kalli

Hi all,

was sick of guessing so I put together an excel sheet which does the calculations for you.
You can enter tyre diameter, diff ratio, gear ratios (1st, 2nd ...) to determine:
how fast will I go with a given rpm in my gears
how high will my engine rev in which gear when I go a given speed

here's what it looks like. I had a hard time picking the right gearing for my rear axle, so I hope this might make it easier for others.

It shows that with my current tyres my top speed will drop from 115 to 90 if I use a 4:11 instead of a 3.2:1.
Lots of options to play around with.


Are there a few volunteers to compare this with what they know about their car to see if this thing actually works correctly ?
It seems to make sense for my car. All ratios are entered as single number so (4.11:1 is entered as 4.11 and 3.2:1 is entered as 3.2)

Thanks for any help
You can download the excel sheet here:
http://www.southcoasters.ie/tire_rpm_speed.xls


Kalli





[IMG]local://upfiles/52145/665F820927074A32AF326DC76BBE7FCE.jpg[/IMG]

Norm Peterson

The spreadsheet will be slightly more accurate if you include the factor 0.965 in the tire circumference formulas in column C. Tire revs per mile will increase and theoretical road speeds will drop accordingly.

The 0.965 takes into consideration that the effective rolling radius of a tire isn't equal toeither the tire's unloaded radius or its loaded radius, but lies somewhere in between. This factor isn't really a constant value either, but it's good enough for most practical purposes to assume that it is. The physical mechanism is that as the tire rolls, the tread compresses slightly in the circumferential direction as it passes through the contact patch. IOW, there is always a little sliding and squirming going on there.

On edit, a good way to verify the sheet is to use the diameters and the revs per mile data that are available on the 'Specs' pages for the various tires on the Tirerack.com site. A further step from there might be to see how close an outsidediameter computed from the nominal tire size designation is to the actual OD and perhaps work up an option to compute this stuff straight from the nominal tire size. In any event, don't worry about "errors" in the revs per mile of 5 or so. Tread wear from brand new to the wear bars amounts to more of a difference than that.

HTH


Norm

kalli

Hi Norm,

thanks for your feedback.
Your 0.965 is the 3% I added for the revs/mile.
1/(0.0965)=1.036=3.6%

Thank you for explaining this. I checked the amount of revs I calculated for a specific tire against what the manufacturer said. And I was always missing 3%. So I simply added it.

Thanks for clarifying this. I'm not too concerned about 2 or 3 mph or 100rpms as the speedo in a car can't be trusted anyway.

I'd love if a few who know their ratios would run and compare it to clarify if this thing is actually correct.
As mentioned above people go for 4.11 without knowing that the top speed will drop dramatically and that they're revving the **** out of the engine when they're cruising at 60 with a 1:1 gearbox ratio.
I have a 3.2:1 and I find that revs high already at 60mph

So please, if you know your values ... try it out

Kalli

Norm Peterson

Got it. Essentially the same end answers, slightly different paths to get there is all.


Norm

roostracing

depending what cam and how your engine was built will determine if you like the motor revving up or not...a completely stock 289 will not like revving 3500 rpms at 60 mph...however, same motor but with a nice streetable cam and free flowing exhaust it may like a 3.80-4.11 gear...ive had 4.11's in a 3 speed manual tranny (still 1:1 in 3rd) and the 4.11's felt perfect for the street cam i put in the 289...(66 fairlane)...i'm considering a 3.55 or at most a 3.80 because it will be a little easier to get the car to hook up vs. the 4.11's

67mustang302

http://richmondgear.com/101032.html

racin66coupe

sry to hijack this thread
ok, i think im a little confused....
so you will get up to speed faster with 3.50 gears in the rear than 4.11's right? or am i completely opposite?

67mustang302

Lower gears(numerically higher, 4.11 is lower than 3.70 is lower than 3.25 etc) give you better acceleration, and you get up to speed faster, but top speed in each gear is reduced.

racin66coupe

ok, thats what i thought at first but then i read on some thread that it was the opposite. i guess i read it wrong.... man i feel stupid lol

kalli

nah. it's not you. it's the fella who told you a 3.2 will bring you to speed quicker than a 4.11 ;-)

it you check the same on my calc you will see that the top speed reduces the higher the number for the gearing goes.

anyone compared results with his own car so far ? it fit nicely for mine ....

kalli