This question is actually about my 69 Nova, but I figured you guys are smarter than the Nova guys because you're driving Camaros. (Don't tell them I said that)
Anyway I have a 4.88 gear with 27" tall tires on it now. It needs new tires and I'm wondering how much a 29" tall tire will change the gear ratio? I know theres a formula for this, but I don't know it and probably wouldn't understand it anyway- I'm illiterate in math. Will a 29" tire kill the gear and make it feel like a 3.73, or is the change not so noticeable, say like going to a 4.56? How about a 28" tire? I need to lose a little gear, but I don't want to kill all of it. The car has a 355" small block, a TH350 with a 3000 converter, and I generally shift at about 6000 rpm.
Thanks all- sorry for the long post. Pete

Pete

There are some calculators on the Richmond Gear site www.richmondgear.com (http://www.richmondgear.com) that will figure r.p.m./ratio/tire height. I played around plugging in different tire sizes at the same r.p.m./gear ratio and it didn't look like it would change that much. The 29 inch tires would bring it down to the 4.50 range and the 28 inch around 4.70. I'm sure there is a different formula, but this seemed to work. I've been fartin' around with it trying to come up with a better combo than my 24 inch tall tires with 4.10 gears!! http://www.camaros.net/forum/biggrin.gif Hope this helps.

Guess I should explain how it worked... Use the tire size that you want to change to with your gear ratio to come up with the r.p.m. The speed remains constant. Then plug in that r.p.m. with your current tire size to come up with the gear ratio for that speed.

Mark

[This message has been edited by mbrekke (edited 10-20-2000).]

Pete, assume you are going 60MPH. Here are the calculations:

with 27 inch tires and 4.88 gears 3643RPM
with 29 inch tires and 4.88 gears 3392RPM

The effect of getting 29 inch tires is about the same as keeping your 27 inch tires and having a rear end ratio of 4.56 to 1 (3404 RPM @ 60MPH).

I hope this answers your question. The Formula is:

RPM = (MPH x Gear Ratio) x 336 DIVIDED BY
the tire diameter in inches

so: (60MPH x 4.88) x 336 = 98,380.8
then: 98,380.8 divided by 27 = 3643RPM
OR: 98,380.8 divided by 29 = 3392RPM

Thanks Mark
Man your motor must sing with that short of a tire- first gears gotta be awesome!
I'm not even sure if I can fit that tall of a tire, but I cant drive the car on the highway with the 4.88s and the tires it has now.
Pete

Thanks SR71
Yes that does answer it. Now I just have to see if I can actually stuff that much tire under my heap.
Pete

Yeah it sings alright! With all the rubber I've been laying down on our road, the tires are probably down to 23.5 inches by now! You should have seen the grin on my sons face when I took him for a "ride"!

Mark

LOL!!!
Same here- my boy is 5, how old is your son?
He loves driving around in it. Only problem is he wants me to paint it black with green flames- just like Gravedigger! I hate to tell him no, but, yuck!
Pete

He's 13. Last time I took him for a ride like that he cried!!! (he was only 4 or 5 then) Already seems to be a diehard Pontiac fan. He wants to find a 68 Firebird to restore. Seems to have the right idea!

Mark

Pete, What are you going through the lights at? and what is the RPM at peak HP? The 27" tire and 88's are pretty steep now, although the little motor cars like that RPM. If you are running out of motor now at the top, then the 29" tire would drop about 500 Rpm off the motor. Depends on the horsepower the car makes and where. The mile an hour is your horspower indicator. 5.14's and a 29 1/2" ET street tire would do about the same as you are now. I don't know your engine but, that would keep you in the ball park. My nichols worth Galen

------------------
S/B Chevy Lover
67 Camaro street car
10:50's @ 128 on motor w/383 cid
79 Z28 w/383 9.1:1 3.73:1 Restoring stock apearance
13:40's @ 101 stock exh.& intake& Qjet
smog legal
Camaro Lover

Galen
I've yet to take it to the track so I dont have any real numbers to work from. I did have someone do a desktop dyno workup, and it gave the following figures for my combo:

