How much will it change my gear ratio to drop 3" in tire diameter?

[hodgesdodges]

Ok,don't laugh!!This is for my 93 Caravan 3.0...I have a grudge race coming up against my nephews "2000 Honda SI 5 speed"...I have driven it,and I think I can take it!!!Now my van is not dead stock.I took all the seats out but the drivers.I took out the old air box,put in a straight pipe with cone K&N filter,and ran fresh air to it.Also adjusted the timing,put on bigger throttle body,took off old stock muffler system,and now run no cat with a 40 series Flowmaster(now sounds like a big rice killer).And I installed bigger badder plugs and wires...Now this thing is pretty surprising above 3000RPM,but comes off the line like a dog.So I'm wanting to change my gear ratio for the "big race"!I am running a 14" rim with about 3 1/2" of rubber,I'm wanting to swap on a pair off 13" rims with a low profile(about 1 1/2)tire,so that would drop about 3" in diameter...How much do you think this will help??? PS...I run the 8th...

[PRO]

1st let me ask,do you race with or without the kids in the van? your a madman,i like that!! A basic rule of thumb is .12 change in ratio for each inch added or subtracted.3x.12=.36 ,so add in ratio.In 93 with overdrive I think you have 3.89s so 3.89 +.36=4.25 .......PRO......

[Duffys Ramcharger]

I like the idea of a modest van as a sleeper!! Check out www.dakota-truck.net and click on the tire and wheel calculator. That'll give you all the info you want and then some.

[JVMopar]

Here is how you calculate it:

Final Drive Ratio = (RPM x Loaded Radius) divided by (MPH x Tranny gear ratio x 168.068)

The loaded radius is the distance from the ground to the center of the rim.

[PRO]

Nice formula JV but you must know a given mph and corresponding rpm.I tried that website Duffy its ok,too bad you cant select any gear ratio,and their tire hiegths are way off,a 235/75/15 they list at 28.88"!!!most run at 27.5",but if you know your tire hiegth you can just pick a size that has the same hiegth as your tire.According to both formulas if you have 3,89s H.D.s then you have 4.34s (JV)or 4.31s(Duffs),my rule of thumb says 4.25 and took way less thought!!!!!! good info guys thats what i like about this site....input..input..input......PRO.............. ....ps let us know if you win

[Moparjimbo]

go to www.richmondgear.com they have cool calculators for this and all sorts of other gear ratio related stuff.

[djswwg]

HodgesDodges, you is one SICK puppy! But I hope you smoke the Hoooonda. So whens the big race? Do you have a line-loc to hold the rear tires so you can do big smokey burnouts, or do you just use the parking brake!?!? har har har............front wheel drive, ya gotta love it....djs

[Race1Mopar]

I absolutely love it hodgesdodges,
ok, Here's another way of looking at it. The height of the tire can be obtained by this formula: 265x75x16 converted is (265/25.4) x .75 = one side wall. Add two side walls and the rim height and that is the height of your tire. (25.4 is a constant) The height of a sidewall is 7.82", two sidewalls are 15.65 plus the rim height of 16" means this tire is 31.64"

Why does this matter? Let's say I drop 2" of this tire for a race and I have a 3.55 rear end. To find my new theoretical rear ratio I would take (old tire height/new tire height) x ratio=theoretical ratio. So (31.64/29.64)x3.55 = a theoretical rear gear of 3.79.

Be sure if you go with a smaller diameter tire to try and go wider. The extra gear may help you go faster... as long as you can plant it and not sit and spin which would definately be bad... hope this helps!! Now go forth and eat ricers!!!!