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Jul 21st, 2025 | | | | Joined: Nov 2005 Posts: 1,501 'Bolter | | 'Bolter Joined: Nov 2005 Posts: 1,501 | I am trying to get optimum rpm at cruise. I have a 46 chevy on an s10 frame with approx 300 hp 350 with 400 trans and 3.42 rear. I am trying to get the right diameter tires. I will be using 15 inch wheels. I have used the ratio calculator and I don't know what rpm I should be shooting for at 55 mph. Examples, 25.5 dia tire. 2478 @ 55
26.1 dia tire. 2422 @ 55
28. Dia tire. 2257 @ 55
Last edited by homer52; 12/03/2016 7:24 AM.
1941 Chevy stock complete
1941 GMC resting peacefully
1946/1947 Chevy Street rod on s10 frame complete
1945 GMC panel truck in line for restoration
1941 Plymouth stock complete
1941 GMC COE in restoration process
1941 Chevy Coe uncertain future resting now
| | | | | Joined: Jan 2015 Posts: 910 'Bolter | | 'Bolter Joined: Jan 2015 Posts: 910 | I have a 2001 suburban 325 4spd OD on 16" wheels, 3.42 and if I remember correct, it's 55 mph at 1,500 rpm. And, when driving my usual 70 to 75 get 20.7 mpg highway.
I am taking a ride upstate tomorrow with the family for a Christmas tree and will record the rpm at various mph. I will also measure the tire's diameter.
So if you are wanting good mpg I would go with a wheel tire combination that would give a 30 - 31 tire diameter. Jeffrey
| | | | | Joined: Feb 2000 Posts: 4,886 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 4,886 | Figure out where your torque is, and aim for it. My 250 inline with 100 hp likes to run at 2350 to 2450 rpm. I would suspect yours to be in the 1600 to 1800 range. | | | | | Joined: Jul 2014 Posts: 333 'Bolter | | 'Bolter Joined: Jul 2014 Posts: 333 | 25.5" diameter tire seems a little short. I have some cheap Cooper Trendsetters on mine, 235/75/15s on the back, and according to Cooper's website, the diameter is 28.66". You might try something closer to that diameter and see if it gets you closer to where you want to be. | | | | | Joined: Feb 2004 Posts: 28,675 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 28,675 | Do you have the specification card on the cam that's in your engine? The most economical cruise RPM will be just slightly above the torque peak. If you get it lugged down too far, the power valve in the carb will open up and kill your gas mileage, plus the acceleration will suck until you struggle up to the RPM where the engine starts making usable torque. Try running a vacuum gauge and see which size tire allows you to accelerate in top gear with the least amount of vacuum drop.
Jerry
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| | | | | Joined: Nov 2005 Posts: 1,501 'Bolter | | 'Bolter Joined: Nov 2005 Posts: 1,501 | At this point I have no solid info on the cam except it is supposed to be an RV cam, go figure. I guess I will just have to try a pair of tires and see what happens.
1941 Chevy stock complete
1941 GMC resting peacefully
1946/1947 Chevy Street rod on s10 frame complete
1945 GMC panel truck in line for restoration
1941 Plymouth stock complete
1941 GMC COE in restoration process
1941 Chevy Coe uncertain future resting now
| | | | | Joined: Mar 2010 Posts: 10,059 Renaissance Man | | Renaissance Man Joined: Mar 2010 Posts: 10,059 | I guess I will just have to try a pair of tires and see what happens. That statement right there may well be the most logical and practical one ever written. With a custom set up like yours, you have too many unknown variables to deal with to try to figure it out on paper. Examples: Stall speed of the torque converter. Gear ratios of each transmission gears. Camshaft profile. I believe you figured out the best plan. Carl
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
| | | | | Joined: Oct 2004 Posts: 128 Shop Shark | | Shop Shark Joined: Oct 2004 Posts: 128 | I'd be inclined to go with whatever looks the best. With a light vehicle and a 350 close is probably good enough. John | | | | | Joined: May 2004 Posts: 1,312 'Bolter | | 'Bolter Joined: May 2004 Posts: 1,312 | Try this website: wallaceracing.com/Calculators
Automotive calculators of all kinds including tire size/gears/trans/ to rpm
| | | | | Joined: Sep 2001 Posts: 29,262 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,262 | | | |
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