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Does anyone know what the axle ratio might be on a 73 F250? If I read it right, the tag said, 24J. I ask because at highway speeds it feels like the RPMs are way up there. I've considered doing some work to lower that since I don't use the truck these days for hauling or towing. I would just like to enjoy some casual driving and not have the engine working so hard. I appreciate any assistance!
Which tag are you looking at? The door tag, or the tag on the differential cover? If the differential cover tag is still present, it will probably list the ring & pinion ratio. Is your truck 4wd? If so, I'm sure you know you'll need to change ratio's up front also.
I'm sure that RPMs seem to be high with 4.10s today after maybe years of higher geared vehicles, with overdrives. Back then, not so much. They just woke things up in high gear at highway speeds. Gas was not really cheap, but the numbers sounded less. I knew a guy with 396 /375 in a '69 Chevelle would drive from Coeburn to Lynchburg and he was pretty quick about it until he got the fat RD ticket. He run 4.56s.
I figure your F-250 has a Dana 60 series rear, and like said ... if it's 4wd it's gonna need to swap both Ft & Rr axle ratios. My FIL's '77 F-250 had the 4.10s and a 4 spd, was a 400, it was thirsty, but was OK out on the road ... but limits were 55 and about all his use was off interstates ... and they too had 55 mph limits. If yours is 2wd, a single re-gear of the rear axle or an OD install would lower RPMs. I know that Dana 60s can have like 3.54 gears, not sure if lower is available? Changing that 4.10 to a 3.54 will maybe lower RPMs by 400 or 450 at 60 mph from your likely near 2,950 to 2,500.
Cheaper trick is to up your tire size, I had 4.10's in my 79 F250 with 33's and a 300, was perfect, got 18mpg when I drove it 1800 miles to my BIL's in MO.. 35's would probably set you just about right IMO.
With so many things making noise on these trucks, never go by sound alone. Exhaust, cooling fans, wind noise, road noise (different with different tires) all play a part.
So either get a tachometer, even a temporary one, or use one of the online calculators to determine your ratio vs speed vs engine speed.
For example, with the 4.10 ratio (assuming it was never changed by any PO's) and tires that are actually 31.5" tall, your engine speed at 65mph with a manual transmission would be just above 2800 rpm.
With an automatic typical of the day (3-speed w/o overdrive) you'd be just over 2900 rpm or so.
Neither of those engine speeds is worthy of concern. At least not for the engine... For your sanity and wallet in the gas stations, maybe you can worry!
But nothing wrong with cruising along at 2850 rpm or so from the engine's standpoint.
But also, if you have not already, check your speedometer. If you're not running stock size tires anymore, your speedo would be off somewhat.
Verify with a GPS app on your phone, or a known good speedo in another car.
X2 on a taller tires helps with eng RPM at cruise speed. had some 265's on some 16" rims and went back up to 33's on some 16.5 rims. Sure helped my 4wd 77 F250 351M auto.