Max. HP 362 @ 5500 RPM
Max. Torque 383 Ft. Lbs. @ 4500 RPM
At 6000 RPM it shows 347 HP w/ 304 Ft. Lbs.
Its projection was a 12.50 @ 94 MPH. I've never used a desktop dyno before so I have no idea if they're accurate or not. I'm hoping to get this thing to the track in the next week or so.
Motorwise its fairly simple- a 355" with 10:1 TRWs on a stock forged crank and rods, 2.02 camel humps, a .525 lift hyd. cam, a Victor Jr. with a Holley 650, and a GM HEI with an Accell Super Coil. Exhaust is Hooker Super Comps with a 3" system with Flowmaster 40s and a crossover.
I would like to lose a little of the gear, as this is first and foremost a street car. Cruising down I-95 with the motor screaming is just a little nervewracking. But fun!
Pete

Hello Pete, Yes they are fun, even buzzing in the upper RPM range on the highway. That is why we have a driver in the garage isn't it. I remember the days when I drove the car I raced everywhere, dates and all. Changed that in a hurry. But the 88's and the 27" tire are going to put you over the 5500 peak I think. There is no advantage at all of spinning the motor past peak hp. The 29's would be about right to get the motor slowed down a bit. The chevy heads and the small cam won't like the RPM. What is the L.C. and duration at .050" Also Pete, that Victor manifold is more of a hamper to you right now than a help. You have the gear to overcome some of that but that is only getting the car to move easier. Remember what the plenum looks like on it. The floor is flat and the runners are back away from the throttle bores line of sight. The victor has been a good manifold through the years but I used to put a "turtle" epoxied to the floor. Brodix still makes them for your manifold. I helps break up the tendancy to puddle especially when the motor sees low rpm, such as cruising on the street. The super victor that Edelbrock has out is a good piece. The runners extend under the carb. and there is little floor area. Vic. Jr. had Wilson help desighn it who also helped Maskins with his companys manifolds(Dart). The brodix hv1 is also that way. They all are modeled after the original Bowtie which is still agtreat piece. You would benifit much from an RPM manifold. The airgap would still give you the neat apearance of the Victor and keep the heat away from the Plenum/carb. area. In the least get the Brodix part I mentioned. They are about 60 bucks now I think. A 383 in place of the 55 would be a kick for you. Even the little smog legal one in my 79Z28 makes over 460 ft.lbs torgue and 376hp@5200. The hp is down because of the manifolds. I wasn't going to use headers as I keep that one stock apearing(a blast to drive though). Easy to make 500 hp and still have fun on the street. Then the victor would get more comfortable. Anyway enough rambling, I like the 68 up Nova's. They are a Camaro in sheeps clothing and easy to get the suspension right. Could hook up in a car wash if done right. The rear well area could take a 10" tire. Move the shock inboard and then instead of the optimum 4 7/8 back space there is room for a 15 X 10 w/ a 5.5 back space. When you taking the car out to show it's stuff? It will run good, only might run out of motor about 1100' but that's a fixable thing. Galen

------------------
S/B Chevy Lover
67 Camaro street car
10:50's @ 128 on motor w/383 cid
79 Z28 w/383 9.1:1 3.73:1 Restoring stock apearance
13:40's @ 101 stock exh.& intake& Qjet
smog legal
Camaro Lover

hey pete b and mbrekke,
You guys should have seen the smile on my dad's face when I took him for a ride in my camaro, lol... Keep roastin guys

That's an interesting spin on the story. http://www.camaros.net/forum/biggrin.gif

Mark

Hey Tireblazin- same here with my father in law- I think he liked it!!!

Galen- I know what you mean about the low rpm stumbling, sometimes it does it a bit. But I figure I'll sacrifice a little low end driveability for the upper rpm fun. I know exactly what you mean about the turtles- I built one out of epoxy in my manifold. I guess I should also mention that I worked for Wilson for 6 years. I know this setup isn't optimum for streetlight to streetlight driving, but it seems to run fine around town. The car has no power accesories at all, brakes, steering are all manual. The motor might see a bottle in the future, so I wanted to stay away from a dual plane.
I have 8.5" Centerlines out back and I know I can fit a 10" tire under it, I just have to make sure about what height tire will fit. I'm in the process of moving the shocks in now.
I've got a real motor that's slowly going together, but it'll be awhile before I can afford to finish that one.
Thanks for the input everyone- Pete