
1948 Chevy COE 2 Ton 8'x15' Flatbed
1950 GMC 354-24 2 Ton 8'X12' Flatbed w/Dump Hoist
1953 GMC 454-30 3 Ton 8'x14' Flatbed w/Dump Hoist
1953 GMC 454-30 3 Ton Cab and Chassis
1942 Clarkator 6 MILL-44 Heavy Aircraft Tug
1942 Ford (9N) Moto Tug with 1/2 yard Loader
1947 Oliver OC3 HG-42 Tract-Crawler Bull Dozier w/6' Blade
| | | | | Joined: Aug 2013 Posts: 255 'Bolter | | 'Bolter Joined: Aug 2013 Posts: 255 | My GMC 450 firetruck with 270, Clark DD 5 spd and single spd rear, don't know gears, will hit 50 on flat ground, flat out. 9.0 x20 tires. | | | | | Joined: Apr 2001 Posts: 268 Shop Shark | | Shop Shark Joined: Apr 2001 Posts: 268 | How fast to you want to go and are you prepared to make additional improvements to your truck.
Remember they didn't have freeways back then, so most of the old trucks were designed for milder speeds say 35 to 45 MPH. Freeways were just coming into play in the mid to late 50's.
That is not only gear ratios, but also stopping power, i.e. BRAKES, wheels and tires. Front axle bearings, sway bars, wheel alinement camber, etc. etc.
Dance like no one is watching,
Sing like no one is listening,
Love like you've never been hurt.
1948 GMC FC101 1/2t Pickup w/270 and SM420
1948 GMC FC253 1t Factory 80"x9' Flatbed Dually
1948 Chevy COE 2 Ton 8'x15' Flatbed
1950 GMC 354-24 2 Ton 8'X12' Flatbed w/Dump Hoist
1953 GMC 454-30 3 Ton 8'x14' Flatbed w/Dump Hoist
1953 GMC 454-30 3 Ton Cab and Chassis
1942 Clarkator 6 MILL-44 Heavy Aircraft Tug
1942 Ford (9N) Moto Tug with 1/2 yard Loader
1947 Oliver OC3 HG-42 Tract-Crawler Bull Dozier w/6' Blade
| | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | I had a '53 GMC, 302 w/ head milled .060", 6.6 rear, 20" tires, added a 5831 Brownie, would go 65 easily, faster than that, things start flapping.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Sep 2008 Posts: 872 Shop Shark | | Shop Shark Joined: Sep 2008 Posts: 872 | My 1.5 ton G506 with a 90hp 235 and stock gears, 7.5x20 tires lists a max goverened speed of 48MPH with transfer case in high. That's with a direct drive 4th gear and 6.67:1 rear end. | | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | 261 engine, or GMC 302, Chev 292-both need to move radiator(larger) to front of cowl support. OD trans., no options for rear axle-substitute modern dually rear, depends on bolt pattern-6, and 10 lug easy- D70, 80, GM 14 bolt-8 lug only. Disc brakes and shocks(if not there) on front. Tighten-up all steering apparatus, substitute Saginaw power steering/linkage. 22.5 tires and wheels.
All these ideas in this forum, recently.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
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