{"id":9087,"date":"2026-08-19T22:00:00","date_gmt":"2026-08-19T13:00:00","guid":{"rendered":"https:\/\/everinterest.net\/en\/?p=9087"},"modified":"2026-08-10T13:10:33","modified_gmt":"2026-08-10T04:10:33","slug":"f25-roof-box-fuel-economy","status":"publish","type":"post","link":"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/","title":{"rendered":"Roof Box Fuel Economy: I Measured the Real Cost Over 24,000 km"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">Another ski season went by without me getting out anywhere near as often as I wanted. Today I finally admitted it was over: the roof box came off my BMW X3 (F25), roof bars included.<\/p>\n\n\n<p class=\"wp-block-paragraph\">Once you are married with kids, hobbies like skiing quietly slide down the list. We did get out a few times with the little one, so the box earned its keep.<\/p>\n\n\n<p class=\"wp-block-paragraph\">The one thing nobody argues about is roof box fuel economy: the thing drinks fuel. I had written about that on feel alone before. This time I had a full season of fill-up records, so I could finally put a number on it.<\/p>\n\n\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_86 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#Why_roof_box_fuel_economy_always_suffers\" >Why roof box fuel economy always suffers<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#Frontal_area_of_the_car\" >Frontal area of the car<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#Frontal_area_of_the_roof_box\" >Frontal area of the roof box<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#The_weight_penalty_is_almost_irrelevant\" >The weight penalty is almost irrelevant<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#Real_fuel_economy_13_tanks_with_13_tanks_without\" >Real fuel economy: 13 tanks with, 13 tanks without<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#With_the_roof_box_fitted\" >With the roof box fitted<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#Without_the_roof_box\" >Without the roof box<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#The_result_about_9_worse\" >The result: about 9% worse<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-9\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#What_that_means_for_other_cars\" >What that means for other cars<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-10\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#What_it_costs_in_fuel\" >What it costs in fuel<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-11\" href=\"https:\/\/everinterest.net\/en\/f25-roof-box-fuel-economy\/#Take_the_roof_box_off_when_you_are_not_using_it\" >Take the roof box off when you are not using it<\/a><\/li><\/ul><\/nav><\/div>\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Why_roof_box_fuel_economy_always_suffers\"><\/span>Why roof box fuel economy always suffers<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p class=\"wp-block-paragraph\">A roof box does two things to your car: it pushes more air out of the way, plus it adds weight. Only one of those really matters.<\/p>\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Frontal_area_of_the_car\"><\/span>Frontal area of the car<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n<p class=\"wp-block-paragraph\">The F25 X3 catalogue figures give a rough frontal area if you take the overall height, subtract the ground clearance, then multiply by the width.<\/p>\n\n\n<p class=\"wp-block-paragraph\">(1,675 mm height \u2212 210 mm ground clearance) \u00d7 1,900 mm width = 27,835 cm\u00b2<\/p>\n\n\n<p class=\"wp-block-paragraph\">That ignores the tyres, but it is close enough for a comparison.<\/p>\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Frontal_area_of_the_roof_box\"><\/span>Frontal area of the roof box<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n<p class=\"wp-block-paragraph\">Mine is an INNO Shadow 16 (BR1200BK). It is discontinued now, though <a href=\"https:\/\/www.innoracks.com\/\" target=\"_blank\" rel=\"noopener noreferrer\">INNO<\/a> still sells the same shape under a newer name.<\/p>\n\n\n<p class=\"wp-block-paragraph\">810 mm wide \u00d7 340 mm tall = 2,754 cm\u00b2<\/p>\n\n\n<p class=\"wp-block-paragraph\">Add that to the car:<\/p>\n\n\n<p class=\"wp-block-paragraph\">27,835 cm\u00b2 + 2,754 cm\u00b2 = 30,589 cm\u00b2<\/p>\n\n\n<p class=\"wp-block-paragraph\">That is a <strong>9.9% increase<\/strong> in frontal area. The roof bars add a bit more on top, but I gave up trying to calculate those.