So i got some Konig rewinds 15x7 with 205/55 tires. I have a 85 with stock motor with a magnaflow muffler and with my stock 13's i really couldn't push more than 100 and last night I was cruising on the Interstate with a 2002 wrx and I kept up till 130. Now I'm not sure where that speed came from but it was shocking to me. Does the lighter wheel really make that big of a difference? Oh by the way Konig Rewinds Rock on the first Gens....Pics soon

 

Or your speedometer could be off based on the diameter of the new wheels...

 

But the overall diameter is the same. I made sure of that. Wouldn't it be the same?

 

Your speedometer reading is now off because of the tire size change. Wider tires also create more rolling resistance.

A stock 12a motor is unlikely to reach a top speed of 130MPH.

 

But my buddy and the wrx pulled over and talked afterwards and it was the same with them who all had stock rims. I was topped out on my tach so is the gearing capable of that?

 

TTIWWP!

 

Everybody knows that nicer rims add horsepower....

 

Yeah what Kentetsu said

 

Are you sure the diameter is the same as stock? I have 245/50/15s on the widebody and my speedo reading is 10% less than actual speed. I would think that you would need to go to a 40 or 45 aspect to have the same diameter as stock. There are some tire ratio charts online, find one and compare the stock ratio to what you have now. The only possible explanation I can see atm if the tires are putting your engine in it's 'sweet spot' before the torque/hp curve drops off. Another thought is, does the WRX have stock tires on his car, maybe his speedo is off.

Best way to tell if the diameter is the same size is to look at the the revs/mile of each tire, i.e. how many times the tire makes a full revolution to travel a mile. Most tire shops have these charts.

 

you gained 30mph from

Edit: a rim swap?

 

anything over 120 in a FB is scary IMO

 

Stock = 894 revolutions per mile
New= 870 revolutions per mile

So you're going faster than the speedo says.

100mph = 102.75mph

So what did your speedo say?

 

SSSSHHHHHHH...........
Don't tell anyone about the SUPER SECRET rims that add HP to our cars.....
Everyone will want a set and we won't have the upper hand...
L8R

 

im runnin 195/65 r15 and my spedos under 10% also.

 

Your speedometer would be reading about 1.5% too fast, meaning that if your going 60mph according to the speedo, your actually going 59mph.

Got that from this site here: http://www.miata.net/garage/tirecalc.html

 

Has anyone maxed out in 5th gear before? If anyone has, how fast were you going?

 

I have, I went to 7200 in 5th. With the tires I had on it at the time I calculated it to be ~235 km/h (140 mph).

 

when i had the 83 limited wheels on my car it was capable of doing 120. mr. po po clocked me so I'm pretty sure i was really going that fast.

 

Ok I'm glad I'm not the only nut here. It was quite strange for me. When I was going that fast, I honestly didn't believe it but when we all pulled over and asked how fast I didn't think it was possible. But with a slight down hill to the highway, I think that might have helped.

 

This is a very good tire size calculator http://www.miata.net/garage/tirecalc.html
tells you speedo difference in % and miles

 

Yes, a down hill will help you out... a lot.

 

Ok I have to respond to this!

I have an 83 GS with the stock wheels and tires, not even the good ones, off the top of my head I think theyre 185/65r13's. The previous owners had taken remarkable care and maintnence of the car, so everything was in really good shape.

Me and a friend were going to Boise from here.. I had just put on a holley intake. I-84 is a wonderful highway

He said it would take 8 hours, and I said we'll make it in 4 watch!!

So for 1 hour straight I was running between 110 and 135 mph.

On level ground the car maxed out at 120 with 2 ppl. and a bunch of stuff in the back.

I distinctly remember going around a corner and coming out of that turn at 120 mph. Right there was about a 2 mile straightaway at a very nice decline down hill.

WE HIT 133 MPH RIGHT AT 7000RPM

I was soooooo happy that the needle kept going past the top speed mark! I then
realized the car would hit 150 at 8000 rpm, but unfortunately didnt have that much power. At 133 the car handled like a dream, even hitting the next turn at that speed. The road leveled out and it was back to 120 mph at 6500 rpm. My right foot did not come off the floor at all

 

And on a side note (not that anyone cares), running a tank of gas above 6000rpm I got about 12 MPG's at the speed limit it was 23 MPG

 

Speedo accuracy regardless of different

Diff Gears
Rolling wheel/tyre diameter
Gearbox gears
Speedo cable drive gears

Is absolutely ******* awful for a late 70s/early 80s car.

Don't be stupid and read the speedo like its Gospel.

I tested my car on a long drive. In australia we have Red marker pegs on the road every 1km on some stretches of our road. I worked out 135-140 was around 100kph on my car.

I have known inaccuracies of, different rolling diameter tyres, diff gears, wrong speedo drive for my rolling diameter (pretty sure it expects wheels around 14", and I have 13"s on lows

 

I have the same rim/tire combo and they are actually 4 pounds heavier each than the stock "+" wheels & tires. Take that back , I have 205/50 not 55. The car feels a little slower due to the wider tires, but is def. more stable in corners. This tire size was supposed to be closest to original rotational size but it does make my speedometer read slow. Per my gps I am running 3-4 mph faster @ 60 on the speedo. I have hit 130 (per the speedo) in an FB before, it took a while and hills helped. Scary as hell!