29 replies to this topic

[Doug007]

For a 1.6 with a 7200rpm rev limit the math goes to 114.9mph in 4th gear @7200rpm (same tire diameter of course)...

I'm not sure if the true tire diameter is 23.07 with load on the tire...

[Keith Novak]

At higher speeds, the tire diameter does change due to the rotating mass. Look at a dragster on launch. Tire stiffness is obviously a factor but ever notice in some street cars that at highway speeds the speedo starts to read deceptively low?

[BreathlessPerformance]

For a 1.6 with a 7200rpm rev limit the math goes to 114.9mph in 4th gear @7200rpm (same tire diameter of course)...

I'm not sure if the true tire diameter is 23.07 with load on the tire...

This is getting strange. My dyno shuts off after it hits a certain MPH and in the Miata it is 115 yet I get conflicting reports that range from 105 to now 114 mph.

I know when I was at Road Atlanta on the straight I would bump the rev limit 7200 just as I reached the end and my GPS unit told me about 111mph

[Doug007]

There might be some confusion over rev limits from different years and tolerances. With a 23.07" tire diameter, 1:1 4th gear, and 4.3:1 rear end:

7000 = 111.7mph
7100 = 113.4mph
7200 = 114.9mph
7500 = 119.7mph...

We all use the same size tires so whatever the true tire diameter at 110mph+ should be the same for everyone. 3/32 in diameter has a 0.4% difference in overall diameter

[Keith Novak]

True. There are a couple different possible error sources when trying to compare one persons numbers to another. The measuring equipment (dynos don't always measure the exact same HP as one another either) the actual wheel size, the RPM, etc. My speed is based off measured tire diameter and RPM measured at the drive shaft recorded on a black box. Tach is measured off an electrical input. Doesn't surprise me a bit that one person's measured speed would vary from mine a little bit.

[Jim Creighton]

On a recent trip through the desert of Nevada, my GPS said 83 MPH, son in laws read 81 MPH and car speedo said 85 MPH. So, we did a measured interstate mile, recorded the time and calculated the actual. Wouldn't you know, it was 82 MPH.

Anyhow, it doesn't make one bit of difference how fast you go on a race track as long as you are going faster than everyone else!!!! Anyone who walks around talking about going x amount of MPH on a race track is not racing, just driving fast.

[Glenn]

On a recent trip through the desert of Nevada, my GPS said 83 MPH, son in laws read 81 MPH and car speedo said 85 MPH. So, we did a measured interstate mile, recorded the time and calculated the actual. Wouldn't you know, it was 82 MPH.

Anyhow, it doesn't make one bit of difference how fast you go on a race track as long as you are going faster than everyone else!!!! Anyone who walks around talking about going x amount of MPH on a race track is not racing, just driving fast.

Good OLE VASCAR! LOL Been caught by that trap before.

[BreathlessPerformance]

thats a good one.

[Ron Alan]

Ron a couple 32nds will not be worth anything. The most you will see between full 8/32 and a cased tire will be a mph. Then it comes down to being consistent enough to even get to the point of discrepancy. That's not where the problem is.

Can I see your long hand notes on this equation Charlie?

[Keith Novak]

It is pretty academic but not always. If you add 8/32 to tread depth, double that to get the change in diameter. Making it simple here, divide 23.5 by 23 and you get 1.02. Mulitply that by 95mph, (the speed I attempted T10 at Portland when I rolled the dice and tried to catch people out on drys while I picked wets) you'll find out that you get about 97mph. That suddenly became very important because 95 is about as fast as people go through that turn on drys so 97 on wets was definitely too hot for wets and there's not enough wet grass between the track edge and the wall to stop. 2 mph can be important if it's 2 mph too fast.