Picked up a IRM IMSA this past week and am looking for offset and sizes for the rear wheels. I will be going with either 18 or 19s in the rear....Atleast 10 inches wide but not sure what offset I will need to put the wheels/tires at a perfext fitment without spacers.

Good question, I was thinking same thing as I look at possibly doing same thing to my chop. Time for a new look soon!

JEGS, Summit, and Speedway sell a jig that bolts to the hub and is adjustable for diameter and offset. Look at their catalogs, it should be easy to fab something up once you see how its constructed.

Maybe this can help. I have an IRM IMSA and had wheels custom made by Louie @ Centerline in CA. These are from their competition race series but are street legal too. He made them with custom rear spacing and bolt pattern ( so no spacers needed ).

5.5" R/S 15X8

6.0" R/S 15X10

[This message has been edited by IMSA/GT (edited 09-24-2016).]

https://www.summitracing.co...&sortorder=Ascending

Got this info from Don Kraus's IMSA 427 build page, his car is an 88 so I don't know how much difference that would make.

quote Adam,for your information,you can use a 10 1/2 inch rim with a 6 1/2 inch backset on a 17 inch rim or bigger.6 inches would be better but 6 1/2 will work but the tire will be close to the strut.The outside of the tire will just miss the fender as it goes up by about a little less than a half an inch.If you try anything else it won't work,believe me I've tryed it. Don

Here is a picture of how I measured myself without any fancy tools when I bought my HRE's off ebay. I taped a string with a washer or something tied to the bottom of it for weight, to the side of the 1/4 panel to use as my outside fender reference. I am running a 20x10 with a 7 inch backspace on a stock bodied 87 with coilovers and my tire is SUPER close to the spring adjuster, but my car is also running uprights from a Pontiac Transport and S10 hubs, so I don't know if any of that would apply to a Fiero with stock suspension.

[This message has been edited by zmcdonal (edited 09-23-2016).]

The concern is that the rear fender lip rolls inward and up to accomodate the stock wheel well liner. That limits the offset of the wheel. Ideally you would run a 15,16,17 or 18x10 wheel with 0 offset. The tire would be exactly level with the edge of the fender but if you hit a bump, you will destroy your fender with the tire. Thats why IMSA/GT has a 6" backspacing so his wheels are slightly tucked into the fenders to allow suspension travel without hitting anything. The other option is to open up the fender well and then you can use a 0 offset wheel.

The race version bodies had the fenders that were cut open above the molding:


And the street version had the fenders cut open below the molding:

[This message has been edited by IMSA GT (edited 09-23-2016).]

quote Originally posted by IMSA GT: The concern is that the rear fender lip rolls inward and up to accomodate the stock wheel well liner. That limits the offset of the wheel. Ideally you would run a 15,16,17 or 18x10 wheel with 0 offset. The tire would be exactly level with the edge of the fender but if you hit a bump, you will destroy your fender with the tire. Thats why IMSA/GT has a 6" backspacing so his wheels are slightly tucked into the fenders to allow suspension travel without hitting anything. The other option is to open up the fender well and then you can use a 0 offset wheel. The race version bodies had the fenders that were cut open above the molding: And the street version had the fenders cut open below the molding:

Thanks for the information. Most wheels show there offset in MM and not inches. Just so I understand is backspacing and off set the same? Also looking at wheels to get a rear set that are dead on is gonna require being made. I am getting a set of 16 Centerlines made for the IMSA so they will be a good starter set...But would like to get an aftermarket set also..

Backspacing is simply the distance between the wheel mounting surface (hub flange) and the back edge of the wheel.

Offset is the distance of the wheel mounting surface from the centerline of the rim.
A 45mm positive offset means that the wheel mounting surface is 45mm towards the front (outside edge) of the wheel rim. (This is actually confusing, because a more positive offset number, all other things being equal, will have the effect of tucking the wheel farther under the car.)
A zero offset means that the wheel mounting flange is right in the center of the wheel rim.

[This message has been edited by Raydar (edited 09-24-2016).]

quote Originally posted by MstangsBware: Thanks for the information. Most wheels show there offset in MM and not inches. Just so I understand is backspacing and off set the same? Also looking at wheels to get a rear set that are dead on is gonna require being made. I am getting a set of 16 Centerlines made for the IMSA so they will be a good starter set...But would like to get an aftermarket set also..

Raydar gives a good explanation. For me, offset is just a foreign pain in the ass even though I figure it out in a few seconds. Backspacing is a very simple measurement in inches where offset is usually from the Asian wheel market. Most American wheel manufacturers like Centerline use backspacing when you order because it is very simple to understand. For offset, take the mm, convert it to inches, and add or subtract it from half of the wheel width depending if the wheel specs say + or - offset.

For example, take 25mm and convert it to inches and it is .984 inches so we can just say 1 inch. Then take an 17x8 wheel and divide the width in half which is 4 inches. Add the 1 inch that you converted to half of the wheel width which is 4 and it becomes 5 inches so you will have a 5 inch backspace on that wheel. Or....you subtract it from the 4 inches and have a 3 inch backspace....as I said depending on whether the wheel is sold with positive or negative offset.

[This message has been edited by IMSA GT (edited 09-24-2016).]