Tire/Rim size and backspacing ?

[ApullinV6]

I'm looking at a rim that you can get as:

Rim Size... Bolt Circle....MM Offset...... Back Spacing

15x8...........5-4 3/4..............-18.......... 3 3/4
16x8...........5-4 3/4...............-11..........4
16x8...........5-4 3/4................ 8...........4 3/4

I'm looking at Mickey Thompson ET Street sizes:
235/60R15, 275/60R15 or the 255/50R16

Which rim and tire would I be better off going with that will sit underneath the car nicely without rolling the lip or notching the frame?

What is the deal about the MM Offset? I'm familiar with back spacing not the offset??

 

[UNGN]

The first 2 would work great, the last one would likely need a spacer.

Offset is from the theroretical centerline of the wheel to the mounting face, while backspacing is from the inexact rear lip (this can vary slightly from wheel to wheel of the same offset).

Positive offset is more backspacing, negative offset is less. Our cars like zero to slightly negative offset.

 

[Monte]

The first 2 would work great, the last one would likely need a spacer.

Offset is from the theroretical centerline of the wheel to the mounting face, while backspacing is from the inexact rear lip (this can vary slightly from wheel to wheel of the same offset).

Positive offset is more backspacing, negative offset is less. Our cars like zero to slightly negative offset.

^^^


I would go with the 275/60/15 or 255/50/16.

PM me when you are ready to order some wheels or tires!

 

[Pronto]

275/60s are taller so they might actually slow you down depending on your car's combination of parts.

 

[ApullinV6]

SO.. if I go with a 16x8 with 4" back spacing and the 255/50r/16. I wouldn't have to notch or roll the fenders right? I want to keep my car lower with tires inside fenderwells.

Thanks for all the imput guys..

 

[UNGN]

SO.. if I go with a 16x8 with 4" back spacing and the 255/50r/16. I wouldn't have to notch or roll the fenders right? I want to keep my car lower with tires inside fenderwells.

Thanks for all the imput guys..

GTA's are zero offset and about 4.3" of backspacing and everybody runs 255/60's with these (though they can be close to the frame on some cars).

Straight 4 inches of backspacing would fit better.

 

[UNGN]

275/60s are taller so they might actually slow you down depending on your car's combination of parts.

With street tires, I've never seen a car slow down switching to 275/60's vs. 50's even with a stock turbo.

The 60's hook so much better than the 50's you can leave with higher boost and won't blow the tires off as easily.

Maybe if the car is so slow that it can't get out of its own way.

26" slicks vs. 28" slicks at the track are a different story than street tires. With those the mechanical advantage of the shorter tire comes into play.

With street tires, traction trumps mechanical advantage, every time.

 

[Pronto]

With street tires, I've never seen a car slow down switching to 275/60's vs. 50's even with a stock turbo.
I have, several times with both street tires and slicks. Again it also is the combo. Much depends on the driver and track prep too. From what I've seen until your running fast into the low 11s or better, the taller tire isn't always the answer to traction.

 

[ApullinV6]

I just figured that the 50's would give you more of a foot print, but I guess it's all to do with the roll out??

 

[UNGN]

I just figured that the 50's would give you more of a foot print, but I guess it's all to do with the roll out??

It's multiple reasons the 60's are better. The taller side wall is more forgiving to the initial shock at launch. The contact patch is slightly longer and the mechanical disadvantage of the taller tires mean they are delivering LESS force to the ground, so you can launch with higher boost for a given track condition. If you blow the tires off, they are much easier to gather back, too.

On the top end, they keep the car in it's peak HP range longer and the you are less likely to get into the rpm range the stock cam will nose over.

If the car can spin the tires off the line, it will benifit from 28" tires over 26" tires. If a TR is revving over 5000 RPM in the 1/4 (no matter what mph), it will benifit from 28" tires.

If you run at a prepped track that is so sticky your shoes come off if you walk on it, 28" tires might slow you down .1 second max. If you have plans of running faster than 115 in the 1/4 with the stock cam, FORGET 26" tires.

Loose converter always go 28"

With a Big cam 26" might be better

Slick Track always go 28"

Ported heads 26" might be better

 

[UNGN]

With street tires, I've never seen a car slow down switching to 275/60's vs. 50's even with a stock turbo.
I have, several times with both street tires and slicks. Again it also is the combo. Much depends on the driver and track prep too. From what I've seen until your running fast into the low 11s or better, the taller tire isn't always the answer to traction.

You are only running 107 mph.

