Peterbilt frame

All of it! Tape measure works best. Unlike passenger car or pickup, all trucks are custom built. Frame length has way more variables than model and wheelbase. On newer trucks, you can find overall frame length thru parts system. Last six of VIN and I will check if yours is listed.

 

Yeah unfortunately there's a couple factors in play that could affect the total length. Amount of overhang at the tail end is one. Some trucks its short, others it could stick a few feet past the back of the tires. All depends on what job the truck was spec'd out to do.

 

Add the extra length of the wheel base change to overall length of the frame you have now and you will be good. So just add 20” to whatever you have now. That should keep overhang around the same.
Just keep in mind that the single axle wheel base measures different. Front Axle hub center to rear axle hub center. Mark that out to make sure your rear overhang ends up being what you want. Add or subtract to overall length to adjust that.

 

Sorry. Disregard the single axle measurement part for your app. I read single in your reply and brain farted. Took it as single axle not single rail.
All other measurements still apply.