ChicagoBurbs

I've ordered new 18" wheels and tires for my 07 TLS, and came to find that there is a slight difference in tire diameter which according to 1010 tires chart shows there will be a difference in speedo accuracy. Can this be recalibrated, or am I stuck dealing with a speedo that's a little off?

Would any of you guys think this is a big deal or should I just not worry about it?

NOTE: BOLD represents the new tire size below.


Stock Tire - 235/45R17 Tire 1 - 235/40R18
Section Width:9.25 in 235 mm Section Width: 9.25 in 235 mm

Rim Diameter: 17 in 431.8 mm Rim Diameter: 18 in 457.2 mm

Rim Width Range: 7.5 - 9 in Rim Width Range: 8 - 9.5 in

Overall Diameter: 25.32 in 643.12 mm Overall Diameter: 25.40 in 645.16 mm

Sidewall Height: 4.16 in 105.66 mm Sidewall Height: 3.70 in 93.98 mm

Radius: 12.66 in 321.56 mm Radius: 12.70 in 322.58 mm

Circumference: 79.54 in 2020.3 mm Circumference: 79.79 in 2026.6 mm

Revs per Mile: 821.4 Revs per Mile: 818.8
Actual Speed: 60 mph 100 km/h Speedometer1: 59.8 mph 99.6 km/h

Speedometer Difference: - Speedometer Difference: 0.317% too slow
Diameter Difference: - Diameter Difference: 0.32%

LKLD

I was in my dealership about 3 wks. ago and asked my service advisor the same question. He said yes they can.

I've done it before on a Dodge truck I owned. All they need to know is the new revolutions per mile. They plug in under the dash and put the new info in...about 5 minutes. I'm sure it's about the same for Honda/Acura.

ChicagoBurbs

Thanks for the quick reply. I'm really glad to hear it's that easy to fix, and that is can be done electronically. I was afraid it would take some mechanincal adjustment that is more objective, depending on the guy who makes the change. Now the big question is how much this should cost. Doesn't sound expensive.

Xx06SickspdTlxX

Ive never heard of such a thing on honda's and acura's but i could be wrong, i dont think the HDS has the software or capability of recalibrating the speedo to different rim or tire size, because aftermarket rims and tires arent factory spec anyways. Well thats just my 2 cents, i will ask some more experienced techs at work tomorrow.

ChicagoBurbs

Appreciate your input as well...but it seems they OUGHT to be able to make this change, especially since the tire size I'm installing is an exact match to the tires Acura dealers install when you order the Aspec or Chrome-Look wheels from Acura accessory stock. I would assume (perhaps incorrectly) that they would do this as a matter of routine when they swap to the 18's at the dealership.

Xx06SickspdTlxX

See when you but the aspec wheels you get an 18" wheel with a smaller profile tire, all you are doing compared to the stock 17" rims is adding 1" more in rim and down sizing in tire. The size of the aspec with the recommended tire size and the stock 17" wheels with stock tires should be the same or even near in size, which the speedo will remain the same or be close to factory spec on the speedo!

ChicagoBurbs

Appreciate your explanation, and I understand completely. However as you state, the 18" replacement tire is very close to the OEM 17" tire, but there is still a difference as shown here:

Actual Speed: 60 mph 100 km/h Speedometer1: 59.8 mph 99.6 km/h
Speedometer Difference: - Speedometer Difference: 0.317% too slow
Diameter Difference: - Diameter Difference: 0.32%

I still think the dealership SHOULD be equipped to adjust for that difference so the speedo reads acurately since they actually sell and install the tire size I am purchasing.

Xx06SickspdTlxX

I see there is a very slight difference but from the automakers eyes is very minimal, i will find out tomorrow if there is any ways of correcting this!

Bearcat94

Call me stupid, but in this case I don't see the point.

Lets say your hauling ass out 88 at 120 mph. When the trooper walks up to your car are you gonna say, "Gee officer, I didn't realize how fast I was really going. I just got new tires and my speedo is off a little - it only showed 119.6."

If it was 2 or 3 mph, yeah, maybe you want to get it recalibrated, but 2/10ths? I wouldn't waste the time.

260 HP

iTrader: (3)

I have to agree that pursuing this will be pointless because the difference is so minute. Even as your tires are wearing down, the difference in overall diameter is larger than the numbers you presented here.

The overall diameter difference between full depth tread (10/32") to legally bald tread (at 2/32") is 0.5". I think you don't want to adjust your speedometer as your tires are wearing down, do you?

jdeec

This(diff. wheel sizes) also effects the number of miles reported by the odometer, correct?

leedogg

I also think its too minimal to affect the odometer. However if it WAS a significant difference, I would wait until you actually DID get a ticket then go get it tested and provide proof to the judge your odometer was off.

ChicagoBurbs

I think you guys are right, and the difference is so small it's insignificant.

However since I'm going in for service Wednesday I will ask them if they do adjust the speedos when they install the factory 18's...just so we all know.

Thanks again everyone for your input.

S PAW 1

That's funny. I asked my dealership the same question, and they said "No".
Electronics/computer can't be adjusted.

skyy406

I work at an Acura dealership in sales and asked the same questions or several techs and the service manager. No the speed ometer cannot be re-calibrated.

94eg!

First off, a difference in tire size does change all instruments weather it's big or small. There is no arguing that fact. If your tire is 3% taller (103% of original), your speedometer is reading 3% low, as does your odometer. This means if your speedometer says your going 100mph, your actually doing 103mph. And if your odometer reads 100,000 miles, you really have 103,000 miles on the car. It works the exact opposite if you have smaller tires (instruments read higher than they should). To know the exact amount for your exact tire look up it's diameter in inches, and divide it by the original tire diameter of 25.5" to get your percentage difference.

EXAMPLES:
245/40/17 = 24.8"D = 97.3%
245/45/17 = 25.7"D = 100.8%
255/40/17 = 25.2"D = 98.8%
235/40/18 = 25.5"D = 100%
245/40/18 = 25.7"D = 100.8%
245/35/18 = 24.8"D = 97.3%
255/35/18 = 25.0"D = 98.0%

I wouldn't really base your calculations on the revs-per-mile rating as that really depends upon the load on the tire itself. Comparing tire diameter "should" be more accurate.

And as far as recalibrating the speedometers (and no it cannot be done), there is a VERY good reason that automakers don't allow this. That reason being ODOMETER FRAUD! People could simply self program the speedometer to run with a tiny tire and the car would suddenly stop racking up miles. For demonstration purposes, lets say you tell the computer you have a tire with a 50% smaller OD than stock when you are in fact running the stock tire. Now when you rack up another 50,000 miles on the odometer, you've really gone 100,000. Now for a real-world example lets say you tell the computer your running a tire thats 3% shorter than stock, yet you've actually installed a tire that's 3% taller. All the sudden your odometer is 6.2% fraudulent and that's quite a big difference...

I know a lot of people would sacrifice an accurate speedometer to keep from racking up miles on their car...