hiwy50 Posted April 14, 2016 Share Posted April 14, 2016 I have a 1964 B67 that against the governor, 2000 RPM, runs 52 MPH. The truck is equipped with a 4.25 rear end. I want to run 65 MPH. The speed, RPM, ratio formula tells me that 65 at 1800 RPM needs 3.40 gearing. With a 711 engine pulling a 1 car trailer is there power to pull this gearing and is this ratio available? Thanks Richard Quote Link to comment Share on other sites More sharing options...
Gmerrill0516 Posted April 14, 2016 Share Posted April 14, 2016 Do you already have an overdrive equipped transmission? That would be a viable option if not. I would assume you have plenty of power to a light load at 65...particularly if you get enough forward gear options like in a quadbox. Someone else here can give a more qualified opinion and info on available rear ratios Quote Link to comment Share on other sites More sharing options...
Freightrain Posted April 14, 2016 Share Posted April 14, 2016 (edited) Sounds like you have a direct transmission. My triplex(single over) with 4.62 gears would go just about 65 mph flat out. You will have to change rear end housing to get a better ratio. I think 4.00 was the fastest you could get in a B model double reduction. I would find an overdrive transmission, that will get you near 70 mph with your present ratio. Right now I have same triplex, 4.10 gears and it will go about 72 mph in OD. 65 mph is about 1800 rpm in OD. You will have plenty enough power to pull a single car trailer. My trailer is 14K lbs and it will go 65+ easily. Get it on hills and it starts to suffer, but in all not a problem running with traffic. (I've had it 80+ mph on level ground-my aux box gives me 85 mph). Edited April 14, 2016 by Freightrain Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment Share on other sites More sharing options...
j hancock Posted April 14, 2016 Share Posted April 14, 2016 Using the numbers you supplied of 52 mph, 4.25 ratio and 2000 rpm indicates a tire size of 37 inches. What size rubber are you running? Quote JimIt doesn't cost anything to pay attention. Link to comment Share on other sites More sharing options...
Hobert62 Posted April 14, 2016 Share Posted April 14, 2016 If your going of the speedo check to make sure it's correct. Something dosent quite seem right. Mine had a 4.60 rear with direct 10 speed tranny and 10.00 x 22 drive tires and according to GPS it would run around 55. My 295/75/22.5 steering tires measure 39" with 80psi Quote Link to comment Share on other sites More sharing options...
HarryS Posted April 15, 2016 Share Posted April 15, 2016 My '60 B has the same rear ratio with 10:00-20s and direct duplex topped out at 60. I changed the trans to 9sp OD and it now runs up to just over 70. Quote Link to comment Share on other sites More sharing options...
Underdog Posted April 15, 2016 Share Posted April 15, 2016 Using the numbers you supplied of 52 mph, 4.25 ratio and 2000 rpm indicates a tire size of 37 inches. What size rubber are you running? My F with 4.25 axle and 20 inch rubber will run 64 mph. Duplex direct. I think your speedo is off. Quote Link to comment Share on other sites More sharing options...
hiwy50 Posted April 15, 2016 Author Share Posted April 15, 2016 New to Mack trucks. Tires are 11-22.5, 41 inches tall. Transmission is 5 and 2. Last drove with GPS, guess it was wrong. Installed speedometer gear so now speedometer works. Driving this weekend to ATHS show, will check to se if possibly not shifting out of low range. Thanks for all replies Richard Quote Link to comment Share on other sites More sharing options...
Hobert62 Posted April 15, 2016 Share Posted April 15, 2016 Quote Link to comment Share on other sites More sharing options...
Hobert62 Posted April 15, 2016 Share Posted April 15, 2016 If you steped up to a 12r 22.5 tire. Quote Link to comment Share on other sites More sharing options...
Hobert62 Posted April 15, 2016 Share Posted April 15, 2016 Sorry they where at 2100 rpm. Quote Link to comment Share on other sites More sharing options...
hiwy50 Posted April 30, 2016 Author Share Posted April 30, 2016 Quote Memory seems to failed me in the 7 mo. since driving truck. Governor set at 1800. Told I need to shim hi pressure spring. Any idea of thickness shim washer to increase RPM to 2100. Quote Link to comment Share on other sites More sharing options...
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