Questions about measuring a wheel's backspacing - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**71CHEVYSHORTBED402** · 07-30-2025, 12:16 PM

I wonder why this fellow is measuring backspacing like this as opposed from the flat surface?

If we we're trying to determine the wheel's offset, would you still measure from that area shown by the tape measure, or would you measure from the flat surface?

Thank you

Sicklajoie · 07-30-2025, 01:23 PM

Measuring backspacing and offset are 2 completely different things.
When doing backspacing, you measure off the point that is flush against the hub, which in this case is that lip on the wheel.

HO455 · 07-30-2025, 02:57 PM

That is where I would measure from to determine backspacing or wheel offset. Think of an aftermarket alloy wheel that has only a flat surface. Both wheels need to be measured from the same reference point for the measurement to be accurate for all wheels.

https://www.efxtires.com/news/how-to...IAI2BTWObZFHL-

**71CHEVYSHORTBED402** · 07-30-2025, 07:12 PM

Quote:

Originally Posted by HO455

That is where I would measure from to determine backspacing or wheel offset. Think of an aftermarket alloy wheel that has only a flat surface. Both wheels need to be measured from the same reference point for the measurement to be accurate for all wheels.

https://www.efxtires.com/news/how-to...IAI2BTWObZFHL-

That's an interesting video because it's a vintage wheel with the lip shown in the OP. Unless he's doing it wrong, he's measuring from the flat surface not the lip, for backspace. I don't know why determining offset would be different, short it's a different thing.

Don't have a hub in front of me, but considering the lips are taller they fit in grooves or there's a gap between the hub and wheel center where the lugs are? Seems to me you'd want both groove/lip contact and contact on the center hub, and if that's the case, measuring from the groove or flat surface produces the same results - just guessing from here.

Only talking a small difference flat surface Vs lip, be nice to know for sure though. The guy who measured that OP came up with a 4.25" backspace for his 15 x 8 GM rally. I came up with 4.375 on the same wheel, measured from the flat surface.

**71CHEVYSHORTBED402** · 07-30-2025, 08:07 PM

Well don't I feel so silly. Here's a wheel hub, I believe the answer is it doesn't matter if you measure from the lip or the flat surface, seems the end result would be the same or close enough. - Unless I'm missing it, which happens, it looks like the inner lip fits into a groove, and the outer surrounds the hub.

**Richard** · 07-31-2025, 03:43 AM

The end of tape measure is sitting on same surface as "flat surface" not in the curve? Do not see the problem. Pretty simple, back space is from inside wheel mounting surface to tire mounting on wheel. Offset is difference from center of wheel (wheel mounting surface) using same inside of tire mounting surface. Where I was confused initially is that higher positive offset equals more back space. There are several online converters availble if the the math is confusing.

SCOTI · 08-01-2025, 09:55 AM

You measure from the point of contact on the back of the wheels mating surface. Some wheels are perfectly flat; some aren't.

Measure from the surface-to-surface contact point.

**mr48chev** · 08-02-2025, 02:42 AM

As people who ask questions on this board tend to have a serious adversion to opening links that we who answer questions take the time to post this is the info from the link that HO455 posted. Back spacing measured with the red arrows in the diagram.

**Richard** · 08-02-2025, 04:28 AM

Now see where question the points to with this particular wheel. In picture the raised beads are the wheel mounting surface, what you would measure to. The slightly lower "flat surface" section is most likely just to allow a tension on the lug nuts with this wheel. Not an engineer so I do not think like one.

SCOTI · 08-02-2025, 10:00 AM

Quote:

Originally Posted by Richard

Now see where question the points to with this particular wheel. In picture the raised beads are the wheel mounting surface, what you would measure to. The slightly lower "flat surface" section is most likely just to allow a tension on the lug nuts with this wheel. Not an engineer so I do not think like one.

Exactly.

07-30-2025, 12:16 PM	#1
71CHEVYSHORTBED402 Senior Member Join Date: Feb 2009 Location: Northern Nevada Posts: 7,692	Questions about measuring a wheel's backspacing I wonder why this fellow is measuring backspacing like this as opposed from the flat surface? If we we're trying to determine the wheel's offset, would you still measure from that area shown by the tape measure, or would you measure from the flat surface? Thank you Attached Images __________________ Tony 1971 C10 Custom Deluxe SWB 402/400TH A/C I developed an assembly information kit for restoring my truck from nuts and bolts. It's written in Assy. order, short & simple, packed with all OEM hardware ID; castings; part ID; 100s of part numbers; wiring simple - Resource, and trivial too. Much info. applies to all 67-72 GM, A/C vehicles, esp. trucks W/O saying. Full search ability, including to 700+ images of illus., parts, charts, Assy., points of interest, cab, bed, & front clip cart plans- Specials and cores were serviced out and R excluded e.g. front/rear glass, body/paint, engine core, rebuilt hinges, steering box, trans, etc. The project was in-line with long former professions developing process, policy, specs, demo, written for novices, admin., policy, engineers, development, systems & test - Public & govt 2, gross. Sell soft copy cheaply, PM if interested.Build thread:https://67-72chevytrucks.com/vboard/...d.php?t=730025

