First time poster here.
I could use some expert opinions. My 1997 f150 has the 4.2 motor and over sized tires....265/70/16 that the previous owner put on. I have had the truck for 8 months and it's been very reliable. I don't keep more than about 250lbs in the bed and don't have a passenger with me. I have used several internet resources and have learned that these tires should only add about 2 to 3 mph to my speed at 60 or so. My speedometer is off by 20%. So 40 on the speedometer is actually 50, and 60 is actually 72, and so on. I am aware that the computer should be calibrated and this would be over. But that is not the end of the story.

The real issue is this: Engine RPM is low at cruising speeds. Other posts indicate that "normal" is 1900 rpm @ 70 MPH. If I cruise at 1900 rpm (which I never do) my actual speed is 80 MPH. When I cruise at 70 MPH, my rpm is 1700. I have the 3.08 rear and 4r70w transmission which has a final ratio of 0.70, tire diameter is 30.16. When I plug these numbers into a spreadsheet, everything is consistent with what I experience on the road.

When my speed is 50 my rpm is only 1200, so I am not near the engine's power band. I give it throttle and experience no acceleration. Give it more, and it comes out of lock up and I get some response. A little more and it comes out of overdrive and starts to accelerate. Cruising at 60 to 65 on the interstate is a continual in and out of lock up to keep the speed up, and often out of 4th to accelerate.

I own a set of 3.73 gears that were not used in my weekend toy. I am contemplating installing them in this truck. Spreadsheet results indicate that at 50, my rpm would be 1450. At 60, my rpm would be 1750. At 70, my rpm would be 2050. That would certainly move me closer to the powerband when cruising and give a little nicer throttle response. I average about 13 mpg and have read both opinions......higher rpm means more fuel consumed; or higher rpm means more efficient under load.

What do you all think? Do others of you experience the same thing??

Thanks.

 

Well if you have them,,,,put them in.

Sounds to me like the 3.73s would work great for you. You'll have better acceleration and it wont hurt your mileage much at all.

I run 4.10s with stock size tire and at 70 the RPMs are about 2500.
When I run my 23" wheels (about 32" tall tire ) my RPMs are about 2200 at 70 MPH.

Oh,,and welcome to the site.

 

Thanks for the input. You are confirming what I expect will be the result. Will changing the ring and pinion effect the speedometer? That would depend on what triggers the sensor. Does anyone know how the speed sensor works? If I understand correctly, it is on the differential. What rotation does it sense? The ring gear, the pinion, the drive shaft, one of the axle shafts, or what? I doubt that my gears have any sort of trigger on them.........I wouldn't want to disable the speed sensor in the process. I am perplexed about how my speedometer can be so far off in the first place. If it was re-calibrated in the past for wheels, they would have had to be enormous!