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EFV-8 Club Forum / General Ford Discussion / highway speeds and rpm

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Posted By Discussion Topic: highway speeds and rpm     tomlud 06-29-2015 @ 2:11 PM reply profile send p.m. Member Posts: 23 Joined: Jan 2011 I have a 51 ford 8ba custom tudor which is stock with the exception of pertronix ignition, headers and dual exhaust. Three on the tree with overdrive and I think a 4:10 rear end with 670x15 radial tires. I usually make 2 or 3 major shows a year which are about 300 miles round trip where most of theses miles are unavoidably interstate. To keep from being run over I usually drive 60 to 65 mph and what i'm wondering is what rpm's am I turning at these speeds with O/D, and if these speeds for a sustained period it too much wear and tear on the engine. Since I've owned the car it has had new pumps, re-cored radiator, carb rebuild, new coil and fuel pump. Not sure what if anything ever done internally to the engine before I bought it 5 years ago. The car runs great but I suspect its sweet spot may be something under 60 mph and don't want to overtax the engine. Sorry I rambled so long but would really appreciate all comments especially what rpm's are at those speeds with O/D. Thanks. Tom JT Ford 06-29-2015 @ 7:39 PM reply profile send p.m. New Member Posts: 112 Joined: Oct 2009 You should be turning 1929 rpm's at 55 mph and 2104 at 60 mph. This would be clocked speed, not the speedometer. My l949 speedometer is 5 mph slower than ac ual speed. I run mine at 55-60 gps speed most of the time. carcrazy 06-29-2015 @ 8:04 PM reply profile send p.m. Senior Posts: 1653 Joined: Oct 2009 Assuming the aspect ratio for a 670x15 radial tire is the as that of a bias ply 6.70x15 bias ply tire. Your engine speed at 60 MPH in overdrive will be 2140 RPM. At 65 MPH in O.D. it will only be 2320 RPM. These should be safe for a good running Flathead which holds good oil pressure and is not overheating. len47merc 06-30-2015 @ 4:04 AM reply profile send p.m. Senior Posts: 1165 Joined: Oct 2013 Tom - a number of assumptions have to be clarified for any of us to give you an absolute answer to your question. For example, the actual percent reduction of your overdrive (perhaps '51s only had one option - ?), the actual ground-to-center-of-hub radius of your properly inflated tires, is the rear end actually 4.11 (you indicated "...I think..."), which gear set does your transmission actually have, etc., etc.. The best answer, if you are interested in making the very minor effort to obtain the 'actuals' (and do not wish to install a permanent tach), is for you to take an old-school inexpensive engine analyzer, cut the wires if necessary and extend them 3-4 feet, reattach some alligator clips and run the wire either through the firewall or, as I did the second go on this when changing to a taller tire, simply out the vent window and in the side of the closed hood, and properly connect them to the coil/engine. Have someone ride with you and take readings at GPS confirmed speeds of 60-65 mph. This will give you the best info you are looking for. (Note - if the engine analyzer is really as old school as the one I modified to do this and has a pointer-needle indicator, be sure your assistant keeps it vertical to take the reading as the weight of the needle is designed into the unit and laying the unit down flat will produce a higher-than-actual reading - for what it's worth) I suspect as JTFord and carcrazy stated you will find, if you are running an assumed 4.10 with a 33% reduction OD and 670s (albeit radials - smaller OD/radius) you likely will be in the 2,150-2,400 rpm range at those actual speeds. Let an inexpensive engine analyzer tach confirm the actual for you - this will give you peace of mind in knowing you are both operating your car at a safe-for-long-distance rpm range and no prior owners have changed gear ratios (trans, rear &/or OD) that may have you operating much higher rpms than you would otherwise think. I run bias ply 710s and the speedo is off of actual (reads slower) by almost exactly 10% through the gauge's range (i.e., at 60 mph speedo reading the GPS shows 65-66 mph). I try to operate my stock '47 with 3.78 rear gears and no OD at a max 55 speedo reading - 60 actual per GPS - which produces ~2,500 rpm, the ~optimum torque rpm for the 59AB motor, and have no problems or concerns running it for long distances at this rpm. The '47 was driven 165 miles each way at this speed to and from the Eastern Regional in Charlotte a couple of weeks back, the return in 95+ degree temps, and the engine performed fine. You should be fine for continuous 150 mile runs (300 miles round trip) at whatever rpm in the low-to-mid 2,000s you find you actually are running at. If you find you are actually running in the upper 2,000 rpm range (doubtful but worth the effort to find out) I'd look at either slowing my cruising speed or sleuthing out where in the gear train your assumption base is off and make the necessary changes. Steve This message was edited by len47merc on 6-30-15 @ 5:57 AM tomlud 06-30-2015 @ 5:08 PM reply profile send p.m. Member Posts: 23 Joined: Jan 2011 Thanks guys for your feedback. I always appreciate the knowledge on this forum from guys like you.

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