Can someone please tell me what is the largest tire size that will work on the factory 16 inch 1991 Formuila rims without "tweaking" the wheel wells? If it matters, I don't mind having different sized tires on the front versus the rear. So I guess the question is what's the largest size for the front and the largest for the rear?

Have a Great Day!!


. - Vern

 

either way the front and back have diffrent offset.. biggest id say is 255, 275 doesnt look good tho yes can be done..

i had 255 on the rear when i got the car .. looked good ..

 

Will 255's still fit the factory rims? I wonder what the affect on the speedo would be (probably not much?).....

Thanks for the reply!


. - Vern

 

As long as the OD of the tire is the same, the spedo will not be affected. I think it is normally a 25.7" OD.

 

Thanks, 91formulaSS, guess I'll have to research the O.D. change. Seems I remember that a 255-50 will be slightly larger diameter than a 245-50, something about the "50" part being larger on a 245 tire than a 255, probably some ratio or percentage of the width. If I find the answer I'll try to post it back here.

Thanks for the reply!

. - Vern

 

I might be wrong but I always thought that a "50 series" tire was the sidewall dimension in millimeters. So as long as they are a "50 series" on a 16" rim it will have the same OD. I have 274-40-17 and it is the same OD as the stock 245-50-16

P.S. I like your car. I have the same one only it's dark green, and it doesn't have a stroker. just yet

 

the 50 in the sidewall is refering to it being 50% of the tread width. So a 70 sidewall is 70% of say the 255 that is the tread width..

 

Thanks for the responses, folks. Based on the second value (the "50" in a "245x50-16" size) being the percentage of the tire width, I checked the math on 89formula#1's reply that a 245x50-16 is the same diameter as a 275x40-17. Amazing, but 91formulaSS is exactly correct in his reply about a 245x50-16 being the same as a 275x40-17 as far as the speedometer goes:

245X50-16 radius calc.:
================
SIDEWALL: 245mm * 50% = 122.5mm
WHEEL : 16in * 25.4 mm/in * 1/2 = 203.2mm
Total Radius = 122.5 + 203.2 = 325.7mm
Circumference = 2 * (PIE) * (radius) = 2 * 3.14 * 325.7 = 2045mm

275X40-17 radius calc.:
================
SIDEWALL: 275mm * 40% = 110mm
WHEEL : 17in * 25.4 mm/in * 1/2 = 215.9mm
Total Radius = 110 + 215.9 = 325.9mm
Circumference = 2 * (PIE) * (radius) = 2 * 3.14 * 325.9 = 2047mm

A 2 mm difference in circumference, which is what controls the "error" in a speedometer calculation (not the diameter), is negligible. If a 245x50-16 is what the speedo is calibrated for, going to a 275x40-17 would result in a little under 1 % error in the speedo (assuming they were that accurate to begin with!) So instead of going 100 MPH like your speedo says, you might really be going closer to 101 MPH with a 275x40-17 tire. Or maybe be about half a MPH at 60MPH. In other words, they're the same.

Which means if you go with a 255x50-16 (radius of 330.7mm, circumference of 2078mm) as a replacement for the factory 245x50-16 rear tires, your speedo could be off by about 1.5%, which is pretty close as well (less than 1 MPH at 60MPH).

Sorry for the length of the post, but I wanted to show everyone the math so they could understand how all this works now that I've been shown the correct way to do and understand it. Appreciate the replies!

My original question was what's the largest width that will fit the factory 16x8 rims. From what I've heard, sounds like a pair of 255x50-16 rear tires will be going on the Formula 383, may go with the factory 245x50-16's on the front to keep the wheel well clearance. Since rotating requires remounting tires on our cars, plus the fact the rear tires will probably wear faster anyway <GRIN>, I don't see too great a problem with the two different sizes...

Again, Thanks for your time, and Have a Great Day!!!


. - Vern

 

I recommend not to put tires wider than 245 on stock rims. I did it last summer (255/50/R16) and once in turn with rears sliding one tire gone off from rim. So, when pressure drops a little bit for some reason, your tire can go off from rim during hard turn. It's no good.