transmission swap

[5500 GMC]

doing a transmission swap and when checking out the calculations for engine RPM to MPH, I am having a problem with the results, here is why
currently the rig has a V6 with a NP 540 5 speed with a 5th gear ratio of 1.00 with 39.5 in dia tires and at 3400 RPM its goes aprox 65 MPH
the calculations I keep coming up with show it would be going aprox 120 MPH ??
does anyone know why this could be
planning on swapping out 5 speed manual to a 4 speed automatic with 4th gear being .75 to 1, this would according to calculations be doing 72 MPH at 1500 RPM is that even possible with a 2000 RPM stall converter
Thoughts anyone
thanks

[Ed Snyder]

Originally Posted by 5500 GMC

doing a transmission swap and when checking out the calculations for engine RPM to MPH, I am having a problem with the results, here is why
currently the rig has a V6 with a NP 540 5 speed with a 5th gear ratio of 1.00 with 39.5 in dia tires and at 3400 RPM its goes aprox 65 MPH
the calculations I keep coming up with show it would be going aprox 120 MPH ??
does anyone know why this could be

Thoughts anyone
thanks

There are various calculators available on the internet, including this one:
http://www.csgnetwork.com/multirpmcalc.html
You'll need to know your ring and pinion ratio though.

[5500 GMC]

thanks, I have tried that one and it also calculates out to about 1800 RPM at 60 MPH
and I know the current set up is doing 3400 RPM at 60 MPH not 1800 as the calculator shows
rear end is 3.20 to 1

[GMC_Dude]

A couple of thoughts. If the online calculators give you an answer you know to be wrong the most likely reason is bad inputs. Given the inputs you've listed, I suspect your ring and pinion ratio is way off. On A 5500 GMC truck I would expect the ratio to be more in the 6.10 range. Does this truck have a two speed rear end? That needs to be factored in as well.

Darrell Hein
San Antonio

[James]

With a 6.10:1 rear axle, 39.5" tires, and a .75:1 transmission (in 4th) and engine at 2550 rpm you should get about 65 mph. I think this is doable. You are only dropping approx. 850 rpm for the same mph. You will loose some pulling power in 4th.

[5500 GMC]

Thanks for the info !
GMC guy got me thinking and i re-figured the ratio by turning the wheel 10 times not 1 and doubling the drive line revolutions and came up with 6.20 as a rear end gear ratio
works out to 63.18 MPH at 2500 RPM
I do not anticipate there being a problem with pulling power, the new engine( 383 V8 452 ftlbs) has almost 100 lbs more torque than the existing one(351C V6 318 ftlbs) at 2500 RPM
new transmission 4L80e
thanks again for helping me to see the light at the end of the drive shaft