89 Metro XFI - MPG Decrease w/ Larger Tires

[mavinwy]

As long as your sppedometer and odometer were accurate before, then yes....the calculation will be off by the same percentage.

Another way I have seen people work with the tire size change (other than changing the speedo gear in the tranny of course) is to use a GPS to figure how far off they are at a given speed on the speedo (say 60MPH for ease of calcuations) and figure the % from there....

Jim

[TestDrive]

Originally Posted by SirKeats

sweet!!!!

i totally didn't even thing of the odometer being off. thanks gents!!!

i don't recall the old size off-hand. will look when i get home and post up.

so what i would do is use the calculator above to determine the difference and then use this to calculate the odometer different (i.e. the figure provided will work for both the speed and distance calculations)?

thanks again! much appreciated!

Tire size calculator should get you close to the correct figure.

For exact calibration, compare against the mile marker posts on a ten mile stretch of freeway.

eg. Reset your trip meter at the instant you go by mile marker 237. Watch for either mile marker 247 or 227 (depending on your direction of travel). Note the trip meter reading just as you go past. 10 miles traveled divided by trip meter reading = odometer/speedometer correction factor.

[MetroMPG]

Or use Google Earth: Using Google Earth to verify your odometer accuracy - MetroMPG.com

[SirKeats]

quick follow up question...

the stock tires say: 155 77S R12

the calculator provided doesn't have the 77 option. i found another one that let me manually type in figures though.

just currious though... does that 77S mean something else maybe? every other calc offers only 155/75 or 155/80 options. just thought the 77 was a bit weird so figured i should check first.

FYI - new tires are 155/80 R13

thanks!

[Coyote X]

stock for the 12 inch tires is 145/12 or 145-80/12

Hunt around the junkyards for a 17 tooth speedometer gear off a 96 up metro to fix the error and not have to deal with calculating the mileage all the time. The 12 inch tires had a 18 tooth gear so a quick and dirty way I think might work to correct the speedo/odometer error is (actual speed = measured speed * 18 / 17)

[bhazard]

My stock tire size is 145/SR12, and Im running 155/80/13's up front. Multiply your miles by 1.077 and youll get your correct mileage. Its actually a pretty big difference.