OD. thats the hose size id.

I replaced my old lines when I restored the car but I still have the 3/8 line and sending unit. I don't know why but I kept all the old parts I could.

B1MAXX, you are correct, learned something today.
So we're better off with a 3/8" flex fuel hose with a larger I.D.??

Joe

I have 4 or 5 old original pumps, I know one is off a 68 GTX, all are 5/16” non Carter pumps.

possibly, but in my experience the pumps were the limiting factor from the factory with 3/8 steel line, except maybe the hemi pump...I never had one of those. My stock 440 pump would cut in and out under full throttle bonzai blasts.

After finding my original fuel line from my car, I found that it is 5/16. I also measured the original fuel line on my 70 hemi charger and it is 5/16 also. I personally think that both sizes were used depending possibly on what was available at the time.

The outside diameter of the stock fuel lines are not what the insides are, a 5/16 O.D. may be less than .300 I.D., less around corners
I try and use a 1/2 O.D. thin wall steel tube for my half way serious race cars, a lot larger than needed is way better than a tiny bit to small
If I can't fatten the mixture up enough to slow the MPG down in the 1/4 mile I don't have enough fuel delivery
My new heads up no prep car will have #12 AN from the fuel tank to the crank driven mechanical race pump on the front of the gear drive for the centrifugal superchargers and other accessory needed to be completive

I don't buy this one bit. Maybe the 69.but not the 70.
I should clarify.... in regard to the tank to pump.