High rpm for mph
 

Hello, I'm just finishing up a project started by someone else, and has been sitting in storage for 6 years. It's a 57 chev pu on a shortend 76 suburban frame. It has a rebuilt 350, rebuilt th350 with shiftkit and stock torque converter, backed by a np205 transfer case, rebuilt 14 bolt rear with locker and rebuilt dana 60 front with posi running 4.56 gears. The problem is that now I've got it to where I can run it on the road I've found that it is turning 3000 rpm and going just 41 or 42 mph. Please tell me if I'm not right on this but with tires (285/75/16), a np205 in hi range running 1.00 ratio and a th350 in 3rd gear running 1.00 ratio I should be going roughly 60mph. No overdrive and no underdrive should be running straight 4.56 final right? I am sure it's not in lo range cause I feel the difference between lo and hi. And I know it's shifting into 3rd gear both manually and automatically. My timing is 8btdc and my plugs are showing perfect for fuel mixture. Starts and runs great other than high rpms for mph, and still runs warm some, but think that's still just from being rebuilt. Could you give some ideas of what to check or could possibly be the problem. Everything seems to work like it should, kickdown, shifting, fluid level, etc. All input would be greatly appreciated. Thanks

Re: High rpm for mph
 

This may or may not be of any help to you other than to confirm whether or not you have an issue to resolve...

But, my truck has a 350, SM465 4-spd (1.00 ratio in 4th gear) coupled with NP205 T-case (same as you). I am running 35" BFG Mud Terrain HM2's pushed by 3.73 gears. My truck turns just about dead on 2400 rpm at 60 mph... About 1800-1900 rpm at 40 mph.

You are definitely geared lower than I am with rubber and gears... But a 1000+ RPM difference at 40 mph?? That seems like quite a difference...

I am no transmission expert.. But just based on seeing others get help from those who know their stuff when it comes to transmissions, I would be wondering if your converter is is failing to lock up in 3rd????

I may be able to work some numbers into a spreadsheet a member hooked me up with to see what RPM you should be turning on paper....

Re: High rpm for mph
 

That sounds about right. Your tires are too small for your gear ratio. I run 39.5 with 4.56. I would suggest 35" tires or go down to 3.73 or 4.10.

Re: High rpm for mph
 

Something seems off... I put in all of your gearing and tire specs, and according to the spreadsheet you should be turning 1867 RPM at 40mph in 3rd gear. However, in 2nd gear it shows 2837 rpm at 40 mph.

Possible I didn't plug some info in correctly, but I went over it twice and got the same result.

According to the sheet, you should be right at 2800 RPM at 60 MPH. Your tire size translates to 32.83" in height. So, 33's.... Mathematically, the spreadsheet makes sense to me...

Are you sure you have the correct ratio adapter for your speedo??? Have you verified your speed with a GPS or something similar?

I had no ratio adapter at all in mine and my speedo read right about 45mph when I was actually doing 60mph according to a GPS speedo app on my phone.

Re: High rpm for mph
 

Red ride, yes I am using a gps speedometer app on my phone and using a tom tom navigation just to verify. And both are spot on reading speed on our family vehicles. So I know that speed reading is correct.

Re: High rpm for mph
 

Copy that.

Still seems like that is sky high for RPM..even with your gearing and tire size, as in I think something might be wrong.

I will leave any further advice to the transmission experts here. I don't want to risk passing bad advice due to taking a less-informed guess.

Re: High rpm for mph
 

My '63 with 35's would turn 3k at 70. I ran it like that until I put in the 4500. Then it dropped down around 2400- 2500. I think that the 400 was on its last leg and slipping as well.

Re: High rpm for mph
 

Thanks for helping guys. Found the problem was the tach wasn't correct. Reading as a 6 cyl. Now mph-rpm are correlating. Thanks for input.

Re: High rpm for mph
 

Glad you got it sorted out bud. :)