I am building a 37 Buick Coupe and am getting ready to order my rear end next week. I am putting a '65 nailhead 401 in it. What rpm should I aim for in the 65-70 mph range? I intend to drive this a lot including freeway driving. I was thinking 2000 rpm at 70 mph? (I will be using a T56 6 speed.)
t56 is double overdrive .50-.63 overdrive. 2000 rpm 3.90 gears .6 overdrive 28 inch tire = 71 mph https://spicerparts.com/calculators/transmission-ratio-rpm-calculator
Be prepared to suffer large gasoline bills.......these Nailheads are guzzlers....and need 92+ octane. My '35 with a 401 and 3.70 gears gets about 10 MPG on the highway.
(Target RPM x Tire Height) / (MPH x 336) = Final Ratio needed. So (2000 x 28) / (70 x 336) = 2.12 and to get 2.12 with a 0.50 sixth gear you need 4.24ish gears.
I used the American Powertrain website calculator. I will have 27" tall tires. I was more interested in people's experiences with nailheads as to where they like to run rpm wise at cruising speeds.
Where it likes to run depends on a lot of things. An important one is cam choice. You go too low with RPM you can hurt your fuel mileage. With the 6 speed you have some extra options. With what you are suggesting I doubt you will use 6th gear very often.
I run a T400 sitch pitch behind my 401 with a 2.79 rea gears. Its really good on gas and has lots of get up and go. Around town though it runs in 2nd until I reach 30 mph.
Nailheads are torquey and with max torque at a low 2800 it must also be pretty strong at 2000. I’m not sure what the transmission ratio is in 6th but if it is .63 I think a 401 would be OK with a 3.70 gear. Looks like .63 OD/ 2000 RPM/3.70/27” tire is 69 mph. My only question is about the tire size, I’m thinking you might want a bigger tire to fill the fender on a ‘37 Buick, do you already have the tires? You might want to hold off on ordering a gear if you might be changing tire diameter. Spending time on the distributor advance curve and also using vacuum advance will pay dividends for your fuel mileage so invest some time on the tune up.
We have been trying different tire sizes for weeks. The fronts were the biggest issue as we wanted the biggest tire we could get but there is actually not that much room inside the fender. I refused to widen the fender as I want to keep a stock body. To get best turn radius the front will be 215/50 18. That was after mocking up every offset we could think of and still have the wheel centered in the fender.
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