Start line ratio .

[bn69stang]

With  S L R  being  trans 1st gear times rear gear ratio , tire height comes in to affect when ? , at what point does it come into play and change  S L R . . Bud

[Joe-JDC]

Roll-out of tire, or circumference of tire always affects final drive ratio.  It therefore affects gear multiplication in every gear.  You will find that you have to start somewhere, and take a particular tire size that fits your vehicle, then change gear ratio in differential to make the most of that combination.  I used a 10.5x29x15 on my '69 Mach I, and with a JPT C-4, I found that my 452MR would cross the line at 7200 with 4.57:1 gears.  If I wanted more rpm, I would go to 4.88:1. You usually have to pick a size and then try it to see how it works.  Go to the track and check out what everyone else is using, and you will save yourself some trial and error.  Engine RPM(stroke/cam/valve springs/carburetion), horsepower, torque, vehicle weight, aerodynamics, all play a part in gear selection.  Just don't select a gear that maxes your rpm out at the 1000ft mark when you still have 320ft more to go.  Joe-JDC

[bn69stang]

Thanks Joe , i currently have 3.89 s and a 27 inch tall tire and was wanting to go to a 28 inch tall tire . I know that changes rpm in every gear , but just started thinking if there was some formula that added or subtracted at a certain height of tire .. Bud

[Joe-JDC]

There are lots of desktop calculators for that, just have to google it.  I found that I could figure it out using algebra, once I knew the rollout on a tire, or given rpm limit of an engine, you can work backwards to figure out what is needed.  Tire height is not always a good indicator, but rollout cirumference in inches will give you exact numbers to work with---until the tire grows at rpm--as in slicks.  Joe-JDC.

[bn69stang]

Thanks again Joe , i found one site where they were using 32 inch tire as a base , so 32 divided by 28 , time s 2.87 times 3.89 . It added 1.14 to the formula , but not sure about that but any way i appreciate your input . I will keep looking to see what else pops up .. Bud

[65er]

I've been working on this spreadsheet off and on for my own entertainment.  It's all accurate as far as I can tell but I can't guarantee that you won't find an error.  Put in the info you want in blue and the orange boxes tell you pretty much whatever you want to know about it.  I was using it to compare different gear sets with different transmissions as you can tell. 
Also if you already know your rollout or tire height, you can fiddle around with any of the tire inputs to make the other stuff show whatever you want.  I think based on my measurements of my Goodrich Radial TA tires mounted on 9" wheels, my 255/60-15 tires came out to 273.3/49-15.  I made actual measurements of the section width and rollout, then filled in the rest to make the rest of the numbers come out right.

I kind of consider this as my driveline dream sheet, lol.  Have fun...

https://docs.google.com/spreadsheet/ccc?key=0AqPlAHiKgTwmdHZhdkF3UXlyc0dLSURGVWRCMDVBdmc#gid=0

[bn69stang]

Interesting Wade , thanks for the input .. Bud

[plovett]

28/27 = 1.037

1.037x3.89 = 4.03

So you'd need about a 4.03 rear gear with 28" tall tires to keep the same overall ratio that you had with 27" tall tires and the 3.89 gear.  Is that what you're asking?

paulie

[Ratbird]

I've used this one Bud. It's quick and easy. The bottom half seems best to me because it allows for your final gear ratio - 1.0 on my 4 speed.

http://www.csgnetwork.com/multirpmcalc.html

Dave

[bn69stang]

Yeah Paulie thats one way to look at it i. guess or , 32/28 = 1.14 times 2.87 times 3.89 = 12.72  S L R . If a 32 inch tire is a baseline ? Thats what im not sure of . And Dave i have that site , and look at it every now and then .. Thanks guy s   Bud

[plovett]

I don't think there really is a" baseline" tire diameter.   Whatever you happen to have at the moment is a baseline in my opinion.

I think I see what you're getting at now.  You want a number for starting line ratio that includes the tire diameter?  Thinking out loud here........the tire is essentially the last gear in a whole sequence of gears.  So we need a ratio to compute the starting line ratio.  But a ratio between what and what?  Circumference (or diameter) of the tire and what?  I guess you could use your 32" tire as an arbitrary standard.

I dunno.  I guess I just look at it differently.  I would tend to just calculate the rpm at whatever speed you're interested in, like mph at the end of the quarter mile, or whatever you're interested in, and adjust tire diameter, gearing, etc. to optimize it.   I wouldn't worry about what the actual number for overall starting line ratio is.

Still, I see the value in what you're saying.  My personal opinion is that the "standard tire diameter" should be closer to an average tire size.  32" is pretty tall.   27" or maybe 28" might be more to my liking for a standard.

JMO,

paulie

[bn69stang]

Yeah i agree with you Paulie , 32 is a bit high , but i cant help thinking that if it changes rpm in your final gear , it changes  S L R  in the beginning . so a 275/60-15 is 28.10 inchs tall and the 255/60-15 is 27.04 inchs tall , and the nitto n555r  drag radial is 27.7 inchs tall . In my case i lowered the rear of my car with a 5 leaf mid eye spring , and from the ground up to the quarter panel lip at the center of the wheel is 26.5 inchs so i think i ll try the nitto s at 27.7 and see how it works with my set up . I guess i need to remember that my car is a street car first , and i appreciate your input and thought on this .. Bud

[My427stang]

Paulie steered you right, a 3.89 with 27 inch tall tires and a 2.78 1st gear will have 7.43 mph per 1000 rpm, the equiv after the 1 inch tire change requires a 4.035 rear.  Many ways to get to the answer, but an easy calculator to download and play with is Gearcalc by Peter Ogden

http://locost7.info/gearcalc.php

I like it because it will display speed in gears, percentage drops after shift, tire change effects etc, without being too fancy and it allows you to put in measured diameter or tire size as well as any gear and tranny input

[bn69stang]

Thank you Ross , i will go over that and see what looks like it will work best for what i want to do , i currently have 3.50 s , 3.70 s , and the 3.89 s but nothing bigger , so may be ordering another gear set .. Bud

[My427stang]

Which tranny do you have in the car?

Personally, I wouldn't worry about SLR on a Mustang with a 28 inch tall tire unless it was under 10.3-ish.  If you are somewhere around there, before you do a gear change, look at the program's output of cruise rpm at different speeds.  Although the car will probably like a 4.10 or 4.11 in the lower gears, you may not like it in high gear.

FWIW, with 28 inch tall tires, my 1st gear is about as deep as I would want it using 4.10s and a 2.87 1st.  That's 11.76 if the reference point helps.  If it were any deeper in normal street driving (shifting at low rpm), 1st would be too short.