It could be significant a 25 inch wheel travels approx. 78.5" inches per revolution, where as a 30" wheel travels approximately 94.2" per revolution. That is almost 20% increase. I will have to try a few things, I believe the speedometer can be reset. Its made by dolphin gauges. Right now I think it is close to correct.

 

Overdrive cars usually got a higher numerical rear axle ratio. I'd bet it's more like a 4.11.

You may be time and money ahead to start from scratch with a more modern axle, like a Dana out of a Jeep Wrangler. You may have to narrow it, but it is not real likely you'll find a late 40's/early 50's axle that is in cherry shape. There is excellent aftermarket support for the Dana axles. It also isn't as heavy as a 9" Ford, which would ride like a cement mixer in your light car.

 

The rear I started with came out of a 1950 Ford with OD. I guess it was a 4:11. I then installed a ring and pinion out of a 1951 ford-o-matic. It helped a lot, was told its a 3:70, but the book shows no 3:70 It could be a 3:89. I think there is a 3:30, and that would cut the RPM down. I suffer form always trying to gear everything too low, I end up buying lots of gears. BY the way, I'm in Ruidoso.

 

I'm presuming that you mean "12.5 in." is the actual radius of the wheel/tire, so that would make the rolling distance approx. 39.290 inches plus or minus. (circumference = pie are squared)

Your right about the high rpm, those gears were not made for modern roads. I like to run a motor just a little above peak torque not high revving
.

 

Your tach or your spedo or both must be a fair bit off. By calc for rear axle ratio based on your readings: (assuming 1:1 high gear tranny)
@30 MPH RAR= 4.46:1
@40 RAR= 4.09
@50 RAR= 3.86

It would be pretty simple tho to check the actual rear end ratio. Simply jack up the center of the rear end until both wheels are off the ground, mark a spot on the tire and the driveshaft. With the car out of gear, turn the tire exactly one full turn while counting the driveshaft rotations. If the driveshaft turns slightly more than 4 turns, the RAR is 4.10, if it turns slightly less than 4 turns then the RAR is 3.89, If it turns 3 1/2 turns then the RAR is 3.50, etc etc etc. Then using a correct speedometer, either a calibrated one in the car or a gps based app on your smart phone, you can calculate the actual RPM at any speed.

 

I think you mean pi X diameter and n this case 3.14 X 25= 78.5 inches per revolution.


I do not like to hear an engine run too fast. I would like to cruise this roadster a 70 mph.

 

according to this calculator (plug in any 3 specs calculate the 4th) Tire Size, RPM, Speed, and Differential Ratio Calculator
you could do 70 mph at 3500 RPM with a 3.72 RAR
or 70 MPH at 3000 with 3.2 RAR

 

I feel your pain about gearing. When you get right down to it, the problem is only having 3 gears in the trans. With all the long drives in NM, a highway gear would be welcome. But then you hit those canyons down your way, and if you have to downshift coming up out of them, you're stuck going 45 or so. A gearbox with more choices is the best answer. With your light weight, and lower wind resistance, you can get by with a little taller gear than a truck, but you'll still need to compromise. For our altitude, I'd shoot for 2300 - 2400 RPM at 60. That's a good efficient operating point.

 

AX, good information. I was reading where max torque is 196 ft lbs at 2000 rpm. max hp is 110 at 3800 rpm. The chart one member posted showed a 330.1 ring and pinion set being made for the 4209 rear. That setup would 47 mph at 2100 rpm. Not too bad.


I will be leaving New Mexico for Florida in 5 weeks. I will return in May, 2016. At that time I will try to locate a lower ratio. I will contact Numbers Dummy and see if he can guide me. He may know where I can buy a set of gears.


I'm pretty sure I have the 389.1 rear, and I just wasted money. I am also going to have the car weighed. I can cruise all around at 15 mph smooth as glass. The engine has torque like a tractor. The Edelbrock 500 CFM carb is working out better than I expected. I have a Bubba's distributor due here in a week or so. The centrifugal advance is supposed to be a big improvement. We shall see. Later mods will include a warmer cam and a set of aluminum heads.




Off the subject, In 1955 I had a 46 ford convertible that would do 100 mph. I know the engine was screaming, I now wonder what the rpm was, but it would do it on the speedometer.

 

It looks like a 330.1 is the tallest gears they made for my differential. That old be close to what you recommend. The overdrive that was in the 50 ford is still available, I just don't want to do all that work.