I went through 3 sets of 50's because I listened to the "you have to run 11's blah blah blah", spread by people who race at tracks where stock turbo cars pull the front wheels.

I switched to 60's and never looked back. At street nights in the real world, they are much faster. I went from 12 teens/12.0's with one 11.9 to consistent 11.6-7's by switching to 28" tires. Picked up 3 mph, too.

That was on prepped tracks. On slippery tracks they can be the difference between a spinning 13.0 second run or an 11.9.

 

[Pronto]

ApullinV6, I run 235s because they fit my stock T rims better. Your looking at nonstock rims so you will have more choices. Maybe you can find someone that has the size tires your looking that would let you try them on your car to see how they look and fit. As far as which size will be best for ultimate traction on the street or track the answer to that is dependant on a few things as UNGN has noted. Personally, I like the look of a 26" tire on the G-body especially on the street. You'll have to decide what's best for you.

 

[ApullinV6]

I think mostly I'll run 1/8 mile track, very seldom do we get 1/4 mile times around here.

Local track is o.k., nothng to brag about for traction.

Pronto, I'm like you.. I think a 26" tire (Short and Fat would look sweet), but as UNGN has pointed out. I don't want to loose performance..Man now I'm really confused.

Loose converter always go 28"

Got a Pat's converer on the way, 3000 stall

With a Big cam 26" might be better

GN107 cam from Postons

Ported heads 26" might be better

Heads are ported slightly with a # angle valve job

My only problem with the 275/60 is the rim I want, I don't think I can get the backspacing in anything but 3 3/4" with either a 15x8 w/ -18mm offset or a 15x7 with -5mm offset, Which would be better?

 

[Pronto]

From the tire manufacture's sites, they recommend a 8" rim on 275/60s or 50s. That's another reason I use the 235s. My rims are only 7". This is on a T, GN rims might be different. I tried a 275/50 and it bulged out too much and rubbed on my frame. On the other hand I have seen the 275/50 on 7" rims on cars that didn't rub. The 60s will has less bulge and will be even better on a 8" rim. This does get really confusing (and expensive) but like I said, if you can get a set of used tires or someone that will lend you some to check out, that might help.

 

[Realspool]

^^^


I would go with the 275/60/15 or 255/50/16.

I believe my Bf Goodrich Drag Radials are 275/60/15 on 15x8 Weld wheels.After a long trip 3 weeks ago the tire got underneath the quarter panel and pushed about 3 inches of the quarter panel where the lip is and bent it towards the tire.I must say there was a little weight in the back with 2 big dogs and some things in the trunk because I was moving out of my previous area.Actually just yesterday I bent that part of the quarter panel back to where it was with ease.I was lucky it was not worse then it was. Also I have Air Bags that would have helped me avoid that happening but when I checked the PSI in both airbags it was only around 13 PSI.I put air in them and now there at 35 PSI and the quarter panel is about 2-21/2 inches higher then when I made the long 70 mile trip.Now I don't have to worry about hitting a bump and having the tire rub the quarter panels and disfiguring them.The 15x8 Welds also have the right back spacing because I ask this question to many guys that know alot about the Turbo Buick Regal.I think the previous post about the backspacing was what mine are.I don't know if I would go with a 16X8 unless I used them for just racing use.

 

[UNGN]

275/60's will work ok on 7" rims. The 50's cup up in the middle at low pressures and don't maintain their contact patch on a 7" rim, but the 60's are fine.

3 3/4" BS should work fine with either wheel (7" or 8") and with both 50's or 60's.

You may need to be careful with the inner fender trim with the 8" wheels and the 60's to make sure there are no sharp edges to catch the tires.

If you race a lot, I would go with the 60's. They are more consistant than 50's.

What turbo are you going to run?

 

[ApullinV6]

What turbo are you going to run?

Undecided as of yet..


Just installed a 3200 converter from Pat's and it really woke the car up, I need some tires now. I was thinking about either a 44, 52-53, or 61 turbo? I want to maintain streetability but be able to fly down the track to..

 

[Realspool]

I would go for a TE-44 or TE-60 Turbo.I had a bigger turbo on my car a while ago and it absolutely sucked on the street.When I went with the TE-44 Turbo with my modified 12 inch 3000 Stall,the car had great street manners(minimum turbo lag) and it hit really hard and the change was dramatic.I am planning on getting a 9.5 3200 converter for even better street manners.A great converter that will give it more pep,spool the Turbo faster,instead of powerbraking it all the time to get boost quickly.I would not go smaller then a 44 or bigger then a TE-60 for my street car ever again.