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What is the Largest Wheel and Tire Combination I can get under my stock 914 fenders?
Last Updated: 11/15/2010
One of the most commonly asked questions is, “ How much tire can I get under these fenders?” Well the 914 is not the most exact car ever built, and wheel well space varies a lot from car to car. The limiting part is the rear. A quick and safe answer is 15x6 inch wheels and a 195/60-15 or 195/65-15 tire. If you’re willing to do a little measuring, you might get a little more under there. If you’re handy with tools a 225/50-15 on a 15x7 inch wheel will fit. That’s what I run. Want to know how?
First some basics. You need to know how much space you’ve got, so you can figure out how much more you need.
Measuring your wheel well. Loosen the lug bolts on the rear wheels of your car. Jack it up. Put jack stands underneath and remove the rear wheels. Glue/tape/bolt/somehow stick a piece of angle iron/ wood about 24 inches long and an inch or so wide, and thick enough to not bend easily on the flat part of your brake disc, right where your wheel bolts to. I used a piece of 1-inch angle iron 1/8 inch thick because that’s what I had. I stuck it in place with a magnet, but I could have drilled a hole in it and bolted it on with a lug bolt. Get the idea? Good. This will be your reference from which you’ll measure your wheel well. Grab a 6-inch ruler or a tape measure and measure the distance from your reference to the inside of the wheel well. Do it in several places. Write them down. Measure the distance from your reference to the inner portion of the outside wheel well. Write them down. Usually the smallest inner measurement is toward the front and the smallest on the outside is the lip 2/3rds of the way toward the rear. Measure the other side. It will not be the same! Now you know how much room you’ve got. Realize this is with the suspension at full droop, at normal ride height this will be a little different, and with your suspension compressed and the body rolling it will change some more. The only way to check all the variables is to drive the car with the setup under it. To get a better measurement you can disconnect your rear sway bar (if you have one) and unbolt the bottom bolt of your rear strut so you can move the trailing arm up and down. Still you won’t account for body roll and suspension bushing deflection, but you’ll get a better number. And unless you’ve got to have that 7-inch wheel and that 225 tire, you ought to be ok.
Now what do the numbers mean? The inside number tells you how much backspacing you can run. Backspacing is the perpendicular measurement of a wheel from the inside of the flat surface that goes up against your wheel, to the outside of your rim. Don’t forget to add your tire bulge to that. To measure a wheel, lay a straight edge across the wheel and measure up from the pad in the center. Every wheel manufacturer in the world understands backspacing. This number is as important as wheel width.
Let’s look at what you’ve got now. You’ve got the total distance from the wheel mounting surface on the rear, to the inside of the wheel well. Subtract from that some room for the whole assembly to move around and for the tire to deflect. We’ll call this figure room for tire movement. (rftm) On an older car with tall tires and the original bushings: 1 inch, on a dedicated track car with new polyurethane bushings you can whittle that down to ½ inch.
Total distance - rftm = total backspacing
Total backspacing – tire bulge = wheel backspacing
Plugging some numbers into that would make it look like this:
6 ½ inches total – 1 inch rftm = 5 ½ inches total backspacing
5 ½ inches – ½ inch tire bulge = 5 inch backspacing for the wheel. So you can use a wheel with a 5-inch backspacing.
Now do the same for the outside of the wheel well. Add the two numbers together and you get your maximum wheel width. If you want a little extra space you can flatten your outer fender lip. If you cut the lip every three to four inches it will roll a little easier. Depending on your rear springs, tires, sway bar and how aggressively you drive, you can cut the outer rftm figure down to as little as ¼ inch.
Application: 914 all
Credits: Pat Steiger