<\/p>\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"The_weight_penalty_is_almost_irrelevant\"><\/span>The weight penalty is almost irrelevant<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n<p class=\"wp-block-paragraph\">I weighed up the hardware too, just to be thorough:<\/p>\n\n\n<ul class=\"wp-block-list\">\n<li>Roof box: 19 kg<\/li>\n<li>XS450 mounting feet: 2.7 kg<\/li>\n<li>TR141 fitting hooks: 0.88 kg<\/li>\n<li>XB130 rear bar: 1.44 kg<\/li>\n<li>XB123 front bar: 1.36 kg<\/li>\n<\/ul>\n\n\n<p class=\"wp-block-paragraph\">Total: 25.38 kg (about 56 lb).<\/p>\n\n\n<p class=\"wp-block-paragraph\">That takes the car from 1,870 kg to 1,895 kg, an increase of roughly 1.3%. Honestly, carrying one extra passenger changes more than that. Aerodynamics is the story here, not mass.<\/p>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Real_fuel_economy_13_tanks_with_13_tanks_without\"><\/span>Real fuel economy: 13 tanks with, 13 tanks without<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p class=\"wp-block-paragraph\">I kept the box on for about three months. That came to 13 fill-ups, so I took the 13 fill-ups immediately before fitting it as the comparison set.<\/p>\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"With_the_roof_box_fitted\"><\/span>With the roof box fitted<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n<p class=\"wp-block-paragraph\">The box went on with the roof bars at the end of 2020, so these records run from late December 2020 to mid-March 2021.<\/p>\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Fill-up<\/th><th>km\/L<\/th><th>Distance (km)<\/th><th>Fuel (L)<\/th><\/tr><\/thead><tbody><tr><td>1<\/td><td>16.70<\/td><td>1,019<\/td><td>61.01<\/td><\/tr><tr><td>2<\/td><td>18.28<\/td><td>1,014<\/td><td>55.46<\/td><\/tr><tr><td>3<\/td><td>19.08<\/td><td>815<\/td><td>42.71<\/td><\/tr><tr><td>4<\/td><td>18.31<\/td><td>1,034<\/td><td>56.46<\/td><\/tr><tr><td>5<\/td><td>18.20<\/td><td>990<\/td><td>54.40<\/td><\/tr><tr><td>6<\/td><td>18.37<\/td><td>1,037<\/td><td>56.46<\/td><\/tr><tr><td>7<\/td><td>16.24<\/td><td>853<\/td><td>52.52<\/td><\/tr><tr><td>8<\/td><td>16.44<\/td><td>873<\/td><td>53.09<\/td><\/tr><tr><td>9<\/td><td>18.90<\/td><td>1,022<\/td><td>54.07<\/td><\/tr><tr><td>10<\/td><td>19.60<\/td><td>942<\/td><td>48.06<\/td><\/tr><tr><td>11<\/td><td>17.95<\/td><td>767<\/td><td>42.73<\/td><\/tr><tr><td>12<\/td><td>14.26<\/td><td>781<\/td><td>54.76<\/td><\/tr><tr><td>13<\/td><td>17.72<\/td><td>948<\/td><td>53.50<\/td><\/tr><tr><td><strong>Total<\/strong><\/td><td><strong>17.65<\/strong><\/td><td><strong>12,095<\/strong><\/td><td><strong>685.23<\/strong><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n<p class=\"wp-block-paragraph\">Not one tank cracked 20 km\/L. The best was 19.60 km\/L.<\/p>\n\n\n<p class=\"wp-block-paragraph\">Over 12,095 km on 685.23 litres, the overall figure was 17.65 km\/L. For anyone thinking in other units, that is 5.67 L\/100 km, or about 41.5 mpg (US).<\/p>\n\n\n<p class=\"wp-block-paragraph\">Winter brings strong headwinds plus longer warm-up idling, so conditions were not identical to autumn. With 13 tanks on each side, though, I think the sample is big enough to mean something.<\/p>\n\n\n<p class=\"wp-block-paragraph\">I also have aluminium tape stuck on the box in a few places to reduce drag. It went on from day one, so I cannot separate its effect. Without it the numbers might have been worse.<\/p>\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Without_the_roof_box\"><\/span>Without the roof box<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n<p class=\"wp-block-paragraph\">These are the fill-ups from before the box went on, running from late September 2020 to early December 2020.<\/p>\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Fill-up<\/th><th>km\/L<\/th><th>Distance (km)<\/th><th>Fuel (L)<\/th><\/tr><\/thead><tbody><tr><td>1<\/td><td>20.18<\/td><td>1,026<\/td><td>50.85<\/td><\/tr><tr><td>2<\/td><td>21.06<\/td><td>806<\/td><td>38.28<\/td><\/tr><tr><td>3<\/td><td>18.87<\/td><td>630<\/td><td>33.39<\/td><\/tr><tr><td>4<\/td><td>17.31<\/td><td>792<\/td><td>45.75<\/td><\/tr><tr><td>5<\/td><td>20.00<\/td><td>800<\/td><td>40.00<\/td><\/tr><tr><td>6<\/td><td>21.03<\/td><td>1,015<\/td><td>48.26<\/td><\/tr><tr><td>7<\/td><td>18.59<\/td><td>1,004<\/td><td>54.00<\/td><\/tr><tr><td>8<\/td><td>20.00<\/td><td>859<\/td><td>42.95<\/td><\/tr><tr><td>9<\/td><td>19.28<\/td><td>1,253<\/td><td>65.00<\/td><\/tr><tr><td>10<\/td><td>19.56<\/td><td>1,193<\/td><td>61.00<\/td><\/tr><tr><td>11<\/td><td>19.10<\/td><td>1,182<\/td><td>61.88<\/td><\/tr><tr><td>12<\/td><td>18.42<\/td><td>1,002<\/td><td>54.41<\/td><\/tr><tr><td>13<\/td><td>19.76<\/td><td>679<\/td><td>34.37<\/td><\/tr><tr><td><strong>Total<\/strong><\/td><td><strong>19.43<\/strong><\/td><td><strong>12,241<\/strong><\/td><td><strong>630.14<\/strong><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n<p class=\"wp-block-paragraph\">12,241 km on 630.14 litres works out at 19.43 km\/L, which is 5.15 L\/100 km, or about 45.7 mpg (US).