tire size and tach readout @ 80mph

[Willy beamin]

Trying to get a feel for tire size / rpms - 80mph just happens to be my highway cruising speed. This is for everyone / big tire / small tires / medium tires - post up - please be careful....I'm running 35x12.5x17

 

[Markuson]

If I get a ticket, Ima sendin yoo the bill...

(Never mind that I speed all the time...)

Edit: Deleted other posts so as not to clutter results.

 

[Willy beamin]

we all do - gas is cheap and we have a ginormous V8

 

[Willy beamin]

I suggest you sit this one out then - spend your loot on a bike rack - the GOV gets enough as is

 

[gaijin]

I see two large sources of error in your experiment:

- Torque Converter must be locked up for any meaningful RPM numbers - it is obviously NOT locked up in one or both of your first 2 pics.

- You are reading RPM at an INDICATED 80 mph, not ACTUAL mph. As tire size changes, so does the ACTUAL mph at any given INDICATED mph.

Wouldn't it just be better - and more accurate - to compare the revs/mile for any given tire?

Just my $0.02.

HTH

 

[Markuson]

I see two large sources of error in your experiment:

- Torque Converter must be locked up for any meaningful RPM numbers - it is obviously NOT locked up in one or both of your first 2 pics.

- You are reading RPM at an INDICATED 80 mph, not ACTUAL mph. As tire size changes, so does the ACTUAL mph at any given INDICATED mph.

Wouldn't it just be better - and more accurate - to compare the revs/mile for any given tire?

Just my $0.02.

HTH

Another way to compare that would at least be consistent...would be to drive at exactly 2200 RPMs in your tallest gear...and note the indicated speed. Then you could do basically the same comparison, but the other way round. RPMs will show correctly regardless of tire size...and indicated speed then becomes the differentiator.

 

[Willy beamin]

You guys sure are nice about telling me my math and science are OFF - ha
Thank you

 

[gaijin]

You guys sure are nice about telling me my math and science are OFF - ha
Thank you

Your question is a good one - how do different tire sizes affect RPM at any given speed?

The only problem is the method.

I found out all I need to know about speedometer accuracy, how the speedo differs from the odometer, what speed I'm actually travelling at when I set the cruise control to a given speed, etc. by using a standalone GPS unit.

If you have, or can borrow, an automotive GPS unit like a Garmin, Tom Tom, Magellan, etc. that shows speed based on GPS position (they all should) then you will have an accurate reference speed to compare everything else to.

Easy test #1: Set your cruise control to 60 (or 80 if you feel the need for speed) and see what speed the standalone GPS unit says you're travelling. The difference is your speedo error - and don't be surprised when it shows your speedo is more accurate with larger than stock tires.

Easy test #2: Reset the trip odo on both the LC and the GPS unit and drive around for a few hundred miles. If you have larger than stock tires, you will find that you actually drove farther than your onboard odo thinks you did.

Phone-based GPS apps are not as accurate as dedicated standalone GPS units, but might give you a rough idea.

HTH

 

[Markuson]

Your question is a good one - how do different tire sizes affect RPM at any given speed?

The only problem is the method.

I found out all I need to know about speedometer accuracy, how the speedo differs from the odometer, what speed I'm actually travelling at when I set the cruise control to a given speed, etc. by using a standalone GPS unit.

If you have, or can borrow, an automotive GPS unit like a Garmin, Tom Tom, Magellan, etc. that shows speed based on GPS position (they all should) then you will have an accurate reference speed to compare everything else to.

Easy test #1: Set your cruise control to 60 (or 80 if you feel the need for speed) and see what speed the standalone GPS unit says you're travelling. The difference is your speedo error - and don't be surprised when it shows your speedo is more accurate with larger than stock tires.

Easy test #2: Reset the trip odo on both the LC and the GPS unit and drive around for a few hundred miles. If you have larger than stock tires, you will find that you actually drove farther than your onboard odo thinks you did.

Phone-based GPS apps are not as accurate as dedicated standalone GPS units, but might give you a rough idea.

HTH

I suspect he sorta NEEDS to drive 80 in order to stay in high gear since he's not re-geared to 4.88. Ya, Willy?

 

[dukmon]

Mmmkay...........

Running 285/75/18's

Speedo @ 80, Tach @ 2k on the nose
GPS @ 80, Tach @ 1.9k on the nose

If I have a chance, I'll set the cruise when my GPS reads 80 and see what my speedo says. Traffic was too much to try this earlier today.
Hard to look at so many things at once, but it appears when my speedo was reading 80, my GPS confirmed I was going almost 83.

Or maybe I have that backwards.

Tough day.

 

[Markuson]

Mmmkay...........

Running 285/75/18's

Speedo @ 80, Tach @ 2k on the nose
GPS @ 80, Tach @ 1,9k on the nose

If I have a chance, I'll set the cruise when my GPS reads 80 and see what my speedo says. Traffic was too much to try this earlier today.
Hard to look at so many things at once, but it appears when my speedo was reading 80, my GPS confirmed I was going almost 83.

Or maybe I have that backwards.

Tough day.

Heh. You could have **50 inch tires**...and--if you could get your transmission to stay in top gear for it--your tach would still be at 2k and your speedo would still indicate 80. The test doesn't work because your speedo isn't size-corrected.

 

[dukmon]

But my GPS is...........

 

[Willy beamin]

@Markuson Correct - I'm not re-geared with 4:88's .....what started this... I was talking to @MScruiser about his lag with the 37's compared to when he was running 35's. He's the only guy I know running a larger tire than me. By MScruisers numbers we aren't that far off...which was interesting.

My numbers at 80 mph:
4th gear 3300
5th gear 2300
6th gear 1900
Please ignore my 2nd photo above - all others are matching up - this is tuning as we go

MScruiser numbers @ 80 mph:
4th gear 3000
5th gear 2200
no 6th recorded

Please keep in mind this is two guys hauling ass in big trucks trying to snap a photo - not a precise scenario but a good S.W.A.G.


I realize the first three gears or "in town gears" are labored by the larger tires. The interstate speeds / gears (4th, 5th, 6th) when you're really moving is where my interest is and the variance of - stock / 35's / 37's. I just happen to pick 80 mph as that's what most of the traffic flow is around the interstates I travel everyday.

The root of my question is in my lack in knowing how to calculate various gear ratios and negative nature of running larger tires with stock gears. I'm have been considering diving into 4:88's for a while. First I spoke with Christo as he was running 35's. During our conversation (months ago) he suggested (from what I remember) take a drive and as a comparison switch between 4th and 5th gear (or maybe it was 4th and 6th?) and that would be the difference if re-geared with 4:88's to get a feel - if I understood him correctly...and maybe I didn't. ..this might as well be rocket science really.

What I'd like to understand how much my rpms are suffering w/ 35's over stock sized tires and then what gains would read out if running 4:88's throughout the gear band...yes @ 80 mph. To my knowledge I believe only the JT guys @just differentials are running the 4:88's with (close to) 35's, so If they could chime in that'd be great.

I know my speedo is off but I had to have a marker to check against so thats what I landed on.
If this is a simple deal to understand then please break it down - Because I don't get it
thanks

 

[MScruiser]

The guys with Cangaro Racing also have regeared (I think).

IM Kurt Williams or Dave Connors

 

[Willy beamin]

Good point! I'll do that

 

[bjowett]

Do the math, then compare to what you are viewing on the 200.

Engine RPM / Engine Speed Calculator

 

[Willy beamin]

@bjowett - thank you