Help with final gear ratio
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Joined: Mar 2007
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From: King Of Prussia Pa
Help with final gear ratio
I need help from you gear guru's out there. My 75 started life with a 3:08 rear. The guy before me installed 3:90 gears and a tranny shift kit. The tires are BF Goodrich T/A's 225/70R15 which have a 27.4 diameter. As the speedometer was way off I thought I had a 15 tooth drive gear so I put in a 45 tooth driven gear. However I have a 17 tooth drive gear with a 45 tooth driven gear and the speedometer is still off. With speedometer on 60, the car is actually going 53, verified by 2 GPS's and someone following me. Since the 3:90's are a bit much for cruisin (for me at least)I found that I can put a set of 255/70R15's on the back which have a 29.1" diameter and the speedometer guys say that will be the correct combination to get the speedometer correct. My question is, what will these tires do to the rear gear ratio? I've been told it will decrease the ratio by 6% making it somewhere about 3:67 ratio. Is this correct? I need new tires anyhow.
Thanks in advance
Thanks in advance
Race Director
Joined: Sep 2004
Posts: 11,874
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
ok i understand what you are trying to do.....look at this way first verify your ratio by turning your drive shaft. then you would look at the circumference of the two tires in question.....the larger diameter tire would be so many inches bigger or longer if you straighten it out.....then take the two lenghts and divide them and figure out the percentage that the large tire is "longer then the other" then apply that percentage to the drive shaft ratio......10% bigger would be a 10% reduction in the drive ratio.... i think thats the correct math, but it may need some tweaking
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Joined: Mar 2007
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From: King Of Prussia Pa
Okay, your correct on what I'm trying to do. Straight from BF Goodrich. The 225 tire is 86.035 inches (long). the 255 tire is 91.374. If my math is correct when your divide the smaller by the larger, the answer is.9415698 and so on. if that is correct and that is the percentage, then 3:90 minus 9 % is about a 3.55 gear ratio.
Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
all right let me try the math out in open.....the good news is we are close either way.....the .94 represents the size the smaller is of the larger one so you would subtract the .94 (94%) from 1 or 6%......
or the other way 91.374/86.035 = 1.062 or .062 (6%) longer then the smaller one.
either way the difference is 6% to apply against the 3.66
so 3.90 x .06= .234.......3.90 - .234 = 3.66
or your way 3.90 x .94 = 3.666
so i agree with your math and reasoning and you have an effective rear end ratio of 3.66
or the other way 91.374/86.035 = 1.062 or .062 (6%) longer then the smaller one.
either way the difference is 6% to apply against the 3.66
so 3.90 x .06= .234.......3.90 - .234 = 3.66
or your way 3.90 x .94 = 3.666
so i agree with your math and reasoning and you have an effective rear end ratio of 3.66
Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
if it came in handy we would remember how to do it......see now if they taught us the importance on obscure reasoning as related to vette tires and gear ratios we may have listened more.....
Le Mans Master
Joined: May 2002
Posts: 5,086
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From: Lebanon Township New Jersey
You do understand that none of this actually changes your rear gear ratio, right? If you had a 3.90 ratio to begin with, that is 39 teeth on the ring gear and 10 on the pinion, you still have a 3.90 rear gear ratio. The speedometer reading at a given speed will be changed by the speedo gear, tire diameter etc.
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Race Director
Joined: Sep 2004
Posts: 11,874
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i had a 1960 ford econoline van with a 144 cid 3 spd that had 13" wheels.....they looked so funny i put 15" tires on it.......talk about screwing with an engineerd drive train.....that thing had no power
Safety Car
Joined: Sep 2007
Posts: 4,458
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From: Indy Indiana
core issue here instead of spending 600$ on tires, get you a 15$ driven gear. most parts houses have them.... if your speedo is reading 60 and you are going 53 then you need to slow down your speedo and make the cable spin slower.. you do this by replacing your driven gear with a bigger tooth driven gear.. go with a 18 tooth or 19 tooth just for trial / error...
Thread Starter
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Joined: Mar 2007
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From: King Of Prussia Pa
Thanks guys for all the input. First I do realize that the rear gear ratio does not change, I have a 3:90 gear ratio verified. I am also only changing to a tire that has less than 6 inch more travel with the larger 1.7 inch diameter. I also realize that I can buy a $15 driven gear or also replace the drive gear. However my attempt was to lower my RPM a little and at the same time correct the speedometer. I have more than enough power in the motor, it's a GMPP 350/330 and runs like a bear, the turbo 400 has been rebuilt and the shifts quick and stong. When the time and money come, I will put in 3:55's and freshen the rear, and also install either a 200 or 700 overdrive tranny, both which I have. In the mean time Tirerack sells the 255/70R15's for $90 bucks each plus shipping and mounting. I can also get them locally for the same price. Again, thank 's for the input
Melting Slicks
Joined: Oct 2002
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From: St Louis MO
