I have a 1989 Ford F-350 Crew Cab 4x4. I have about 7,500 miles on my new engine. I use Mobil 1. A couple years back when I bought the truck, it had 35 inch tires on it. At 55 mph my truck runs 2,700 RPM's with my new 33 inch tires. The truck stock from the factory came with 4/10 ratio, I imagine the original owner changed this to accomidate the 35's. When I had the 35's on, my RPM's were 2,100 at 55 mph. Is 2,700 and 3,000 RPM at 65 mph way past the norm? I realize that is probally uses this much gas running this high. Is it bad for the engine? I have to take the cover off the rear to check out the ring and pinon that is in there now? It is probally going to be cheaper going back to buy 35 inch tires than it would to change the front and rear pinions? I would really appreciate it if you could give me a couple of answers and opinions from the experts out there.
-Thank you

 

2,700 rpm is not too high for the highway. Neither is 3,000 rpm. While it's not that great for mileage, it won't hurt the engine. I have a '70 F100 with a pre '70 390 from a car. My 390 was built to run, and it runs great on the highway with 3.50 gears, 31.0 x 10.50 tires and cherry bombs @ 2,300 rpm. Before I put my 31's on, I had 235x75x15's and it turned 2,700 rpm.

Jason

 

In my 72 F250 I have a 460/C6 and 4.10s. With 265x75R16 tires basically 31's. My engine turns 3000 at 65... Perfect for towing with a 3 speed tranny...

 

My 97 F250 hd has 4.10's and 33's. My rpm's are 2100 at 65 and 2800 at 85. I have the 460/E4OD though....

 

Update: Tonight I took the cover off the rear, it is stamped Ford 410 E5TWDA 10418JA , so I am assuming that it is a 410, correct? Now, my truck from the factory originally would come with the standard tires, that are no bigger than 30 inch. This would mean that the RPM's would go even higher, and my speed dometer is correct, this was already checked with another vechile with stock settings. With the 35's the RPM's were around 2100 at 55 MPH. With the 33's the RPM's are 2700 at 55 MPH. I am amazed due to the fact this truck has a 410 rear with it from the factory, has a 3 Speed C6 Auto-Transmission. If the truck came stock from the factory with 265 R16 tires, with the 410 gears, this would have to drive the RPM's around 3000, or 3500, which sounds crazy. This is what I have found, and these are what the results are. So everybody else with the 410 rear stock tires, I have been reading in many posts that their RPM's are so much lower, how could this be?
These are the facts about my trucks. Again, if any experts out there could give me any information, it would be greatly appreciated.
-Thanks

 

I have a 1988 F350 4X4 1 Ton with a EFI 460, and a C6 3 speed auto tranny. My truck is stock, and at 55 I run at about 2500 rpm. I am told it is normal, and it won't hurt the engine, other than bad gas milage. At 65 I am turning at about 2800 - 3000 rpm. I don't know if this helps, but my tires are the 265x75x16. Sorry I couldn't help you out more. But your not alone.

 

I drive an 86 F250, 460 4speed, I don't know what my rear end gearing is, but I run 2500 rpm @ 65mph. I don't recomend running any more than that, (I'm no expert, just an opinion). My truck has 300,000 miles on it, I think. When I bought it the odometer said 50,000, and a few hundred. I found a reciept in it for tires purchased at 175,000mi. So now the odometer says 4000, plus a few hundred so If it isn't 300,000 it's still a lot. I've been putting about 30,000 a year on old 460s, (I pull a trailer that wieghs 6,000lbs), and I find that even an old worn out Ford will just keep on going if you keep it at around 2500 rpm.

 

I regularly cruise between 3-3500 rpms at 60-75 mph in my Bronco (specs below) and haven't had much of a problem.

 

Thanks to all who replied , I realize i guess that without an overdrive that is why the rpms are so high. So either i go back to 35 inch tires, or put in 3.55 or 3. 73 ring and pinions in, because I have the 3 speed trans . I have the 10 inch limited slip rear , How much would the front and rear gears cost only, If I did the job myself? Verses Buying four 35 inch tires? And i guess upgrading to a 4 speed trans would be out of the question I do Know people that have had big problems with those e4od transmissions. Thanks to those again who replied.

 

 

Gears for the 10.25" Sterling run about $250 a set. Some say change the pinion and carrier bearings at the same time. Gear change for the front and rear will run you about $1200 is my thoughts.