Correct gear ratio for tire size.

[drumnerd33]

Ok so i was doing some research on gear ratios because i know alot of us run larger tire sizes on our cars now adays. What i dont like about my 185/60/r14 is that my gears seem really long now, and 5th gear is almost somewhat useless. So i was reading about what some of the other threads on here were saying about the festiva vs aspire mtx 5 speed and it seems there was some misinformation. All of the gear ratios of 1-5 are not the same.... now stay with me here.. 1st 2nd and 3rd are identical, but 4th and 5th are not. here are a couple links on the stock specs of the two transmissions.
1994 ford aspire 2 door mtx 5 speed

1988 ford festiva lx mtx 5 speed

Now i got the stock tire size and entered it into the garage buddy app on android (if you dont have this it is a great app with tons of calculators for mechanical work) and wrote down the tire diameter size= 21.1 or 21.44 for 13 inch rims now i am running the 14 rims with 185/60 which tire diameter comes out to 22.74 thats why my gears are so long now is from the larger tire diameter. Now i went to the garage buddy tire size gear calculator and entered my original tire diameter of 21.1 and the new diameter of 22.74 and (referring back to the links i posted it gives you final drive or differential gear ratio) the festiva diff ratio of 3.77 and it spits out the proper gear ratio for the larger tire diameter, which= 4.06 now once again refer back to the aspire link and see the final ratio of the aspire mtx= 4.058 Wow thats almost perfect to put me back at stock ratio, so my speedo would be almost exact and have shorter gears which = more power and will still be great on the highway.

Now if you look closer at the 1-5 ratios you will see the aspire is a little taller on 4 and 5 so yes you will see a slightly higher rpm on the highway but will be minimal.

[drumnerd33]

Links didnt work gimme a little bit and will post pic of page.

[drumnerd33]

So in conclusion here if you are running the larger rim tire combo the aspire swap would actually fix the speedo being off and bring you back to stock gearing and still give you the mpg you want... the only reason people see less mpg on an aspire is the stock tire size is almost the same size as a festiva stock size..so its a little over geared and that makes the rpm too high on the highway, with taller tires and a aspire mtx5 it should be perfect.

[rmoltis]

Cani lupine put a festy transmission in his aspire.
I think he runs around 2500rpms @60mph.
As opposed to 3000 @ 60mph.
I would rather have the lower rpms at cruising speed to increase engine longevity,
and keep the gas mileage high .
I would at least try stock diameter tires first you would probably be happy.
But if you want the increased torque, lower mpgs, and -16mph top speed Go for it.
If I could increase my torque as simply as putting in a new transmission I would. I love torque.

[drumnerd33]

Well im not sure if its my altitude or what but before i built my motor up i would drive the highway going to work @ 85+ mph and would still have to shift into 4th to keep my speed up.. now with the way more powerful engine i still hit 75 in 4th gear... seems pointless to me if i could be in 5th gear and hold 65-80 i think it would actually save some gas because i wont have to be downshifting to 4th all the time.

[sketchman]

This would fix the speedo being off? For some reason that's not making sense in my brain. Isn't the axle still turning the same speed for a given tire size regardless of how fast the engine is turning?

[drumnerd33]

The differential is a taller gear festy is 3.77 and aspire is 4.058 so this will cause your axles to turn more revolutions... which will bring the speedo back closer to stock, i cant say it will be perfect but will be really close. I have done alot of regearing on jeeps and other rock crawlers and when you run a larger tire you need a taller gear not only for proper power to turn the tires but it brings your speedo back to stock

[drumnerd33]

right now @ 60mph my speedo reads about 64-65 your gear ratio should match your tire size to fix this problem... if you use a tire size gear calculator it will tell you what gear ratio with what tire size gets you back to the right speedo reading... and with my 185/60/r14 i need a ratio of 4.06 and the aspire is a 4.058 so its literally almost perfect... should bring my speedo to read extremely close to perfect.

[blkfordsedan]

I always figured the lower final drive ratio on the Aspire was intended to compensate for the taller tires they used, as well as the increased vehicle weight. Remember that most factory Festiva tires were 145/80r12 and not the 155/80r12 that everyone sells. When I went to 175/50r13 tires (same diameter as 145/80r12) it was a night and day difference. The car was much more "zippy". The cruise RPM increased, but it put it right in the sweet spot of the torque curve. As a result, the car held speed better on hills and MPG actually increased. Big tires are power killers, and just because RPMs decrease does not mean MPG will increase. It's all about the total package.

[drumnerd33]

^Exactly...... sometimes its hard to understand gear ratio and all the numbers and calcs, but im telling you guys this to help... if you are running 14s and have the issue of 5th gear being almost useless and speedo being off.... Here is your fix... and there is plenty of them in the junk yard

[sketchman]

It's cool that it's an option, but it does require a trans swap, so wouldn't it be a lot easier to just buy the right size tires when you need them? My speedo is accurate with 14s and 5th gear is fine too. I guess it's down to what you want to do, though.

