Just wondering if a few of you guys with different gear ratios would mind dropping a line on how many RPMs your running at 70mph - My 03 V-10 has 3.73s and the tires are 285/75 R16s and runs 2000 @ 72mph - this is just for comparison my dad has a 96' duely with 460 and an auto and I beleive his stickers says he has 410s or so and he runs around 3000 @ 70mph. Just want a comparison on the Super Dutys with say the 410s and 456s. I was thinking that I might have been happier with the 410s but don't like running 3000 down the hwy.

 

Mine runs about 2500 at 70 mph, and I have the 410's. wish I had the 373's though.

 

RPM = (MPH x AxleRatio x TransRatio x 336) / TireDiameter.

The engine speed difference between a truck with 3.73's and an otherwise equal truck with 4.10's is about 200 rpm's.

Also note, your speedo is probably off due to the change in tire size, unless you have corrected it. Your truck's actual speed is higher than what is shown on the speedo if the correction has not been made.

Your dad's truck may be cruising at 3000 rpm's at 70 mph due to no overdrive gear in the transmission.

I run 4.56's with 33's and a C6 tranny (no OD and the converter does not lock) and turn about 3000 rpms at 60 mph. I don't drive much at 70 mph, but if I did, my engine would be spinning about 3400 rpms. If I had an E4OD with a 0.71 OD ratio and a lock-up converter, my rpms at 60 mph would be about 2000, and at 70 mph would be about 2400.

With 3.73 gears and 285/75-16 tires, your effective gear ratio is about 3.60 and your speedo should be off by about 5%.

 

I was aware of my spedo difference and since this has come up, do i have to take it to my dealer to get it changed so its right, and how much will it cost?

 

Not sure on the newer trucks, but on the '92-'97 trucks there is a way to reprogram the speedo yourself. You can read about it in the 'tech articles' section of this site.

I left mine unchanged with my 33" tires because I can calculate in my head about how much it's off as I'm driving (about 5%, like yours) and I dont drive so fast as to worry about speed traps much anyways. The mileage difference on the odometer is so small that even after 100,000 miles of driving with it inaccurate, the total mileage will only be off by about 5000 miles. I can handle that.

I only mentioned it to you so that you would be aware that any gear/tire/rpm calculations and/or comparisons you make would be off if you used an inaccurate speedo reading as part of the calculation or comparision.