[Eaglestang]

I have a 1993 mustnag hatch back,
my friend gave up project to it, gave it to me.
he told me it has a C3 transmission, not a A4LD,
I started up , take for a drive, i was going 55, posted state speedlimit.
and engine reving at 3000 rpm.
My friend told me that have to change the 8.8 rear end. I have a 7.5 now,
Usally the C3 came off froma 1987 ford thunderbird turbo coupe,
I learned that has a 8.8 rear end. didnt have the money to swap over. So I went to used tire place put in oversized 14 inch tires (factory oem is 195/75/14) bought2 225/70/14s on took a drive for a mile watched speedometer about very very close to a mile when i drove a mile,
drvien daily to work engine revving at 2500 rpm at 55,
then when I drove on interstate, going 70 alot of cars passing me even trucks,,
i say not making any snese i going that fast and posted limit is 65.

[b_chandler]

First off, you don't need to swap axles. You may need to change your speedometer driven gear to dial in your speedometer, but that's a hell of a lot easier.

This will help you figure out how fast you're going for a given RPM.

http://www.kabamus.com/garage/gears.html

[Yan_Jin]

Originally Posted by Eaglestang

My friend told me that have to change the 8.8 rear end. I have a 7.5 now,
Usally the C3 came off froma 1987 ford thunderbird turbo coupe,
I learned that has a 8.8 rear end. didnt have the money to swap over. So I went to used tire place put in oversized 14 inch tires (factory oem is 195/75/14) bought2 225/70/14s on took a drive for a mile watched speedometer about very very close to a mile when i drove a mile,

umm honestly as far as i an tell your changing of tires didnt do anything to effect the drive of the vehicle. Infact, if you were trying to get a solutions out of the same rim size, and a shorter tire height, the wider tires wouldnt effect the final drive ratio.

I agree with the gentleman above that you should prolly change your speedo gear. but i think that the rear end does play a role in final drive ratio. Where the rotation of the axle is being measured and how that rotation translates onto the road is dependent on the tranny, rear, and tires. if the tires overall diameter is smaller than stock, expect to read higher speeds than you are actually going b/c they are turning at the correct speed to b measured there, but b/c they travel a shorter distance per rotation you will not travel as far per rotation. same goes for the rear. It is ment to rotate with the 8.8 so it will read for 8.8. The 8.8's ratio is higher than that of the 7.5, so Iwould expect that to play a role as well.

im no expert...but that my logical 2 cents.

but yea check out that guys link.

[b_chandler]

Difference in gear ratios and tire sizes can all be compensated for by a different speedo gear. It's just a matter of doing the math to find the right gear.

[Eaglestang]

Originally Posted by b_chandler

First off, you don't need to swap axles. You may need to change your speedometer driven gear to dial in your speedometer, but that's a hell of a lot easier.

This will help you figure out how fast you're going for a given RPM.

http://www.kabamus.com/garage/gears.html

ok how i know the c3 transmission gear ratio, help me here.
also, where to get right speedo gears? swap over a 5.0 spedo?

[Eaglestang]

err..... where get spedo gear or how to figure out, i not know anything about ratio, i just know about engine work... and body work

[b_chandler]

Other than the scale (0-85 vs. 0-140) the speedometers are identical. Changing the speedo won't change your calibration.

Check out Hanlon Motorsports online. I think they have a chart that shows the correct speedo gear for different applications or at least shows you how to determine the right gear.

[cantthink29]

So, is that true for any speedo, or just stock 5.0? I mean, could I go get an aftermarket piece and put it in?

[b_chandler]

Most, if not all aftermarket speedos need to be calibrated. Now if it's identical to the factory piece like the BBK, or Motorsport replacement speedos, there should be little to no fuss involved. If you go with an Autometer or Stewart Warner, you'll be doin' some calibratin'.

[Eaglestang]

Originally Posted by Yan_Jin

umm honestly as far as i an tell your changing of tires didnt do anything to effect the drive of the vehicle. Infact, if you were trying to get a solutions out of the same rim size, and a shorter tire height, the wider tires wouldnt effect the final drive ratio.

I agree with the gentleman above that you should prolly change your speedo gear. but i think that the rear end does play a role in final drive ratio. Where the rotation of the axle is being measured and how that rotation translates onto the road is dependent on the tranny, rear, and tires. if the tires overall diameter is smaller than stock, expect to read higher speeds than you are actually going b/c they are turning at the correct speed to b measured there, but b/c they travel a shorter distance per rotation you will not travel as far per rotation. same goes for the rear. It is ment to rotate with the 8.8 so it will read for 8.8. The 8.8's ratio is higher than that of the 7.5, so Iwould expect that to play a role as well.

im no expert...but that my logical 2 cents.

but yea check out that guys link.

yeah , i learned about that i have a 3.73 rear end and used the calucator to figure out speeds
since engine is strong and can with stand high rpm i about to swap to 15 inch wheels w/ good tires 2256015? or something like that?
i don't care about speed now, just drive work and back