Test of MPG at various speeds: results

[Mike_R]

I was curious what effect different speeds would have on the MPG I would get since here in Utah, the urban speed limit is 70 MPH which generally means the lanes vary from <70 to 80 MPH so when there's not congestion, you often can select your desired speed and pick a corresponding lane. Much of Utah even has speed limits of 80 MPH (thank you UDOT, it saves time on road trips!) which means you can go at least 85 MPH without the risk of a ticket. But I limited this test to 80 MPH as there's not a place near by where I could conveniently drive the same stretch with speeds from 65 to 85 without the risk of getting pulled over. What effect does it have if I choose 80 MPH rather than 70?

Results here, details below.
This graph shows the two runs for each speed averaged together:

This graph shows each run seperately so you can see if a run looks like an anomaly that's potentially not valid. Personally, the 65 MPH Westbound seems like it MIGHT be too high but possible not. And the 75 MPH Westbound seems like it's probably inaccurate as it's not consistent with the trend seen in all other runs - meaning all the other runs showed a lower MPG with an increase in speed but the 75 MPH Westbound does not as it's MPG is better than the 70 MPH Westbound.

I would like to run this test on a different stretch of freeway and average it together with this run to dilute any variables and therefore get more accurate numbers.
I drove a 5.3 mile stretch of freeway at 65, 70, 75, and 80 MPH. One run each way of the freeway at each speed using cruise control. I measured the MPG using the on-board computer. I tried to keep no cars in front of me to prevent differences in aerodynamics between the test.
I ran the freeway in this order to spread out the two runs at each speed as much possible to minimize the effect of variables in between the speeds:
80 MPH Westbound, 65 MPH Eastbound.
70 MPH Westbound, 75 MPH Eastbound.
65 MPH Westbound, 80 MPH Eastbound
75 MPH Westbound, 70 MPH Eastbound.
The car: 2014 CX-5, FWD, 2.4 engine. 65,500 miles. Fairly aggressive all season tires. OEM roof side rails installed but not the crossbars. Chevron 85 octane gas. Windows up, AC on max with the fan on 3. Headlights on.
Things that happen to not be OEM: Air filter, tires, and the engine oil and filter are whatever Jiffy Lube put in when the previous owner took it there.

 

[Unobtanium]

Looks like about 2 to 3mpg more than I get at each speed. I have awd.

 

[Mike_R]

Looks like about 2 to 3mpg more than I get at each speed. I have awd.

Good to know. Do you drive much in rain or snow? Before buying mine, I looked at as much data as I could and 3 MPG is difference I guessed between AWD/FWD in my snowy climate. Small enough that I wish had AWD but the private seller market is very limited here and AWD adds about $1200 per Kelly Blue Book so to get one near my budget, I would've had to wait forever till a base trim with higher than average miles was for sell. I didn't want to wait that long so after 2 months I bought a FWD Touring and I love it but still wish I had AWD.

 

[Kaps]

Your 33.6 is inaccurate. Everything else seems +/- .3 or so off from what I see in TX. Ofcourse mine has 10k Miles. The best mpg you will see is around 30/50 mph.
Did a trip to Austin - lots of 80 mph driving but very close to other cars. Too many cars on the road due to long weekend. I was pleasantly surprised to see about 28 mpg for the trip. On open highway this would be 26 mpg at best. FWD till 16 sucks on the highway big time. Hopefully someone will repeat this for 17 - it should see good real world numbers on the highway. Pretty sure it will beat 16 FWD despite being 90 lbs heavier.

 

[jhu8]

I'm getting pretty much those exact same numbers in my AWD 16 CX5. Actually pretty spot on. I know your data is for FWD though.

 

[thzpcs]

That's pretty cool. Solid information for going on road trips and such.

 

[dougal]

Your 33.6 is inaccurate. Everything else seems +/- .3 or so off from what I see in TX. Ofcourse mine has 10k Miles. The best mpg you will see is around 30/50 mph.
Did a trip to Austin - lots of 80 mph driving but very close to other cars. Too many cars on the road due to long weekend. I was pleasantly surprised to see about 28 mpg for the trip. On open highway this would be 26 mpg at best. FWD till 16 sucks on the highway big time. Hopefully someone will repeat this for 17 - it should see good real world numbers on the highway. Pretty sure it will beat 16 FWD despite being 90 lbs heavier.

Why is he inaccurate? Because he does not get what you get?

 

[Mike_R]

That's pretty cool. Solid information for going on road trips and such.

