Had a blast this weekend up Pacific Coast Hiway (Hwy 1) Beautiful weather, top down, not much traffic. So, remembering a past post regarding rpm vs mph we took a few notes:
60 mph....2200 rpm
70 mph....2600 rpm
80 mph....3000 rpm
90 mph....3400 rpm
100 mph...3800 rpm
(I had lotsa throttle left, but I ran outta road.)
These are apoximate, as I have not had anything (speedo, tach)calibrated. But even if I had it calibrated, the ratio would be close to the same anyway. And it could be +/- 50-100 rpm...but doesn't it seem odd that every 10 mph=400 rpm???
Also...my car originally had 14" wheels...now it has 15s with p255/60...so if my speedo is showing 100, I assume I'm really going, like, 90 right? And how do you calibrate a speedo...is there some mechanical gear or something that can be installed to give a more accurate reading?
Thanks for any help...

Four hundred rpm per 10 mph is correct, at least this is the same as my 88 Silverado Dually. Should take car to a speedo shop and see what they say its off/on in comparison to there calibrated speedo. Select speedo gear from where you are at to where you want to go.

Here is one approach.

http://www.novaresource.org/speedo.htm

Nice convert, btw!!

I made a simple Excel spreadsheet that allows you to input tire diameter, axle gear, and transmission final drive ratio and obtain MPH vs RPM. If you are interested shoot me an email.

If you were at 100 mph with 14"s and now you have a larger diameter 15" you are going faster than 100 mph.

I was on PCH last weekend with the Camaro. Stopped for a while to let my 3-year old son climb the rocks just south of Topanga Canyon. It's really nice there this time of year. No crowds and the weather is great.

Carl,
Well, I guess I had that formula a little backwards...if my speedo indicated 100, I thought my actual speed would be less, not more. OOPS! Now I have no idea how fast I was going! And until I can sort out the rest of my car, I don't need the spreadsheet. I will, however take you up on your offer when I do. I haven't a clue about my rear end gears...I was told by the previous owner it was 3.08 open from a Nova...but I'm not sure I believe him. 100 mph PLUS @ 3800 rpm. The gearing seems much higher than that. I have lots to do!
And, yes, it was gorgeous on PCH this weekend. Are we lucky to live here or what? And thanks to Everett and BPOS. I will locate a speedo shop.
graemlins/beers.gif
Here's the Camaro yesterday on PCH.
http://www.stevewaddington.com/images/beach1.jpg

[ 10-13-2003, 07:37 PM: Message edited by: Steve W ]

Someone told me that from the mph vs rpm I quoted, that I probably don't have a 3.08, more like a 2.73 or something. Would that be correct? Also, it seems to me that any gas mileage advantage to higher gears might be negated by the fact you have to floor it just to get it going! :D

:confused:

Murph,
What I meant was taller gears aren't as quick off the line. You accelerate faster with 4.11 than you do with 2.73...correct? But the 4.11 cranks alot more rpm at 100mph than a 2.73 would. So you save gas at higher freeway speeds with the higher gearing. But if you have the higher gear (2.73) and you want to accelerate off the line faster, you have to hammer the throttle, using more gas. Isn't this correct? Or am I out to lunch?

I think I have to disagree (partly) with the 14" and the 15" wheels and the speed.
It doesn't have to matter if your car has 14" wheels, or 15" wheels. The difference lays in the circumference(?), (or diameter) of the entire wheel including the tire.

The original diameter of the wheels was 14"..... but which tires were on them?
If you go to this site: http://www.miata.net/garage/tirecalc.html you can see that a 14" wheel with 225/75 has a circumference of 85.7" (27.3"diameter) but the 'new' 15" wheel with 255/60 has 'only' 85" (27"diameter). This means that the 15" wheel is actually smaller than the 14". This means the 15" setup accelerates the car faster, but makes the car slightly slower in top speed.

Ofcourse I do not know what the original tire size of the 14" were, but yes, probably the 14" did not had the balloon tires (225/75) and were they smaller in circumference...
Just my .02

Yes, that makes sense...it would be the overall diameter. I never measured either of them, but I do have one of the original 14's in the trunk as a spare and it seems a lot smaller. The car came with it. I'll check it out. What do you think about my question regarding the gearing though?

Sorry for the ill-wording above. I assumed that the 15" wheel also included a larger diameter tire.

With a 1:1 trans, 3.08's, and 26" diameter tires 4000 RPM=100 mph.

So, Carl, if I'm showing 3800 rpm @ an indicated 100 mph, and I know I'm actually going faster than that, (say...115) then my gearing is probably taller than 3.08...correct?
What is the next step up frpm 3.08?
2.73 or something in between? or would you still guess I'm running a 3.08?
I think I want to change the gearing because i want a bit more ooomph off the line. But I want a reasonalble rpm @75 mph too. Suggestions?

Originally posted by CarlC:


With a 1:1 trans, 3.08's, and 26" diameter tires 4000 RPM=100 mph. Very similar to mine! I have a 3 speed manual tranny and if I am not mistaken 3rd gear is 1:1. My rear is 3:08 and I have 15 inch 235/60's in the rear.
55 MPH= 2400 RPM
70 MPH= 2900 RPM
100 MPH= 4100 RPM

Old rule of thumb: with a 26" tire & trans in direct-drive, your tach will tell you your rear end gears at 73mph. (divide by 1000)