Larger diameter tyres V's peformance

[Signature035 Advanced Member Find all posts View Profile Close Signature035]

Last November we replaced the Cooper 265/65 ATR's with new 265/70 ST's.

Did a couple of short trips prior to Christmas, playing in the bush and was really impressed with their performance. (This is my 4th set of Coopers on 3 different 4WD's)

Around town they are slightly noisier, but nothing over the top (actually, quite a pleasant humm) and there was no noticable difference in performance.

We have the old 3ltr diesel, but had a DP Chip installed not long after we bought it as we do a bit of towing and there was a noticable lack of pulling power on big hills when fully loaded. The DP Chip fixed the problem nicely.

We have just come back from 3 weeks up the coast towing our Jayco Eagle Outback and the performance drop was significant. Hills that I previously glided over at 80kph + I was struggling to stay above 60kph. It was as if the DP Chip had been turned off.

I have had the Prado in for its 95k service since returning and all was in order (air filter was changed, but still acceptable). I have checked the operation of the DP Chip and its also good. The only variable I can see is the larger diameter tyres. I was told at the time there may be marginal impact on performance with the 70's, but in this case its enough to take the edge right off.

Just food for thought if you are considering going up a size, particularly if you tow. (Obviously not as relevant to the V6, or D4D owners either)

PS: Can't fault the Cooper ST's though, they are a huge improvement off road over the ATR's.

[WBY 1 Senior Member Find all posts View Profile Close WBY 1]

Re: Larger diameter tyres V's peformance

I'd also add your rolling resistance would be increased with the more aggressive tread, which would be more noticeable when going up hill.

(Obviously not as relevant to the V6, or D4D owners either)

slightly noticeable on the v6. I change between the dunlops (65 profile) and ATZ4's (70 profile) and in a lot of cases its like driving with the handbrake off and on respectively.

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Re: Larger diameter tyres V's peformance

I put 70 profile MTRs on mine recently and noticed the slight loss in performance. Fuel economy went up slightly as well. They are heavy buggers, I tried to lift the tyres once they were put on the rim at the wheel alignment place and I struggled.

If you can, find another PP'er around your area with the grandtreks and swap tyres for a short period. At least you can pinpoint whether it is the tyres or not.

[WBY 1 Senior Member Find all posts View Profile Close WBY 1]

Re: Larger diameter tyres V's peformance

If you can, find another PP'er around your area with the grandtreks and swap tyres for a short period. At least you can pinpoint whether it is the tyres or not.

can do at the moment if youre interested Rob. the roadies are sitting in the garage right now and will do for the next couple of weeks.

[sleeky Junior Member Find all posts View Profile Close sleeky]

Re: Larger diameter tyres V's peformance

I recently changed to BFG 265/70's and also noticed a drop off in performance, but not to the extent that you have. Speedo is now fairly accurate and I have got used to the minor performance difference, probably by driving a bit harder. The result is that fuel economy is not as good. Off road however I really appreciate the extra (if only slight) clearance.

Neil

[Signature035 Advanced Member Find all posts View Profile Close Signature035]

Re: Larger diameter tyres V's peformance

can do at the moment if youre interested Rob. the roadies are sitting in the garage right now and will do for the next couple of weeks.

Thanks for the offer Kenta.... Really I have eliminated all other causes..... and aound town (or off road) its notr noticeable, it's just under load, towing, it is running out of legs earlier than previously...... just the price I am going to have to pay for more off road ability.

[leachy_9 Advanced Member Find all posts View Profile Close leachy_9]

Re: Larger diameter tyres V's peformance

If you haven’t got one already, a good aftermarket exhaust will go a long way to alleviate the issue. The exhaust should not only improve the torque by itself but also allow you to turn up the DP chip without excessive EGTs.

Leachy

[Signature035 Advanced Member Find all posts View Profile Close Signature035]

Re: Larger diameter tyres V's peformance

If you haven’t got one already, a good aftermarket exhaust will go a long way to alleviate the issue. The exhaust should not only improve the torque by itself but also allow you to turn up the DP chip without excessive EGTs.

Leachy

Hey Leachy

Yep, Deffinitely on the shopping list, just waiting for an opportune moment to put the funds aside. Read lots & the Tipan looks like the go,......... Do you think it would return my performance to where it was?

[leachy_9 Advanced Member Find all posts View Profile Close leachy_9]

Re: Larger diameter tyres V's peformance

Hey Leachy

Yep, Deffinitely on the shopping list, just waiting for an opportune moment to put the funds aside. Read lots & the Tipan looks like the go,......... Do you think it would return my performance to where it was?

By fitting a 3" Taipan exhaust with no other mods I got a 9.7% increase in torque and a 3% increase in power. The torque was produced at about 200 rpm lower and the Prado was much nicer to drive. Mine is a D4D but I from what I have read and what others have reported I would expect similar results from a TD.
I would think this level of improvement in torque would compensate for the 6% increase in tyre diameter.

If you do get an exhuast, get an EGT fitted. This will allow you to get the most out the DP chip without excessive EGTs.

When I fitted a Steinbauer chip, having the exhuast with an EGT fitted enabled me to wind the chip up beyond the standard setting. This gave another 6% increase in power over what I was getting with the standard chip settings whilst maintianing EGTs below factory levels.

Leachy

[SteveMc Junior Member Find all posts View Profile Close SteveMc]

Re: Larger diameter tyres V's peformance

Totally agree with leachy,the exhaust & chip retuning will go a longway to compensate for the tyers but those buggers are hard to turn so you still have to think ahead with
the 1KZD Motor!

Cheers Steve
PS Its Still Scary overtaking when towing :roll:

[Bushbasher Out of control poster! Find all posts View Profile Close Bushbasher]

Re: Larger diameter tyres V's peformance

I'm still not sure that you can get more power from a chip but still get better economy? If it was that simple, then why wouldn't Mr Toyota programme his own computer (ECU) to get the power and torque that people demand?

Most peformance chips simply regraph the injector cycle to squirt more fuel in - more fuel = more power. More fuel = less economy. I have been around Ford EFI engines for many years now and keeping the air/fuel ratio as close to the stociometric (I think that's the correct term) ratio of @14:1produces the best power with economy value. To accelerate faster you dump more fuel, simple. You only have to watch your scanguage as you power away from a standing start to see the LP/100k go through the roof.

A loaded up Jayco is no garden trailer. My old outback swan was 1400kg EMPTY. With water tank, gas bottle full, food and clothing, etc it was probably around the 2000 - 2200kg mark. That will eat right into your figures.

I run 70 series BFGs which, as I have stated before, causes the speedo to read 100% accurately. I can still beat the advertised 9.3l/100km easily if I work the throttle with care. I don't think the rolling diameter of these tyres is the big drain on performance that you think it is.

[SteveMc Junior Member Find all posts View Profile Close SteveMc]

Re: Larger diameter tyres V's peformance

With a chip and exhaust your maximizing your fuel burn and exhaust gas dump so you dont need to use as much throtle .
It does work but its dam hard to stay of the throtle ops: as for the rolling diameter of the tyre i dont think thats the
isue, to me the rolling resistance of theise tyres is the isue.

Cheers Steve