speedometer is way off due tire size

[mherman18]

I got the same size tires 235/50 all the way around and now the speedometer is off by 5 mph slow. Is there a way to fix this with a tuner or how can I solve this problem?

Thanks in advance

[Glockafella]

5 MPG slower than actual?

[Glockafella]

My 255s in the rear are making me 5MPH faster on my speedo than actual speed. I prefer it that way...makes me safer with cops running loose writing tickets.

[MrBluGruv]

a smaller than stock tire size should cause the speedometer to read a higher MPH than your actual physical MPH. I'm on 245/45/18ZR all around and I think my speedo reads roughly 5mph faster than real speed at around 70mph. I haven't verified this against a calibrated speedo, but once on the highway I was doing what I thought was 70mph and I looked over and the civic next to me and their digital speedo display read like right at 65mph.

Either way, keeps me from getting a speeding ticket if my speedo says I'm dead on the money. Also reinforces my idea that people are scared of anything that even remotely looks like law enforcement, cause few people dare to pass me even being sometimes up to 5 mph below the posted speed limit. :P

[breeze]

yea i got 235's all around too

[bob6364]

I got the same size tires 235/50 all the way around and now the speedometer is off by 5 mph slow. Is there a way to fix this with a tuner or how can I solve this problem?

Thanks in advance

You need an SCT tuner and change the revs/mile to the correct one for your tire diameter. There are usually several people around here selling older ones for short money.

[Bluerauder]

My 255s in the rear are making me 5MPH faster on my speedo than actual speed. I prefer it that way...makes me safer with cops running loose writing tickets.
Surely you are exaggerating and reversed. ;) The 255/55 are 29.04" tall. The 245/55 are 28.61" tall. That means that the difference is only 1.5%. When your speedo says 60 MPH, you are actually going 61 MPH. Only 1 mile per hour difference at 60 MPH. The 255/55 are working against you on tickets not for you. Unless something else is contributing to the difference.

[fastblackmerc]

Usually the speedo difference is a percentage, ie. the faster you go the more MPH your speedo is off.

[Bluerauder]

Usually the speedo difference is a percentage, ie. the faster you go the more MPH your speedo is off.
Yes, 1.5% based on the difference between 255/55s and 245/55s alone.

.... so 1 MPH difference at 60 MPH (vs. 61 actual)
....and 2 MPH difference at 120 MPH (vs. 122 actual)

[mherman18]

ok i used a gps and and when is was going 40 on the speedometer it was actually 35 70 was actually 65 sorry i think i did reverse it it seemed to be 4-5 mph throughout.

[Bluerauder]

ok i used a gps and and when is was going 40 on the speedometer it was actually 35 70 was actually 65 sorry i think i did reverse it it seemed to be 4-5 mph throughout.



I got the same size tires 235/50 all the way around and now the speedometer is off by 5 mph slow. Is there a way to fix this with a tuner or how can I solve this problem?

Thanks in advance

The difference in speedo should only be about 3 MPH at 60 MPH (vs. 57 actual)

235/50 height is 27.25"
245/55 height is 28.61"

The 235/50 are only 95% of the 245/55 so the actual speed is 95% of 60 MPH or 57 MPH (minus 3 MPH) ....give or take a hair due to tire wear differences.

80 should be 76 (-4.0 difference)
70 should be 66.5 (-3.5 difference)
60 should be 57 (-3.0 difference)
40 should be 38 (-2.0 difference)

[mherman18]

Should a ford dealer be able to fix this or just a hand held tuner plus i lost almost 2 mpg

[Bluerauder]

Should a ford dealer be able to fix this or just a hand held tuner plus i lost almost 2 mpg
If you lost 2 MPG from tires alone .... the first thing that I would check is tire pressure. You could be several pounds low all around.

[bob6364]

Hahahaha....you guys are hilarious,give it a break,"no its faster...no its slower...poo poo you uneducated moron here's a formula"...gawd will 2 more people tel him to buy a tuner so he gets the point.I think all a Ford mechanic will tell you is put the right size tires on there....

[mherman18]

Wow how helpful you are. Great attitude and thanks for the help.

[Stranger in the Black Sedan]

Tuner. The money you just saved on using 235s all around just cost you a tuner

[Dennis Reinhart]

I got the same size tires 235/50 all the way around and now the speedometer is off by 5 mph slow. Is there a way to fix this with a tuner or how can I solve this problem?

Thanks in advance


It is fixed by adjusting the revs per mile value in the ECM any SCT or Diablo dealer can help with this.