My Chevy Express is a Class-B motorhome conversion that started life out as a conventional 155" wheelbase cargo van that Roadtrek extensively reworked, including lengthening 16" by adding on additional frame and building a a whole new fiberglass body integrating some elements from the original van body from the B-pillar rearward on the driver's side, and in the side-door structure on the passenger's side. This new body is wider than the stock van body, with wheel wells that are around 17" wide from the leafsprings to the lip of the arch. The van is also very close to max GVWR.



As brought home April 2026


The little 16" wheels and LT245/75 tires look a little silly in the back, inset quite far, and there's just enough mass overhanging past the wheels to give some pause as to stability of the vehicle. As such I've been looking at options for a dually conversion. Unfortunately the rear axle in my van is not a true full-floater, it looks like despite what the RPO codes say I should have, there's a Dana 60S in 3.73 ratio.


17" clearance body to lip of wheel arch

36" height ground to lip of wheel arch


Obviously there are some real full-floater SRW axles that will fit the van just as-is, but their bearings are still fairly close to each other. The dually axles look like they are wider and have bearings that are further apart from each other, which should be a little better during those moments where an uneven road might load one tire in a pair more than the other. Trouble is, I'm having difficulty finding good information on axle dimensions for the GMT-610 platform. I found an upfitter's guide that GM produced and it gives a bunch of info on frames, mounting points, even wiring and exhaust and spare tire mounting, but nothing on how wide the leafsprings are or where the shock absorbers mount on the axle.

I took some measurements of my van and came up with the following:



Dana 60S dimensions in 2005 Express 3500. Spring perches are 50.5" on center, 48" inside to inside, 53" outside to outside.

On the SRW 3500 it looks like the axle perches are 50.5" on-center, 48" inside to inside, 53" outside to outside, for 2.5" wide leafsprings. The shock mounts are around 34.25" outside to outside measured on the outside, and 30.5" inside to inside, measured on the shock-side of the tab. The shock mounts should be considered approximate, the tape measure had to stretch around the differential case.

If anyone can contribute these measurements for a dually axle for a cab-and-chassis or a commercial-cutaway it would be most helpful. I might end up going junkyarding to see what I can find regardless but if these dimensions are already known it would save a lot of time and might help others down the road.

For what it's worth I know that dually tires would stick out a little past the wheel arches and would require some kind of fender extension in the back. This doesn't bother me, there are motorhome and travel trailer fender extensions that will do that once I have the rest of it sorted. I'd probably also look for the thinnest dually wheel setup that I can find, perhaps something in the 5.5" width range, to have as little stickout as possible.

 

Take a look at this:
BillaVista.com-14-Bolt Bible Tech Article by BillaVista
14-Bolt Bible. It's got a lot of info. I'd think if you wanted one that would bolt into your van it would have to be from a van. Otherwise the length from WMS to WMS (Wheel Mounting Surface) is all you really need to be worried about. You can cut off and move all the other stuff. If you can't weld it personally, get it all mocked up and take it to someone to weld. The 10.5" ring gear is a good size, the wheel bearings and axles are strong, and there is good aftermarket support even if nothing currently uses them. I've got one from a '77 swapped into my Blazer.

 

I believe they were used in these vans up until at least 2015 in the 6.6L diesel. That's not "too old" and about now should be available from a wrecking yard. Would be single rear wheel. Do you really think you need dual wheels? Look on http://Car-part.com for something near you drive there with a tape measure.

 

I suppose that i should have added that keeping the 8 on 6.5 wheel bolt pattern is a must. That eliminates all of the conventional trucks after 2010, where SRW went to 8 on 180mm and DRW went 8 on 210mm.

The GM Upfitter's Guide to the GMT-900 trucks actually has a dimension published for outside-of-leafspring to outside-of-leafspring, and for a conventional pickup truck with a factory bed it's 53", which is what I measured my van's axle at. This means DRW axle will allow the van's springs at their stock width to sit on it, and it should be wide enough for the necessary tire clearance inner-sidewall to inner-sidewall. Shock mounts are wrong but that's comparatively minor. Model years applicable are 2007 to 2010.




I'm incredibly well acquainted with car-part.com, been shopping through them for years. Trouble with looking up GMT-900 parts, GM apparently didn't do a very good job with making it easy to distinguish cab-and-chassis versus conventional pickup, any listings without pictures are questionable if the truck is correct. Cab and chassis trucks have much narrower frames and all the associated hardware is much further inset toward the vehicle centerline.

 

I've had the same issues with car-part.com going way back to finding a 8.5" 10-bolt for a 2WD S10. They existed but were rare. I wish I didn't get rid of that truck.

Yes, the trucks changed but the Vans still have 8 on 6.5. Dodge kept it for quite a while as well. Makes me wonder if a van cab and chassis would still be 8 on 6.5", like the 4500? This is listed for a 2024 Express 4500 and it's 8 on 6.5". Maybe the rear is 6.5" too? https://www.rockauto.com/en/moreinfo...455521&pt=1636. No idea how a van cab and chassis is different from a truck cab and chassis, but you might be in luck with the bolt pattern.

 

This looks for sure like a 14-bolt full floater 10.5"
https://imageappky.car-part.com/imag...k%23%20L-601#1

 

The difficulty is that at least as of the 2005 GMT-610 upfitter's guide that was still considered current in 2021, the cab-and-chassis vans use a narrower frame than the cab-and-chassis trucks. The van literature doesn't list the dimensions including the springs though.

I had an unexpected opportunity to measure a cab-and-chassis GMC Savana dually yesterday, and the dimensions were different than the SRW van. This is the measurement from inside of spring to inside of spring:



The SRW is 48", this is 45-5/8", so the perches are not in the correct spot for use on van. The brakes are very close to the springs too:

And there's a noticeable taper to the axle tube as it approaches the endplate. Even if I chose to cut off and reweld the spring perches I'd have a problem with the brakes possibly not clearing the SRW leafsprings and the outer U-bolt perhaps not finding a good place to seat. I suppose that the 2-3/8" difference exists because of the brakes, going to disc brakes requires more hardware to stick out inboard of the mounting plate than drum brakes do.

So at this stage I'm looking at GMT-900 2007-2010 trucks as the most likely source for DRW axles even though the shock absorber mounts might need to be reworked. I had also considered looking for a Topkick/Kodiak 8-lug axle like a "4500" might use but that's getting a bit extreme.

 

I did look up the GMT-800 truck platform too, I think that GM's upfitter documentation has errors. The SRW regular pickup springs outside-to-outside width is 52.5" (rather than the 53" of the van) but the DRW pickup with bed supposedly has an inside of tire sidewall to inside of tire sidewall of something like 47". Clearly either the DRW dimensions are wrong and copied the narrower cab-and-chassis info into the pickup page, or the DRW pickup uses a different frame. I suspect the former.

But either way the width is wrong by a half-inch, so the GMT-800 years are likely out as donor vehicles.