07-30-2025, 01:23 PM	#2
Sicklajoie The Niner Join Date: Nov 2010 Location: Broad Brook, CT Posts: 2,589	Re: Questions about measuring a wheel's backspacing Measuring backspacing and offset are 2 completely different things. When doing backspacing, you measure off the point that is flush against the hub, which in this case is that lip on the wheel. __________________ "A good racer is one whose head is in communication with his balls." - Richie Evans 1970 C/20 - 402/TH400/3.54s - 74K miles 2003 GMC Sierra 2500 - Wheatland Yellow Niner Progression Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=697183 Classic Trucks article: https://www.motortrend.com/features/...et-c20-driven/

07-30-2025, 02:57 PM	#3
HO455 Registered User Join Date: Feb 2016 Location: Portland Oregon Posts: 12,412	Re: Questions about measuring a wheel's backspacing That is where I would measure from to determine backspacing or wheel offset. Think of an aftermarket alloy wheel that has only a flat surface. Both wheels need to be measured from the same reference point for the measurement to be accurate for all wheels. https://www.efxtires.com/news/how-to...IAI2BTWObZFHL- Attached Images __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

07-30-2025, 08:07 PM	#5
71CHEVYSHORTBED402 Senior Member Join Date: Feb 2009 Location: Northern Nevada Posts: 7,692	Re: Questions about measuring a wheel's backspacing Well don't I feel so silly. Here's a wheel hub, I believe the answer is it doesn't matter if you measure from the lip or the flat surface, seems the end result would be the same or close enough. - Unless I'm missing it, which happens, it looks like the inner lip fits into a groove, and the outer surrounds the hub. Attached Images __________________ Tony 1971 C10 Custom Deluxe SWB 402/400TH A/C I developed an assembly information kit for restoring my truck from nuts and bolts. It's written in Assy. order, short & simple, packed with all OEM hardware ID; castings; part ID; 100s of part numbers; wiring simple - Resource, and trivial too. Much info. applies to all 67-72 GM, A/C vehicles, esp. trucks W/O saying. Full search ability, including to 700+ images of illus., parts, charts, Assy., points of interest, cab, bed, & front clip cart plans- Specials and cores were serviced out and R excluded e.g. front/rear glass, body/paint, engine core, rebuilt hinges, steering box, trans, etc. The project was in-line with long former professions developing process, policy, specs, demo, written for novices, admin., policy, engineers, development, systems & test - Public & govt 2, gross. Sell soft copy cheaply, PM if interested.Build thread:https://67-72chevytrucks.com/vboard/...d.php?t=730025 Last edited by 71CHEVYSHORTBED402; 07-30-2025 at 08:13 PM.

07-31-2025, 03:43 AM	#6
Richard Senior Member Join Date: Dec 1999 Location: Southern California Posts: 3,346	Re: Questions about measuring a wheel's backspacing The end of tape measure is sitting on same surface as "flat surface" not in the curve? Do not see the problem. Pretty simple, back space is from inside wheel mounting surface to tire mounting on wheel. Offset is difference from center of wheel (wheel mounting surface) using same inside of tire mounting surface. Where I was confused initially is that higher positive offset equals more back space. There are several online converters availble if the the math is confusing. __________________ Richard 1972 K10 Custom Deluxe SWB Fleetside My build https://67-72chevytrucks.com/vboard/...d.php?t=800746

08-01-2025, 09:55 AM	#7
SCOTI Registered User Join Date: Apr 2001 Location: DALLAS,TX Posts: 22,572	Re: Questions about measuring a wheel's backspacing You measure from the point of contact on the back of the wheels mating surface. Some wheels are perfectly flat; some aren't. Measure from the surface-to-surface contact point. __________________ 67SWB-B.B.RetroRod 64SWB-Recycle 89CCDually-Driver/Tow Truck 99CCSWB Driver All Fleetsides @rattlecankustoms in IG Building a small, high rpm engine with the perfect bore, stroke and rod ratio is very impressive. It's like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar..... *Cubic inches is like Indiana Jones with a cheap pistol.*

08-02-2025, 02:42 AM	#8
mr48chev Registered User Join Date: Jul 2007 Location: Toppenish, WA Posts: 16,400	Re: Questions about measuring a wheel's backspacing As people who ask questions on this board tend to have a serious adversion to opening links that we who answer questions take the time to post this is the info from the link that HO455 posted. Back spacing measured with the red arrows in the diagram. Attached Images __________________ Founding member of the too many projects, too little time and money club. My ongoing truck projects: 48 Chev 3100 that will run a 292 Six. 71 GMC 2500 that is getting a Cad 500 transplant. 77 C 30 dualie, 454, 4 speed with a 10 foot flatbed and hoist. It does the heavy work and hauls the projects around.

08-02-2025, 04:28 AM	#9
Richard Senior Member Join Date: Dec 1999 Location: Southern California Posts: 3,346	Re: Questions about measuring a wheel's backspacing Now see where question the points to with this particular wheel. In picture the raised beads are the wheel mounting surface, what you would measure to. The slightly lower "flat surface" section is most likely just to allow a tension on the lug nuts with this wheel. Not an engineer so I do not think like one. __________________ Richard 1972 K10 Custom Deluxe SWB Fleetside My build https://67-72chevytrucks.com/vboard/...d.php?t=800746



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