i have a 52 ford f3 with a 215 ohv 6. it has a 9" ford rearend with a 3.25 ratio. im wondering what a safe cruising rpm would be? im thinking that 30 inch tall tires would be the best bet. its no hot rod but i would like to drive it a bit and possibly cruise at 60 mph

 

With a 3.25 rear gear ratio and 30" tall tires, you should have no problem at all cruising at 60+ mph.

 

I'm surprised the 215 can pull those gears with those tires. I had a '53 F-100 with the 215 back in the day, with narrow 16" tires, stock gears, in the days of the 55 mph limit. It didn't have much left at 55. Your truck must weigh several hundred pounds more.

2500 is an accepted safe sustained RPM for just about any engine.

 

yeah i figured 2500 would be safe. the original wheels and tires were 17" 31"tall. the original rearend was a 4.86 (stump puller). i tried a 4'10 and it was still pretty tight then i got this 9" ford 3.25 and tried it with 28 inch tall tires. much better but still pretty tight. next week i will get some 30 inch rubber and try that. i just want to be able to keep up with traffic as i plan on using the truck a bit. i converted the front using f1 backing plates, hubs and drums (yeah i got them real cheap).

 

What exactly is "still pretty tight"? Over the years, we as drivers have been conditioned to the quiet sounds of freeway driving with overdrive transmissions and high gears that allow new engines to basically idle at 70mph, so we can get 25mpg from them. We forget that back in the 60's, it was commonplace to turn engines 2500-3000rpms at freeway speeds, and there were no ill effects. Those kind of engine speeds sound horrendous to us today. It's not that it's a problem, just that we're no longer accustomed to the sound.

I don't have my calculator handy, but it seems like the difference in rpm's you'll see between a 28 and 30 inch tall tire would be minimal, maybe 100 or so, and not worth the expense, imho, if that's your only reason for the change.

 

With 28", 60 mph = 2,341
With 30", 60 mph = 2,185
difference = 150 RPM

You're exactly right, Wayne, unless there is another set of wheels and tires laying around, it isn't worth changing. You're correct about the Old Days, but the other side of that coin is that with only a 100 HP 215 pushing a big F-3 thru the wind and especially dragging it up a hill, you need to be spinning it. Too little RPM and you'll just be driving with the power valve open all the time.