kenv

I just picked up a set of oem CE wheels, with like new Bridgestones for my `87 coupe. The original tire/wheel combo for my car is "16. The CE combo is "17. Now when I go down the road at 40mph, the digital speedo says I`m doing about 65mph. How do I correct this minor problem. Also, everyone I talked to said I would need spacers to run these wheels on my car. Ron at Contemporary Corvette, where I bought the wheels said thats , they will work & look ok without spacers. I put them on, & they look fine to me.

Ken

bogus

there are some of those wheels designed to work on the earlier cars...

As for the error... with the difference in wheel size, versus, tire profile, it should be a wash... what are the 16" tires again? I can do the math...

kenv

Thanks: I`m tire math " challenged " myself

old tire size is 255/50/zr16. New tire size 285/40/zr17


Ken

bogus

last time I tried to post this, I got a database error!

here goes... the old wheels were 24.75" tall, the new ones, 26" tall!

That's a big difference... you will have to change the speedo gear on the tranny... to what? I donno.

kenv

Thanks: I got that database error also, about four times.

Ken

CFI-EFI

I think the old data base error has hit again. P255-50 R16's are approx 26.1" tall. I haven't done the math, yet, I just know the tires. I think the 285/40/17's are considered a "plus 1" and SHOULD be the same diameter.

RACE ON!!!

RED-85-Z51

I just went and measured the P255-50-ZR16's on my car, and the tire diamter is exacly 25.5".

kenv

Thanks: but if they are, why is the speedo way off?

Ken

CFI-EFI

Tires vary from spec, quite a bit, from manufacturer to manufacturer and from mold to mold. The bead to tread height is nominally, the section width (285 or 255) times the aspect ratio (40 or 50). Two tread heights added to the rim diameter make the tire OD, nominally. Mathematically, the 16's should be 26.039" tall. The 17's should be 25.976" tall. The difference is 2 tenths of 1 percent. THAT should NOT be noticeable on the speedo.

An indicated 65 mph at a true 40 mph, is a whooping 62.5% error. There is a bigger problem here, than a tire size switch.

RACE ON!!!

kenv

Hmmm, the only things I replaced were the tires & wheels

Ken

Oh yeah: all the tires & wheels are the same size 9.5 x 17". They are all from a CE, but I don`t know if they are from the "same" CE. They may all be front wheels. I don`t know if this makes a difference.

bogus

this tells me the problem may have been there for a while and you just noticed it?

kenv

Ummm--- no, I just bought them this weekend. Put them on saturday.
speedo worked ok with the "16 wheels. The car has the Z52 suspension, I don`t know if that makes a difference or not.

Ken

kenv

ttt

CFI-EFI

Your speedo problem was NOT caused by the tire change. The speedo does NOT care if the tires or the wheels came off of the same car or if they are off the front of something. The speedometer reading is based on distance the tire travels per revolution (rolling circumference) and the rear axle ratio. Since you made an insignificant change in tire size, your speedo problem has another cause.

RACE ON!!!

PS. It just dawned on me. You didn't happen to inadvertantly hit the display switch on the dash from standard to metic, did you? Forty mph is 64.36 KPH. I'm betting BIG money on this one!

JackDidley

The cluster must be set on metric . Flip the switch.

kenv

DOH! :bb :bb Thanks: You guys were right. I was changing an ip bulb the same day I changed the wheels, & must have hit the english/metric switch. If it weren`t for the knowledgeable folks on this forum, I would have gone down the road in the metric mode for the next six months, going &

Thanks again

Ken

oldace84

Good luck!