Should my speedo be spot on? Mine speedo is about 5mph fast.. I've read conflicting info... is there a formula somewhere that it can be calculated?
I have 4.88's and 315's, Speedo???
- Thread starter rblain82
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bloc
SILVER Star
there are calculators everywhere online
Simple RPM Calculator | RANDYS Worldwide
Edit: a better way to do the formulas is to look up your tire's revolutions per mile then work from there. A small inaccuracy in height ends up adding up to a lot over the distance of a mile and the manufacturers do the math for you usually.
Simple RPM Calculator | RANDYS Worldwide
Edit: a better way to do the formulas is to look up your tire's revolutions per mile then work from there. A small inaccuracy in height ends up adding up to a lot over the distance of a mile and the manufacturers do the math for you usually.
5-8% over sounds about right for 315s and 4.88s. To get back to stock you would have needed 4.56s and 315s or gone to 37s with 4.88s.
Here's a good site for converting the numbers to see.
http://www.grimmjeeper.com/gears.html
Here's a good site for converting the numbers to see.
http://www.grimmjeeper.com/gears.html
I have 315's that are on the smaller side and I am 8% under geared.
So, one easy way to figure this out which will take into account tire deflection is use an app like Speedview on your phone.
Frank
So, one easy way to figure this out which will take into account tire deflection is use an app like Speedview on your phone.
Frank
This is why I am going to 37's. Speedo correction with increased ground clearance as a bonus. 
Plus, the deeper gearing means the drive shaft spins faster which usually makes vibes worse. Bigger tires will help there.
inkpot
SILVER Star
Check the gps on your fone. Most have speed indicator.
I have the same tire size and gearing. At a gps speed of 65, the speedo reads 72. It is a linear relation back to zero from there. YMMV, literally!
By the book you are overgeared. You actual speed will be slower than the speedo shows.
315's should get 4.56, 37's 4.88 in order to be close to stock specs. Again, by the book.
In real life, mane people prefer to be overgeared with 315's @ 4.88
For example I am on 285 with stock gears, that translates into being slightly in overdrive with my real speed being higher than what the speedo shows (by only 3MPH, it would go slightly higher from there on 315's).
As others stated, phone GPS, or a navigation system with GPS speed info should help you until you get the idea and the feel of your speed.
On my DD (non Toyota) I know by how much the speedo is off (as set by the factory, most newer cars do it for "safety" purposes). It always shows a higher speed than in reality and it gives you the feeling you are flying on the freeway, when in fact you are going at the speed limit.
I am sure there is an agreement here between US and manufacturers.
I do have an app on my phone which can code all sorts of parameters, but it does not allow US cars to have their speed altered. European cars can do it, but not USDM. There is a way to do it, as some hackers found out, but I really don't care, I know the real speed since I know how many MPH to add at any given speed.
315's should get 4.56, 37's 4.88 in order to be close to stock specs. Again, by the book.
In real life, mane people prefer to be overgeared with 315's @ 4.88
For example I am on 285 with stock gears, that translates into being slightly in overdrive with my real speed being higher than what the speedo shows (by only 3MPH, it would go slightly higher from there on 315's).
As others stated, phone GPS, or a navigation system with GPS speed info should help you until you get the idea and the feel of your speed.
On my DD (non Toyota) I know by how much the speedo is off (as set by the factory, most newer cars do it for "safety" purposes). It always shows a higher speed than in reality and it gives you the feeling you are flying on the freeway, when in fact you are going at the speed limit.
I am sure there is an agreement here between US and manufacturers.
I do have an app on my phone which can code all sorts of parameters, but it does not allow US cars to have their speed altered. European cars can do it, but not USDM. There is a way to do it, as some hackers found out, but I really don't care, I know the real speed since I know how many MPH to add at any given speed.
When I was running 4.88's and 315's my Marks Box correction was 8%, and that was perfect per GPS. After my Diesel conversion I went back to 4.10's with the Terrain Tamer T-Case OD and still running 315's, and had to adjust the box, which took all of five minutes. Speedo once again 100% correct per GPS, and getting a verified 20 MPG from my last fill-up, with the idiot light coming on at 400 miles. 
Bludozer
SILVER Star
I have the same gear/tire combo as you OP and speedo is about 5mph over too. It doesn't really bother me honestly and I LOVE the way it feels being slightly overgeared. 37s are always tempting (f I'm totally honest with myself I don't really need them anyway) but it would be really tough to give up this delicious over geared feeling and have to go back to stock ratio at this point.
So speaking of overgeared, let me put my two cents in. I am running 37's on 5.29's. I am about 9% overgeared. With the 37's there is A LOT of tire there and with that A LOT of weight to spin. Even with this gearing, I struggle to keep the speed limit while climbing through the canyon in the east mountains of Albuquerque. Now I am not supercharged and I am already at 5000 feet trying to get to 7200 so I have that disadvantage. But because of this, I would recommend that unless you are boosted, dont run 4.88 and 37's.
- Thread starter
- #14
So, let me ask this... how would I figure accurate MPG?? I assume my odometer is run from the speedo, so my mileage would be off correct?
Correct. If, you are reading say 10% high, then your odometer is clocking 10% more distance. You can either divide your distance by 1.1 to reduce it 10% or get your mpg and reduce that by 10%. You can multiply your mpg by .9 if that makes more sense.
Use this site to find out what your actual percentage is vs OEM. Multiply your mileage shown on your odometer by that percentage.
275/70-R16 vs 315/75-R16 Tire Comparison - Tire Size Calculator | Tacoma World
275/70-R16 vs 315/75-R16 Tire Comparison - Tire Size Calculator | Tacoma World
If you have an Ultragauge you can calibrate distance on it to adjust speed at the display. You can either set up a 2 part equation for the ratio disparities between the tire revolutions and gear revolutions or you can zero the trip gauge, drive for a bit, and enter the actual miles traveled. I prefer the math
.
It doesn't correct the vehicle's odometer but will give you a more accurate speed.
. It doesn't correct the vehicle's odometer but will give you a more accurate speed.