<\/p>\n\n\n<p class=\"wp-block-paragraph\">The first four rows are on winter tyres, the other nine on summer tyres (newest first). The best tank of the whole set, 21.06 km\/L, came after switching to winter rubber. My winter tyres are Pirelli Ice Asimmetrico Plus 225\/60R17, second season, plus they happen to be very economical. That is why I was comfortable mixing both tyre types into one comparison.<\/p>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"The_result_about_9_worse\"><\/span>The result: about 9% worse<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p class=\"wp-block-paragraph\">19.43 km\/L dropped to 17.65 km\/L. That is a <strong>9.1% loss<\/strong>.<\/p>\n\n\n<p class=\"wp-block-paragraph\">Frontal area went up 9.9%. Fuel economy fell 9.1%. Those two numbers landing almost on top of each other is more satisfying than it has any right to be.<\/p>\n\n\n<p class=\"wp-block-paragraph\">Same commute, same driving style, same roads. So the honest answer on roof box fuel economy, for a car shaped like mine, is roughly a 9% hit. The measured penalty matched what my right foot had been telling me all winter.<\/p>\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"What_that_means_for_other_cars\"><\/span>What that means for other cars<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n<p class=\"wp-block-paragraph\">Drag scales with speed, so your result depends heavily on how you drive. My mileage is mostly motorway at 90 to 100 km\/h (56 to 62 mph), which is close to the worst case.<\/p>\n\n\n<p class=\"wp-block-paragraph\">Apply the same 9.1% to other starting points:<\/p>\n\n\n<ul class=\"wp-block-list\">\n<li>A car doing 20 km\/L becomes 18.2 km\/L<\/li>\n<li>A car doing 15 km\/L becomes 13.6 km\/L<\/li>\n<li>A car doing 25 km\/L becomes 22.7 km\/L<\/li>\n<\/ul>\n\n\n<p class=\"wp-block-paragraph\">In US mpg: 40 mpg becomes 36.4, 30 mpg becomes 27.3, 25 mpg becomes 22.7.<\/p>\n\n\n<p class=\"wp-block-paragraph\">Your car shape, your speed, your box, your bars: all of it moves the number. Treat this as a ballpark rather than a specification.<\/p>\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"What_it_costs_in_fuel\"><\/span>What it costs in fuel<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n<p class=\"wp-block-paragraph\">Fuel economy translates straight into money, which is why it nags at me.<\/p>\n\n\n<p class=\"wp-block-paragraph\">At my mileage, a 10% swing is \u00a52,000 to \u00a53,000 a month (roughly $13 to $20). Over a year that is tens of thousands of yen, a couple of hundred dollars. Enough to notice.<\/p>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Take_the_roof_box_off_when_you_are_not_using_it\"><\/span>Take the roof box off when you are not using it<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p class=\"wp-block-paragraph\">Removing a roof box is a hassle, plus you need somewhere to store the thing. After running the numbers, though, leaving it up there all year is simply throwing fuel away.<\/p>\n\n\n<p class=\"wp-block-paragraph\">I make a point of pulling mine off as soon as the season ends. Taking it off after every single trip would be ideal. That is a step too far for me.<\/p>\n\n\n<p class=\"wp-block-paragraph\">If you cover serious distance, keeping the car in its most efficient shape is worth the effort, for your wallet as much as for the planet.<\/p>\n\n\n<p class=\"wp-block-paragraph\"><em>This article is also available in <a href=\"https:\/\/everinterest.net\/f25-roofbox-nenpi-hikaku202103\/\" target=\"_blank\" rel=\"noopener noreferrer\">Japanese<\/a>.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Roof box fuel economy measured over 24,000 km on a BMW X3: 13 tanks with the box fitted versus 13 tanks without. The real penalty was 9.1%.<\/p>\n","protected":false},"author":1,"featured_media":9086,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[8],"tags":[1996,1964,11,13,249,1995,1997],"class_list":["post-9087","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-bmw-x3-f25","tag-aerodynamics","tag-bmw-x3","tag-f25","tag-fuel-economy","tag-inno","tag-roof-box","tag-roof-rack"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.3 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Roof Box Fuel Economy: 9.1% Worse Over 24,000 km<\/title>\n<meta name=\"description\" content=\"Roof box fuel economy measured over 24,000 km on a BMW X3: 13 tanks with the box fitted versus 13 tanks without. 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