I am changing from the stock 14in rim to the z28 15in rim and wanna know how much my speedometer will be off? the tires are different. the old ones are 185/65/14 and the new ones are 215/65/15 in the back and in the front are 195/65/15. So if anyone can help me please do.

 

I know you will be reading that you are going slower then you actually are. AS to how much, I dunno. Shurely you can do a search on the web, and find a formula, or a place that you type in your sizes, and it will spit the info back to ya.

BTW, go buy your rear tires, not the front.

 

What year was is your car.
I know the size the speedo runs off of on most of our V-6's is 215/65/15's. So if your car was running 14's I think it should have had 205/70/14 on it from factory, so maybe your speedo will be in now, and was out all along.

Just a thought.

 

what year do you have??? I had no idea any thirdgen came with 14in rims stock????

 

Yeah, there were some 14's in the early models. Uh, well, unless he got confused... he said he's going to a Z28 15 inch rim? Weren't most Z28's a 16 inch rim? Or did some early model Z28's have a 15"?

Check out this link for how to determine the height of your "new" tire/wheel package as opposed to your old tire/wheel package: https://www.thirdgen.org/techbb2/sho...hreadid=132387

 

Yes...thirdgens did come with 14 in rims stock...well...mine did at least...I have 14 in pontiac rally style wheels on my Firebird...they have this huge black center cap that bolts up with the lugs and has pontiac in the middle...they also have trim rings...my original size tires were 185/75r14...i now have 235/60r14 on there...and my speedomter reads 5 miles slower than normal...so if it says 60 i'm doing 65...the only way i found this out was being radared by the portable radars cops set...i noticed that it was off...but yea...amazing that they actually put puny 14 inch rims on thirdgens...i do have 15s...but for the original look...i like the 14s...all i had to do was upgrade tire size...

 

Early models got the 14' wheel standard. 15" was standard on early Z-28s

In 85 the base model got 14", Z-28 got 15" and IROCS got 16" rims.

In 86-up, the base size was 15".

In 88 the Z-28 was dropped and all IROC's got 15" rims (I think 16" was optional).

 

My berli had 185/75/14's but we switched over to 15" z28 rally wheels with 215/65's. I kept getting speeding tickets because the speedo was way off (showing me going slower than I really am) and so my dad clocked me one day. When I was going 50km according to my speedo, I was actually going 62km. So if ya can figure that out in terms of mph then i'm sure it'll help ya.

 

Rim size doesn't matter. you can go from a 13" rim to a 17" rim and never effect your speedo. just make sure that your over all tire height is the same. forget all this noncense about different rims. watch your tires.

 

I just put the 14" rims back on my car. will post stock tire sizes for the 85 bird for all sizes listed in owners manual.

 

I know that the standard wheel size was 14 inch through 1984 at least on both Firebird and Camaro. 16 inch wheels did not become optional until 1985. The first use of 16 inch wheels on an F body was the 1984 15th anniversary Trans Am. They came with the white painted wheels that would become the optional wheel on the 1985 Trans Am. I think that 1986 was the year that the 15 inch rim became standard on the Firebird, and the 16 inch rim became standard on Formula and Trans Am, but I could be off on that one. I believe that the camaro was done the same way.

I do know that there were no factory issued 16 inch wheels before 1985, except on the above mentioned car. The 15 inch aluminum camaro and firebird wheels that most of us have started out being the standard Trans Am and Z28 wheels in the early 80's. As better/larger wheel and tire combos came out, they gave the base models the formerly uplevel rims.

 

tire sizes available to the 1985 Firebird per owners manual:

P195/75R14 (on the car now)
P205/70R14
P215/65R15
P235/60VR15 (the leaking tire on the 15" rims, minus the V)
P245/50VR16 (directional, special rotation instructions)

 

You can put whatever size rims/tires will fit and have an accurate speedo. Just go to http://www.700r4.com/SpeedOCalc/ and order the correct gear for your tranny.