[nitrofarm]

I always figured the lower final drive ratio on the Aspire was intended to compensate for the taller tires they used, as well as the increased vehicle weight. Remember that most factory Festiva tires were 145/80r12 and not the 155/80r12 that everyone sells. When I went to 175/50r13 tires (same diameter as 145/80r12) it was a night and day difference. The car was much more "zippy". The cruise RPM increased, but it put it right in the sweet spot of the torque curve. As a result, the car held speed better on hills and MPG actually increased. Big tires are power killers, and just because RPMs decrease does not mean MPG will increase. It's all about the total package.

Stock Festy 145/80R12 dia = 21.2"
175/50R13 dia = 19.9" this is why your Festy will feel "zippier" with these 13"alternative wheel/tires. http://chadconway.pbworks.com/w/page...re%20Breakdown

right now @ 60mph my speedo reads about 64-65 your gear ratio should match your tire size to fix this problem... if you use a tire size gear calculator it will tell you what gear ratio with what tire size gets you back to the right speedo reading... and with my 185/60/r14 i need a ratio of 4.06 and the aspire is a 4.058 so its literally almost perfect... should bring my speedo to read extremely close to perfect.

Drummer your 185/60R14 specs out @ 22.7" dia . From personal expierence running a Festy tranny in my Aspire's for @ 65,000 miles the max dia I can run is @ 22.6"-22.7".
When I went taller it was just to sluggish on the highway. With 175/64R14 23" dia Blizzacks's the Festy 5spd in my Aspire was just to tall for comfort. My mileage suffered badly(I'm sure some of it was tire drag and the very heavy BMW wheels) but it still was over geared.
My normal "Cruise" speed and commute is 75mph with some very large hills (no CO mountains but Large hills).I drive 102 miles everyday,and with stock Aspire 175/70R13 dia 22.6" tires its just enough motor to feel comfortable. But if I get days with big head winds pulling some of these hills gets annoying in 5th. I would really like to try my Blizzacks with STEEL wheels but I still think its just too tall of a tire for my combo.
So with all that said, for ME a tire that's @ 22.6" dia on my Aspire with a Festy tranny is pretty much maxed out. And @ 75 mph I'm @ 3100 RPM btw. So in a lighter Festy this tire dia should work pretty good with a healthy strong engine.
I'm going to try to get some Honda 15" steel wheels and run 195/50R15 22.7" dia rubber this summer on this Aspire. Also very important to note is that "Printed" tire specs are meaningless! The actual tire specs can vary widely from manufacturer. I worked for Goodyear as a tire salesman so I know this for fact. We would see this ALL the time,so a WISE person will actually measure his overall tire dia. And not just assume its what its supposed to be....
And Drummer how did you get a 14" to fit on your stock hub? Or did you finally pull the trigger and do the Aspire swap? The 14" wheels are hard to get a good tire dia for our MTX cars, the 13" size is easier to find more selection. The 175/70R13 is basically the Aspire size and its still 1.4" larger dia than a stock Festy.

[blkfordsedan]

I'm not sure if the speedo works out 100% on an Aspire trans in a Festy. Logic says it will, but Im not sure math works out between the tooth count on the speedo cable gear that slips in the tranny and the Festy speedometer. I seem to recall there was something odd about the speedometer drive gears between the two versions. I could be dead wrong....there are several people on here that have messed with it and can tell you for sure.
Regardless, the steeper final drive on the Aspire works great to gain back the applied torque lost by running taller tires. I believe the guys running the G25 from an EGT or Capri get like a 4.11 final drive. Some on here have done some cool stuff with swapping internal gears and other stuff.

[rmoltis]

I agree with everyone else.
Keep your 14" tires and have a tire shop put a set of tires on with stock diameter.
You can keep whatever rims you have but the new tires would correct these things.
It would save you a lot more time money and customization.

[drumnerd33]

^yes that is correct, if you are running a festy tranny in your aspire then having a tire my size @ 21.7 diameter does not work that well... that is my problem and now realizing this i see a aspire tranny in my future. Festiva stock diff gear ratio @ 3.77 and aspire stock diff @ 4.05 that will help alot, and according to the calculation i need a 4.06 for my size tires... so the aspire 4.058 is right there.

Now as far as sucks being a tranny swap... yes it does suck in a way, but im looking at it like this, when i changed my gears from 3.55 to 4.56 it is way more work than a tranny swap... now i also installed arb air lockers but still doing the gears takes a bit of time to set them correctly and not screw them up so they will last a long life, so from that stand point swapping a tranny seems like a cake walk.