Tire diameter changes gear ratio how much?

Well I am trying to figure out what tire size to get and was curious to know what kind of difference I would see in my gear ratio if I changed to a smaller diameter tire. I am not sure of the original tire diameter that came on a new 72 Riviera but I currently have 275/60/R15 Overall diameter (28.06 in) with 3.23 gears. I am thinking about getting some 295/50/R15 Overall diameter (26.80 in) I am thinking this should make my gear ratio seem like a lower gear but around what should I expect from this change? Thanks for your help. Davy

 

email me at buickragtop74@yahoo.com and put "geardaddy" in the header and i will send you an exel spereadsheet that i made. it helps you compare gear ratios, tire and wheel sizes and compare them at any given speed.


:3gears:
the answer to your Q is ... it's like changing to a 3.38 gear.

 

That spreadsheet is awesome. Thanks for posting it in the past because I use it several times a month. :TU:

 

Cool I will send you my email I look forward to getting that spreadsheet. Oh and 3.38 that sounds like a good improvement for launching off the line. Now I just have to decide if I can handle the way the stance would look with the smaller diameter.

 

my 74 lesabre has a 700R4 tranny, 2.72 gear in the rear, and 235/75/15 inch tires .... im just lookin for a road that i can try to pull 4grand in OD.... :laugh:

 

not being an expert with gears wouldn't it simply be a simple ratio thing? If the tire is 5% smaller say then the gear would seem bigger by the same %?
28.06 = 3.23
26.80 = 3.38
(Showing my work here, 28.06/26.8 = 1.047 So the second set of tires is 4.7% smaller, making the gear seam that much bigger 3.23X1.047 = 3.38)

check out 1bad69.com and the calc page for a good gear reference

That's just what I figured.

 

That "simple ratio thing"kinda makes since to me. Math is not my strong subject as you can tell. Yuk is trying to help me with the spreedsheet but I have yet to figure it out. About the simple ratio thing, Is that assuming that with my 28.06 tires that my gear ratio is already exactly 3.23? I am thinking that the 3.23 would be for the diameter tire that originally came on the car. Again as you can see math is is not my thing.Thank you for your help as you can see I need it

 

The gears are just the gears I believe, like the relation between the pinion gear to the ring gear. The pinon gear turns 3.38 turns for every turn of the ring gear. That's why a set of 2.56 gears will make the engine turn less turns / turn on the wheels in comparison to a 3.38, thus giving the car a higher top speed, but the motor will have to work harder to get there = less acceleration.
I've got a set of smaller tires I want to try at the track, but I still keep larger one's on for daily driving, to save fuel on the freeway.

I have a set of tires with a circumfearence (sp?) of 85.5 inches, and another set with 90 inches. That's a 5% difference in the difference of how far a tyre will roll/ turn. Now, would the engine have to have a 5% increase in rpm's to achieve the same speed on the freeway?

 

Hey Dave. Here is the way I see it. No matter what tire was on originally, the tire you have on now is what you're used to. So we call that "Dave's 3.23" So if we go to the smaller tire, we would call it "Dave's 3.38" I'm not really sure, but that's the way I look at it.

 

you are correct. once you figure the percentage in tire difference, you are most of the way there.

but geardaddy can let you try lots of different scenarios and see results as fast as you can enter the data.

i used geardaddy to obtain and compare final drive data and then i crunched the numbers with a calculator. just because it was more fun that way. and while along the way i was able to peek at the difference in rpms for givin speeds.


last fall i used the concrete part of the spreadsheet to figure the cement on our new 30X60 garage ... we had less than 10 gallons of concrete left over. :jd:

its all just fun. :bglasses:

 

Making more since, Thank you. Davy

 

However, it will make your wheelspin situation worse...

 

So then if I go from Stock on my 70 GS (26.6) to now having 275/60/15 (28.06), I have essentially converted my 3.55 gearing to 3.37. Yuck! No wonder it is hard to spin the tires with the 4 speed without the benefit of the clutch!!!!!

 

Ok I just went out to my car and looked at the size of the original spare tire in my trunk. The tire size is a H 78-15 which also matches the sticker of the recomended wheel size in the glove box. Now if someone knows how to convert that and get the overall diameter we can find out exactly what kind of gear ratio I have on the car currently.

 

Isn't there like a 15/75/225 number in there somwhere?

 

i think an H78-15 is about the same as a 235/75/15.