Yeah, that's what I mostly wanted the info for. I go on quite a lot of road trips so I wanted the necessary info to prioritize speed vs. gas savings on each trip.

 

[Tccox]

Pretty much matches what I get with my AWD.

 

[Kaps]

Why is he inaccurate? Because he does not get what you get?

See the graph for detail - the 37 is a big outlier. All mpgs reduce with speed but the jump from 37 is really big. Not sure I would trust that number.
33.6 is not higher than what I get - except onsies twosies = never heard anyone with FWD getting 30 or up if doing 70 or more. I beat the combined by a good 2-3 mpg. But 33 is a distant dream. Maybe my tires need to be totally naked or something running 40 psi to even dream of that mpg.

 

[NelsonLewis]

Pretty much matches what I get with my AWD.

Same here.

 

[RDK006]

I've consistently gotten 33-35 mpg on stretches going eastbound from L.A. to Palm Springs (mostly light traffic, from 70-80 mph); going westbound from PS to L.A., I get about 3-4 mpg less.

 

[brillo54]

See the graph for detail - the 37 is a big outlier. All mpgs reduce with speed but the jump from 37 is really big. Not sure I would trust that number.
33.6 is not higher than what I get - except onsies twosies = never heard anyone with FWD getting 30 or up if doing 70 or more. I beat the combined by a good 2-3 mpg. But 33 is a distant dream. Maybe my tires need to be totally naked or something running 40 psi to even dream of that mpg.

From my experience ('16 FWD, 22,000 miles) I think 37 mpg @ 65 mph is quite possible. It seems the westbound runs are wind/terrain assisted while the eastbound runs are being hindered. To the OP, I'm curious what the conditions were. On a calm day, I can go 70 mph and get 32 - 33 mpg, but a 20 mph headwind will take that down to 29 mpg while a 20 mph tailwind will take it up to 35 or 36. From my point of view, the data point I question is the 70 mph westbound. Looks a bit low to me.

My CX-5 is stock. OE tires have 16,300 miles on them (winter tires have 5,700), tread depth of about 7 or 8/32". 3 oil changes @ 5 K, 12.5 K and 20 K (always Mazda moly). tire pressure at 38 psi.

YMMV

 

[Mike_R]

From my experience ('16 FWD, 22,000 miles) I think 37 mpg @ 65 mph is quite possible. It seems the westbound runs are wind/terrain assisted while the eastbound runs are being hindered. To the OP, I'm curious what the conditions were. On a calm day, I can go 70 mph and get 32 - 33 mpg, but a 20 mph headwind will take that down to 29 mpg while a 20 mph tailwind will take it up to 35 or 36. From my point of view, the data point I question is the 70 mph westbound. Looks a bit low to me.

My CX-5 is stock. OE tires have 16,300 miles on them (winter tires have 5,700), tread depth of about 7 or 8/32". 3 oil changes @ 5 K, 12.5 K and 20 K (always Mazda moly). tire pressure at 38 psi.

YMMV

Yeah, the 37 mpg @ 65 mph data point shouldn't be considered by itself, it should be considered along with the 30.1 mpg @ 65 data point for an average of 33.6 mpg @ 65 mph. The assist/hindrance is terrain. There's a slight elevation loss going westbound.

The conditions were 86F, sunny and the only time I paid attention to the wind was in between runs if I had a red light and I it was only lightly breezy during those times.

 

[Kaps]

From my experience ('16 FWD, 22,000 miles) I think 37 mpg @ 65 mph is quite possible. It seems the westbound runs are wind/terrain assisted while the eastbound runs are being hindered. To the OP, I'm curious what the conditions were. On a calm day, I can go 70 mph and get 32 - 33 mpg, but a 20 mph headwind will take that down to 29 mpg while a 20 mph tailwind will take it up to 35 or 36. From my point of view, the data point I question is the 70 mph westbound. Looks a bit low to me.

My CX-5 is stock. OE tires have 16,300 miles on them (winter tires have 5,700), tread depth of about 7 or 8/32". 3 oil changes @ 5 K, 12.5 K and 20 K (always Mazda moly). tire pressure at 38 psi.

YMMV

Thank god you specified your psi. The door reads 34 - most folks will ride 34. If I did 40 psi sure I will get 37-38 but my experience - Running 36 psi - nowhere and on no day ever ever ever I see anything north of 31.5 mpg on a highway - if i drive for enjoyment and keep it 60 mph ..... Still I dont see 32 mpg as well. This comes back to my old comment on 33 EPA - pretty sure it was a stock CX5 Sport without floor mats / skinny driver and 44 psi tires. Otherwise 33 on highway is impossibru.

