So I just got wheels on my car in 295-30/20 and 255-30/20. This should put my rev/mile at about 750, right? Well I have stock gears and Chris set me up with tune. When I went to change my rev/mile to 750 I checked the gear also. Well, I set the rev/mile and I was still off by about 3 or 4 mph at 65 (using gps, I know some may say its not always accurate, but its closer than my speedo). At that point I changed the gear from the stock setting to 3:55 and it didn't change much. So in the end I had to set the rev/mile up to about 800 to get it close within 1/2 mph. What am I doing wrong?

 

On my 285-30/20 tires I used the aspect ratio to calculate the diameter to use for the rev/mile and my speedo was off by 3 or 4 miles per hour. I then actually measured the diameter which was different than the one the aspect ratio yielded and used it and then it was correct ...

 

Yea I was using this:
295mm*0.03937in/mm=11.6in
11.6in*.3=3.48in
3.48in*2+20in=26.97in
(5280ft/mile)/(26.97*pi/12in (ft/rev))=747rev/mile

Still, that doesn't explain why I have to put in 800 rev/mile to get it somewhere close. I mean with the smaller diameter that should effectively change the overall drive ratio, but I don't know where to go from here.

 

Add 4.10 gears into the mix on these:

Front - 255/35-20
Rear - 285/30-20


Fronts and back I split the diff for the number then told the tune I had 4.10's....

Im not sure that did it or just screwed it up but it seem to be pretty darn close.

 

Your 295/30/20 tires must be shorter than calculated, as that would explain why the tire has to rotate 50 more times per mile than you calculated. I measure actual height of my tires to set revs per mile with my tuner, then I check it with mileposts on our local interstate over a 20 mile stretch to double check. GPS verified, as well, so I know that it works.

 

Well I did a quick calc and it seems that in order to get 800 rev/mile my tire diameter would have to be 25.2". Thats something like a 220/30-20. I doubt that is a size!
I understand the measuring, but the difference between stock and what I have is only like 5 rev/mile.

 

So, I just spoke to CR and he said that there is some program that calculates it out. Supposedly 295/30-20 is a 771?? I'm going to try that tonight with the stock setting on the gears and see if that does it. Hope for the best!

 

Use this, it's pretty close http://www.miata.net/garage/tirecalcold.html or if you want a fancy version http://www.bigcustomwheels.com/rt_specs.jsp

 

Just go to the tire manufacturers website and look for your tire. They will have the correct rev/mile listed for your exact tire. No fancy math needed

Just looked at your signature
Tire size --------------------------------------------------------------------------------------Revs
295/25ZR20 95W RD 197550 10.0-10.5-11.0 29.1 11 26 11.6 12.3 1521 50 300 AA A 799

295/30ZR20 101W RD 197540 10.0-10.5-11.0 30.6 11 26.9 11.7 12.7 1819 50 300 AA A 771

 

Grem, thanks for that. CR said 771 too, so I tried 780 and that puts me about 63mph when the speedo says 65. I'm guessing that there is some error from factory that is requireing me to put in around 800 for it to be correct. Thats about 3.7%. Does that sound right to anyone? Does my car just have an A hole speedo? The guys that gps'd your car after the tire size change, were you spot on or just close to like 1-2 mph?

 

Spot on.

 

jojade,
I might be saying something you've done already, but did you try 771? Can you only go in steps of 5? If so, 770 wouldn't be that far off? I'm sure Chris could fix the tune to 771??? Sorry if this has been posted or I misread.

 

No its ok. I did try the the 750 and it read 65 and I was doing about 60ish. Then I tried 780 and it was going about 2-3 under 65. So finally I ended up with 800. Thats got me either spot on or pretty **** close. 820 was over.
The thing I don't understand is why does it take such a high rev/mile to get my car right. I mean I even measured the distance of 1 rev of my rear tire and it came out to 84inches. With that in the mix its still in the 770 ish range. CR said to try the 771(it was low still) and he would rewrite the tune to whatever I wanted. I'm just wondering if something else is wrong with my tune or my car to make it further off than what you guys are getting with normal tire revs/mile.

 

Did you change the setting for your rear gear?

 

Hey Bullitt, I did at first thinking that the tune file could have possibly been programmed for a different gear, but it didn't change anything. Last night I spent about an hour driving with gps trying to figure out what rev/mile worked best between 30-70mph. 800rev/mile seems to work the best.

 

Maybe pull the battery and reflash????

Something maybe sticking in memory for a while <- yes I'm reaching, but you do this on computers when you upgrade firmwares so it may be similar.

 

Have you actually measured the diameter of the tire and then divided the number 20168.114 by the measured diameter ? Like I said in an earlier post when I calculated the diameter from the aspect ratio and used it the speed wasn't correct , but once I actually measured the diameter and used it the speed was correct ...

 

Then just leave it at 800. Make sure it's accurate at other speeds too 30-40-50 mph etc. Most speedometers aren't that accurate anyway.

 

Gremlin: I have reflashed several times to change the rev/mile, but I have not disconnected the battery at all. I may try that though.

Steve: I took a piece of tape and wrapped it around my tire until the ends met and rolled it a few times. Then I took the tape off and measured the circumfrance. I used that to calculate the rev/mile. If you use the size of the tire (295/30) then you get like 746 I think. After my measurement it was about 770ish. Still off!

Bullitt: I think I will just leave it at 800 like you said. My brother is a cop and last night I got him to radar me up to 65 and it was spot on! I'm guessing there is just some error from the factory that wasn't accounted for before. I mean I knew it was off by about 1-2 with the stock tires.

Thanks to all for ideas and help!

 

Well I'm using 800rev/mile and CR just sent me updated tunes to have that as the stock value which seems to have my speed right on. The bad news is, I filled up the other day and read my odometer from one spot to the other and it seems to be off by 3.7%. I used google maps to tell me the distance. Seems weird that the speedo would be right but the distance would be incorrect. Maybe the odometer isn't reading/isn't synced properly with the speed sensors. That 3.7% seems to the difference between the 771rev/mile and the 800. Any suggestions?

The only thing I can think of is either let the speed or odometer be correct.