I've got a 97 3/4 ton 360 5 speed (nv4500) lifted 5 inches on 35s. I've got 3.55 gears now, my tach says I'm turning about 1900-2000 around 70 mph, not exactly sure about how far off my speedo is but the speedo is at 63 or so when it's at 1900 or so. I'm wanting lower gears, can't decide between 4.56 or 4.88. I used an online ratio/ rpm calculator and it said I should be turning 1760 with the 3.55s now and with 4.88s I'll only be turning 2460 at 70 mph, does that sound right? I didn't think I'd need to go that low, I do quite a bit of towing and that's about the rpm I want at that speed but I thought 4.88s would be a much higher rpm than that

 

Yes, those rpms come out in that ballpark for the calculator of 3.55s and 4.88s. However, I entered in 35 for the tire size, whereas your tire size is less than that.

http://www.grimmjeeper.com/gears.html


Measure your actual tire size and it will be like ~34.5 or sometimes smaller based on wear and brand. That should account for the 200ish rpm difference of what you are reading from the gauges to the calculator. Don't expect to match exactly.

 

Your rpms seem a little odd to me. You appear to be revving pretty high for having 3.55 and 35s. With stock gears and larger tires your rpms normally drop quite a bit. I was revving at 1500 with 3.55 and 35s. With 4.56 and 35s its like 2000ish at 65mph for me. I'm not familiar with the 5 speed rpm ratios so maybe that's normal.

 

I'm running a 325/60/20 nitto terra grappler, they measure out right at 35 where they're at now. My tach might not be exactly right either, also like I said. I haven't checked to see how far off my speedo is yet, it could be 10-15 mph off. It's around 1900 when the speedo says 63 or so. I figured with the 4.88s it would be touchin 3 grand or a bit higher

 

On mine, (speedo reasonably accurate.) at 2000 RPM, in O/D, I am doing about 70mph. You need to find out how far off your speedo is..... My guess is, quite a bit.

 

Figured my speedo out tonight, bout 8 mph off