Brian Bowman

Giving my '80 Euro S a facelift in the next few months (new paint to replace the faded silbermetallic, new front spoiler, new badges, etc.) and considering upgrading from the factory slotted dish 16x7's to AT Spa 17 x 8 in the front and 17x10 in the rear. I'm aware there are some issues with the spacers, but the thing I'm most concerned about is my speedometer. I'm guessing that the additional diameter will cause my speedo to read slow? How can this be reconciled/calibrated? Thanks!

-Brian

2V4V

Brian,

The thing you really need to worry about is the clearance. When you "plus size" your tire/wheel combo you will be installing tires that have a different aspect ratio. (If you want a good page or two on tire sizing, try tirerack.com) But I'll try to give you a short bit or two...

Basically, the speedo functions by counting the rate at which your rear wheels turn. The amount of distance covered by one revolution of the tire is turned into movement of your speedo needle. As long as the distance travelled in one revolution of the tire remains about the same, your speedo remains about as accurate as it was before.

When you go from 16" to 17" rims, you go to a tire with a shorter sidewall height. Thus keeping the distance around the outside of the tire ABOUT the same. (it more than likely won't be exact, but within 2-5%)

If you really need to alter your speedo ratio, I'm not sure about your model year. If the speedo is cable driven (don't think it is though) then you would need to do some lightweight math to come up with the new speedo gear. (I highly recommend "Auto Math Handbook" for all those HS formulae long forgotten)

If your speedo is like mine (and I'm fairly sure it is) then it derives input signal from a magnetic sender unit located on the back end of the differential. Things are a little different in this case. If you are electronically inclined, you could tap the signal wire, then build a box that takes the pulse from the sender unit, and modifies the output to what you need it to be. (not terribly complicated, but would require a little circuit design and probably $30 in surplus hardware)

Really though, clearance is the issue. Especially with spacers.

Good Luck!

Greg

Carlos

My only issue was due to incorrect ride hight in front. Once that was solved, no problem.

John V

I mounted 18" rims and tires on mine and have had no problems to date. My take on the spacers was related to rim offset (relation of the rims mounting surface to the rim outer lip). On early cars such as mine, there was a significant amount of space between the rear tires outter side wall and the quarter panel wheel arch inner lip (measured side to side parralel to the ground). My understanding was that the spacers were placed between the rim and the hub to space the tires outboard (increasing the rear track width) and filling up that space for a better appearance. My new rims were already offset such that the spacers are not needed. The outside edges of the tires JUST clear the inner body work.
I agree with the others that you can put any properly sized upgrade (+1,+2,+3) on without effecting speedo accuracy significantly. As long as the tire/rim combination is approved for the 928, and you haven't modified ride height or bodywork, you should be Okay. One note. Check the clearances before you drive on them! My car had been hit in the right rear quarter previously. Whoever repaired it did a great job visually but apparenty didn't pull it all the way out. When I mounted the tires initially, the right side didn't look right(correct). After some careful measurent, I found that sides wheel arch was 1/2" closer to the hub than the left side. I had to redo the quarter but I'm glad I found it in the garage and not on an on ramp.

Randy V

See the attached link for a nifty little calculator that will tell you how your speedometer will behave after installing different sized wheels/tires:

Speedometer / Tire Size Calculator

Brian Bowman

Thanks all for the good information. The bit about sidewall height makes perfect sense... and the calculator is very useful! Thanks again to all who responded...

-Brian

Steve Cattaneo

Brian, Changing tire diameter changes final drive ratio, which in turn affects performance
there is a formula for calculating drive ration.When i find it, will post.


steve c