# Tire change, speedo calc.

I recently put some 33x21.5x15's on the Blazer to replace the 31x10.5x15's. Any one know how far out the speedo will be now? How much more fuel I will use?
How many more/less RPM will I need to have to maintain the same speeds?
Basically, what sort of changes should be noticeable?
Wiz.
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Play around wtih one of these calulators like this one:
This will give you an idea. Of what to expect. To give you an idea before you were probably going 62mph, now you will be going 66mph (2500 rpm, 1, 1, 3.73). But what I like to do even better than that, is to take a drive with a GPS in the car with you. They will measure speed faily accurate. Use your cruise control or hold the speed as best as you can, and see what the GPS reads for speed.

31x10.5x15's.
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the GPS, for sure! We just got a gps setup for the laptop and were surprised that it showed speed... we thought that it was in need of adjustment because it showed us consistently going about 3mph faster than the speedo indicated on a trip from calif to washington and back..
A neighbor checked with dodge (this was a ram) and found that when we went from 255 75 tires to 265 75 tires last month, it caused a 4% speedo difference!

Mac
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Hello Wiz,
You went from a 31" diameter to 33" diameter tire (an increase of 6.45%). Driving on your larger diameter tires and assuming no slippage in the transmission (or same amount of slippage, if any) as before:
If your speedometer indicates 60 mph, your actual speed "X" will be: X = 60 mph x 1.0645 = 63.97 mph If your speedometer indicates 100 mph, your actual speed "X" will be: X = 100 mph x 1.0645 = 106.5 mph
To drive at actual 60 mph, your speedometer indicated speed "Y" should be: Y = 60 / 1.0645 = 56.4 mph To drive at actual 100 mph, your speedometer indicated speed "Y" should be: Y = 100 / 1.0645 = 93.9 mph
If you used to get 20 mpg driving at 60 mph on your old set of tires, driving at actual 60 mph (56.4 mph indicated) could get you: MPG = 20 x 1.0645 = 21.3 mpg
Good luck, Franko

31x10.5x15's.
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Thanks Franko,
You have taken out some of the guess work.
Now I just have to stay at some of those limits and see how my gas mileage works.
Thanks,
Wiz.

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