Even without all the proper info, here's my take on it. If the car is set up properly, the gear will be the same. I run both every year, with my primary racing done on the 1/8. I have never changed gears for one or the other, and I am, well, lets just say,"competetive"
You can run the same gear/tire in the quarter and 8th and you will get the 8th mile split on your time slips. However, to maximize your 8th mile potential requires trapping out at half track instead of running out to the quarter mile point. Most quarter mile cars shift into their 1:1 ratio before the 8th mile mark and just run out the last half track. For the 8th mile you have back that way up so you are trapping out at half track. To do that requires going about a full number larger on the gear. For example, if you run 4.11's for the quarter you would want a 5.11 or there abouts. As an alternative or in combination you can go to a shorter tire. In that example if you ran 4.11's with a 28 inch tire you could run say 4.56's and go to a 26 inch diameter tire. These are just my close guess/examples. To know exactly for sure requires knowing your trap rpms, mph and the size gears and tires you are running in the quarter. Take that info to drag strip plus or drag analyzer and see what it does.
Bob, I have a mild Cleveland and run the 1/8th mile when I get a chance. I like 5.14's. This was with about 28 inch tire. I would shift about 6k and go through the traps a little over. I have also tried a set of 4.56 with a taller tire. Like switching to 4.11's with the 28" tires. My car slowed down. Hope this helps.
I RAN A 5.14 WITH A 29" TIRE IN MY '67 MUSTANG WITH A 4-V 351C USING 2 660 CENTER SQUIRTERS ATOP AN OFFENHOUSER TUNNEL RAM. I SHIFTED ABOUT 7400 AND CROSSED THE TRAPS AROUND 6800... KEEPING THE RPM'S UP SEEMED REALLY IMPORTANTWITH THE 4V HEADS! THIS WAS A STREET CAR THAT RAN 7.60'S WITH A STOCK SHIFTER ON A TOPLOADER..PRETTY CRAPPY BUT...IT'S ALL I HAD.
Going with more gear will no always make a car faster. Example, one of my buddies I race with every week was running mid 6.30's in the 1/8 with 1.38 avg 60' times. He was running a 5.83 gear. He decided he wanted to go 1/4 racing so he took the .83's out and installed a 5.13. The car ran the exact same et, with the same 60' and actually ran 1 mph more. If the car has too much gear, it will accelerate out of its powerband too quickly and not use the HP to move the car. I run a 4.86 in my car using a 4v cleveland. No way would I ever dream about putting anything more than a 5.14 in it, and only then if I were to put a taller (like a 33") tire on it. It would slow the car down. 1/8 mile it goes thru the lights about 64-6500 around 107 mph, 1/4 it goes thru about 72-7300 around 132 mph. If the convertor matches the combo, the same gear will be perfect for either length of racing.
I run 5.43 rear but with 32in tires with a 1.76 PG with 5300 converter. 466 in engine.I only run eighth mile.No interest at all to run qtr. More danger (higher speeds) winding the engine that last half makes me see dollar marks flying away. Leave at 3400, shift at 6000, cross line at 6800. Best ET 5.98 with 60 ft of 1.28.