 

[Unobtanium]

Thank god you specified your psi. The door reads 34 - most folks will ride 34. If I did 40 psi sure I will get 37-38 but my experience - Running 36 psi - nowhere and on no day ever ever ever I see anything north of 31.5 mpg on a highway - if i drive for enjoyment and keep it 60 mph ..... Still I dont see 32 mpg as well. This comes back to my old comment on 33 EPA - pretty sure it was a stock CX5 Sport without floor mats / skinny driver and 44 psi tires. Otherwise 33 on highway is impossibru.

I have compared 34 to 38psi and noted no differences over multiple tanks of gas, regarding mileage and my daily commute.

As to one way vs. the other, I used to live near about a 1.5mi long bridge. In my Z06, I could get something like 29mpg going across it one way, and only got low 20's going across it the other way. Same for other vehicles I drove on it, but the Z06 getting nearly 30 made me do a double-take! And this is a BRIDGE...one would think it would be kinda sorta level. not so much, lol

 

[Kaps]

2psi increase is noticeable for me. In terms of pulling away - its crisper and my highway mpg numbers look good. I dont think a 4 psi difference is zero in effect. Run it at the cost of harsh handling for 2 weeks and then you may see a difference.
If my commute changed to highway a lot - i would put in 38 psi and roll with it.

 

[Unobtanium]

2psi increase is noticeable for me. In terms of pulling away - its crisper and my highway mpg numbers look good. I dont think a 4 psi difference is zero in effect. Run it at the cost of harsh handling for 2 weeks and then you may see a difference.
If my commute changed to highway a lot - i would put in 38 psi and roll with it.

Agreed on harshness. It was notably harsher, with no improvement in mpg, even on the highway.

 

[brillo54]

Thank god you specified your psi. The door reads 34 - most folks will ride 34. If I did 40 psi sure I will get 37-38 but my experience - Running 36 psi - nowhere and on no day ever ever ever I see anything north of 31.5 mpg on a highway - if i drive for enjoyment and keep it 60 mph ..... Still I dont see 32 mpg as well. This comes back to my old comment on 33 EPA - pretty sure it was a stock CX5 Sport without floor mats / skinny driver and 44 psi tires. Otherwise 33 on highway is impossibru.

Just to clarify. I've got a GT so the door reads 36 psi for my 19" Toyo's and I'm only 2 psi above recommended, not 4.

As an example of a typical drive for me, wife and I went to get garden supplies this afternoon. 20 mile drive, much of it a 55 mph speed limit highway (so i went 60) and slight tailwind (4-5 mph). Fuel economy monitor that I have set to display when engine shuts off said 34 mpg. Return trip, similar conditions (now a slight headwind) and speed, same passenger weight (~380 lbs) and 370 lbs of cargo (bagged mulch and crap), AC on and FEM said 32 mpg at trip end.

I've driven my CX-5 as far south as Natchez, MS, west as far as Bull Shoals, AR, to the far north burbs of Chicago. Getting 33 mpg overall with numbers to back it up on Fuelly. No complaints here.

On a lighter note, I've always heard things are bigger in Texas. Maybe gravity is bigger too?? (uhm) (rofl)

 

[finch204]

Just to clarify. I've got a GT so the door reads 36 psi for my 19" Toyo's and I'm only 2 psi above recommended, not 4.

As an example of a typical drive for me, wife and I went to get garden supplies this afternoon. 20 mile drive, much of it a 55 mph speed limit highway (so i went 60) and slight tailwind (4-5 mph). Fuel economy monitor that I have set to display when engine shuts off said 34 mpg. Return trip, similar conditions (now a slight headwind) and speed, same passenger weight (~380 lbs) and 370 lbs of cargo (bagged mulch and crap), AC on and FEM said 32 mpg at trip end.

I've driven my CX-5 as far south as Natchez, MS, west as far as Bull Shoals, AR, to the far north burbs of Chicago. Getting 33 mpg overall with numbers to back it up on Fuelly. No complaints here.

On a lighter note, I've always heard things are bigger in Texas. Maybe gravity is bigger too?? (uhm) (rofl)

Took a quick look at your fuelly records and you sir are getting incredible MPG. I noticed that you drive at least 300+ miles before you fill-up. How many bars are left on the fuel bar before you fill-up? I usually fill-up when there are 3 bars left, so 1/4 of the whole fuel bar. That amounts to around 240 miles covered before filling up.