I have a 98 3.8 with a 7.5 I just put a set of 4.10s in and I cannot find any answers to if I have can have a tune made to fix the speedo or what gear I need, and help would be greatly appreciated.
Unfortunately, you're a year too early to be able to change this in a tune (I'm in the same boat).
Gears might be possible, but there's some caveats:
4.10s on stock tires with the stock main speedo gear (8 tooth) requires a 23 tooth sender gear; what I've read suggest that this combo will destroy itself.
If you're running a manual, you should be able to change the main gear (to a 7 or 6 tooth gear), which would allow the use of a smaller sender gear (21 and 19 respectively). To do so involves pulling the tail off the transmission. From what LMR says, the auto trans cannot swap this gear (I'm no 4R70W expert though).
The other option is a speed cal unit, which is installed between the speed sensor and speedo, and allows adjustments of the frequency being sent. I've not see any Mustang specific ones for sale lately, but someone brought it to my attention that there are generic ones, and I have found a couple that output the correct format for our cars (8000 hz per mile). So far, this is what I'm considering: Dakota Digita Speedometer Signal Interface From what I'm reading on it, this should work, but I do have to say I have not done this yet (I've been waiting to do gears until I could solve this problem).
Unfortunately, you're a year too early to be able to change this in a tune (I'm in the same boat).
Gears might be possible, but there's some caveats:
4.10s on stock tires with the stock main speedo gear (8 tooth) requires a 23 tooth sender gear; what I've read suggest that this combo will destroy itself.
If you're running a manual, you should be able to change the main gear (to a 7 or 6 tooth gear), which would allow the use of a smaller sender gear (21 and 19 respectively). To do so involves pulling the tail off the transmission. From what LMR says, the auto trans cannot swap this gear (I'm no 4R70W expert though).
The other option is a speed cal unit, which is installed between the speed sensor and speedo, and allows adjustments of the frequency being sent. I've not see any Mustang specific ones for sale lately, but someone brought it to my attention that there are generic ones, and I have found a couple that output the correct format for our cars (8000 hz per mile). So far, this is what I'm considering: Dakota Digita Speedometer Signal Interface From what I'm reading on it, this should work, but I do have to say I have not done this yet (I've been waiting to do gears until I could solve this problem).
Yeah it’s an auto and from what I found they all said the 23 tooth would destroy itself in feet and I’m not on stock tires either so that makes it even harder. Thanks I’ll look in to that and might give it a try because right now the Speedo is way off was told it would only be like 10-15 off but when I’m doing 75 it says 100
Yeah, with gearing it's a change in ratios, not a static change. 3.27 to 4.10 means your true speed is 75% of what your actual is; the faster you go, the further off it is.